Exciton-polariton photodiodes

Domain: physics
Published: 2026-01-17
Editorial outcome: major_revision

Abstract
Photodiodes are central to high-speed communication, sensing and light-harvesting devices. While silicon and other inorganic semiconductor-based photodiodes have been widely integrated into commercial technologies, photodiodes from excitonic semiconductors continue to maintain high interest in research. Excitonic semiconductors are superior in terms of absorption as compared to silicon and III-V semiconductors, but have always suffered from poor charge transport and narrowband absorption due to the short diffusion lengths and resonant nature of the excitons, respectively. Here, we attempt to solve both these central issues in excitonic photodiode devices via use of polariton physics and demonstrating photodiodes in the strong light-matter coupling regime. By using a conductive tin-doped indium oxide transparent top electrode that also serves as an anti-reflective coating combined with self-trapping and hybridization of light in inorganic excitonic semiconductor WS2, we demonstrate photodiodes with broadband absorption and internal quantum efficiency values approaching unity near the zero-detuning condition. Furthermore, it is demonstrated that the response speed of photodiodes is enhanced under strong coupling. Our work presents a promising approach to explore the physics and applications of photodiodes based on excitonic semiconductors.

Introduction
Photodiodes and photovoltaics based on excitonic semiconductors have emerged as a potentially low-cost alternative to conventional semiconductors1–9, owing to their markedly enhanced absorption coefficients in photon harvesting processes10–12. In excitonic photodiodes and photovoltaics (e.g., organic photovoltaics), the excited state (exciton) must diffuse to a heterointerface before being separated into free carriers13–15. However, poor exciton transport is often a limiting factor on device efficiency.

Strong coupling between excitons and cavity photons can occur by placing an excitonic material into an optical cavity with a discrete mode in resonance with the exciton energy16–21. The resulting hybrid states are called exciton-polaritons (EPs). EPs, in contrast to excitons, can exhibit long-range transport due to a low effective mass and delocalized wavefunction22–27. Recently, the utilization of strong coupling in organic semiconductor devices has been demonstrated, leading to longer exciton diffusion lengths and enhanced photocurrent28–33. Notably, most of the previously reported studies on organic photovoltaic devices have required external optical cavities for strong light-matter interactions due to the low refractive indices, thus resulting in considerable energy loss from partially transparent metal top reflectors, leading to the inability to obtain state-of-the-art device performance. In contrast, recent advances in open-cavity architectures have demonstrated a viable pathway toward substantial absorption enhancement without such trade-offs34,35.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) feature dielectric screening and strong confinement effects that lead to unusually large exciton binding energies in monolayers (>300 meV), but their van der Waals bonding out of plane also ensures binding energies >25 meV in the bulk. Thus, exciton photophysics governs photoexcited carriers at room temperature36. Furthermore, the large optical constants and strong light-matter interactions enable strong light absorption in the films37–39. TMDCs of Mo and/or W, in particular, show simultaneously strong exciton resonances and large optical constants, enabling self-hybridized EPs and providing a natural platform for investigating photon-exciton coupling without the need for external optical cavities40–42.

Here, we demonstrate a self-hybridized exciton-polariton photodiode with a transparent conductive top electrode also acting as an antireflective coating, enabling strong light-matter coupling with broadband light absorption and polariton-enhanced transport. The strong-coupling effect is observed with large polariton peaks and dispersion behavior in thickness-dependent reflectance spectra. Corresponding polariton peaks are also observed from external quantum efficiency (EQE) spectra, indicating that the EP states enhance the generation of photocurrent. The strong coupling of excitons to cavity photons in the WS2 absorber layer enhances the internal quantum efficiency (IQE) to near unity. The exciton polariton transport achieved improves the response speed of the diode, obtaining the fastest response time of 217 ns. This work demonstrates that strong light-matter coupling in self-hybridized structures can be used to design polaritonic photodiodes with broadband absorption and polariton-enhanced transport for improved quantum efficiency and response speed.

Results
Device structure and working principle
The exciton-polariton photodiode is shown in Fig. 1a. WS2 is contacted by a tin-doped indium oxide (ITO) layer that functions as both an electrode and an anti-reflective coating layer. The thickness of the WS2 active layer varies from 5 nm to 200 nm. The index contrast between ITO and air enables the ITO to be used as an antireflective coating (ARC). However, the large optical impedance mismatch between the ITO and WS2 results in a Fresnel reflection coefficient at the interface that can produce Fabry-Perot (FP) cavity modes in the WS2 layer. The energy of the FP cavity modes depends approximately linearly on the WS2 thickness, and dipole-dipole interactions between the cavity photons and the WS2 excitons result in self-hybridized EPs for WS2 layers of appropriate thicknesses. Therefore, our ITO/WS2 devices can simultaneously exhibit self-hybridized EPs and strong off-resonance absorption, benefiting from the ARC and high imaginary permittivity of WS2 above its bandgap.

A representative device optical image and corresponding photocurrent map illustrate that the photocurrents are spatially distributed uniformly over the entire device range. (Fig. 1b, Supplementary Fig. 1). Generally, the transport of energy carriers in solid state materials is determined by their wave function and their interaction with the environment. As shown in Fig. 1c, conventional photodiodes suffer from three primary energy loss mechanisms: i) the photon with energy E < Eg cannot be absorbed by the material; ii) additional kinetic energy loss of the electron-hole pairs; and iii) recombination of electron-hole pairs. These losses stem mainly from poor charge transport and limited bandgap absorption. In contrast, EPs as neutral bosonic quasiparticles can substantially suppress the Coulomb scattering that plagues conventional charge carriers43. This suppression leads to longer diffusion lengths and lowers scattering rates, thereby opening avenues for enhanced exciton transport and improved overall photoelectric conversion efficiency44–47. In exciton-polariton photodiodes, absorption occurs through exciton-polariton states, the physical nature of which can be revealed by the wavefunction properties: the polariton wavefunction can be described as a linear superposition of photon and matter excitations48,49. This particular quantum superposition state determines the way energy is distributed in the light-matter system and the energy transport process. Specifically, the exciton-polariton state causes oscillations between photons and excitons at the Rabi frequency, i.e., photons are periodically absorbed and re-emitted through the exciton state. Under strong coupling conditions, the effective mass, and therefore the transport, is mainly determined by the photon composition, and the excitons are able to achieve transport through photonic modes, so that exciton transport can be improved to exhibit longer diffusion lengths. The hypothesized principle of photocurrent generation is similar to that of pure exciton photovoltaic devices, i.e., the exciton dissociates into a free charge at the interface, but in strong coupling, the exciton is transported in a photon-like mode and dissociates at the charge-selective contact, one charge being immediately extracted while the opposite free charge undergoes drift-diffusion transport to the other contact. In this study, we leverage the strong absorption and low effective mass of EPs to enhance the performance of photodiodes. In Fig. 1d, we schematically elucidate the relationship between the emergence of multiple polariton branches in the WS2 strong coupling device. The exciton-polariton states can enhance the subgap photon absorption and exciton diffusion length, which can minimize the energy loss and improve the quantum efficiency. While the presence of polaritons in our devices is clearly evidenced by enhanced absorption and spectral quantum efficiency, the overall photodetector performance, particularly the response speed, remains below the theoretical polaritonic limits owing to material and interface imperfections. Interfacial trap states induce nonradiative recombination and hinder carrier extraction, while non-ideal contact geometry may further constrain carrier collection dynamics. Importantly, these are technological rather than fundamental limitations. Advancing crystal and interface quality (e.g., through improved encapsulation and transfer processes), optimizing dielectric environments, and especially implementing carefully designed integrated cavities with higher Q factors and tighter mode confinement could substantially enhance the light-matter coupling strength and absorption efficiency. Together with refined electrode configurations for rapid carrier extraction, these improvements could bridge the gap toward the theoretical performance limits of polaritonic photodetectors.

Strong coupling in self-hybridized polaritonic photodiodes
We performed thickness-dependent reflectance measurements, transfer matrix method (TMM) calculations, and fitting to the coupled oscillator model to confirm the presence of strong coupling in our devices50. Figure 2a shows the TMM-calculated thickness and wavelength-dependent reflectance of our devices. Polaritons appear in the calculated reflectance spectra as reflectance minima that exhibit anti-crossing behavior near the exciton resonance (approximately 1.97 eV for WS2, denoted by the yellow vertical dotted lines in Fig. 2a, b). For each cavity mode order, the dispersion is fit with the coupled oscillator model, in which we include decay of the exciton and cavity modes22,51. The resulting fit is overlaid in Fig. 2a as dashed white lines, and the corresponding bare cavity modes are overlaid as red dashed lines. The Rabi splitting values between the upper and lower polariton branches (UPB and LPB) for the first 3 FP cavity modes (l=1, 2, 3) are found to be ℏΩ = 152.62 meV (l=1), 173.38 meV (l=2), and 171.42 meV (l=3), respectively (Supplementary Fig. 3). The real values of Rabi splitting confirm that the devices are in the strong coupling regime when thicknesses support cavity modes with resonances near the A exciton. For thicknesses below approximately 50 nm, the devices are in the weak coupling regime, as evidenced by the asymmetric Fano resonances slightly blueshifted from the exciton resonance52.

Figure 2b shows experimentally measured and reflectance spectra for representative devices of varying thickness. The corresponding TMM calculations (dashed black lines) match the measured spectra very well. The dispersion of the UPB and LPB is shown by the purple dotted lines. Spectra are offset vertically for clarity. Notably, the absorbance is large and broadband, and the polariton branches can be seen to extend the spectral range of absorption while exhibiting near unity absorption. To further resolve weak or overlapping features, we performed derivative analysis of the measured spectra (Supplementary Fig. 5)53. The first derivatives (Supplementary Fig. 5b) highlight inflection points corresponding to reflectance extrema, revealing subtle spectral features otherwise obscured in the raw data. Representative derivative spectra (Supplementary Fig. 5c) for devices in the strong-coupling regime (151.2 nm, 77.3 nm) display two well-resolved peaks straddling the A-exciton energy, fully consistent with the formation of UPB and LPB. Angle-resolved reflectivity spectra of devices with different thicknesses further reveal the coupling regime. Pronounced anti-crossing features were observed for WS2 layers thicker than 50 nm, consistent with strong exciton-photon hybridization, while thinner devices displayed only a dispersionless excitonic resonance. Coupled oscillator model fits quantitatively reproduce the experimental dispersions and confirm strong coupling across representative thicknesses (see Supplementary Fig. 6, Table 1). More detailed information on the simulation method is provided in the Supplementary Fig. 3 section.

EQE spectra driven by exciton-polariton states
We now consider the spectral response of WS2 photodiodes operating in the strong coupling regime. EQE is an important metric for photodiodes and photovoltaics as it reflects the ratio of the charge carrier flux (i.e., current density) collected at the electrodes to the incident photon flux at a specific excitation wavelength. EQE measurements (“methods”) are performed in the 500 nm to 800 nm range on devices with WS2 thicknesses from 17 to 200 nm (Supplementary Fig. 9). To accurately identify the polariton-related photocurrent features, we performed first-derivative analysis of the EQE spectra (Supplementary Fig. 9b, c). This method sharpens spectral features and pinpoints inflection points that correspond to EQE maxima. Figure 2c shows the coupled oscillator model fit, along with the experimental EQE peak wavelengths extracted from a range of device thicknesses (Supplementary Fig. 9b). Notably, the peak response positions of the EQE spectra in devices with varying thicknesses (scattered dots) exhibit excellent agreement with the theoretical model (dashed lines). In particular, the evolution of their wavelength positions with thickness strictly follows the characteristic dispersion relation of polaritons, with the peak positions corresponding to the resonance modes of the UP and LP branches, respectively. This consistency between experimental observations and the theoretical model provides solid evidence that the polariton state enhances light-trapping efficiency by modulating photon-exciton interactions, clearly demonstrating the crucial role of polariton engineering in improving photodiode performance. Figure 2d shows the EQE spectra of three representative devices, one in the weak coupling regime, one near zero detuning for the first-order polariton, and one near zero detuning for the second-order polariton. In these strongly coupled devices, both the UPB and LPB exhibit spectral separations from the bare exciton resonance that exceed their respective linewidths. This clear spectral separation enables unambiguous identification of the polariton branches. The responses below the bare exciton energy of the strongly coupled devices can be observed, and the peaks of the responses correspond to the reflectance minima (attributed to UP and LP branches) reported in the previous section (Fig. 2b). These features are absent in weakly coupled devices, confirming that all these responses are dominated by the exciton-polariton states. Based on this, we can predict that the strong-coupling device can harvest more photons in the spectral region around the bandgap due to the formation of exciton-polariton states.

Further, we note that the response of the photodiodes is broadband, highlighting the benefit of the ITO ARC. Simultaneously, the cut-off wavelength of the device is extended (Supplementary Fig. 11).

Compared with conventional excitonic materials, the narrow-band absorption properties due to the strong exciton binding effect and the ability to achieve high absorption efficiency only at specific wavelengths are avoided.

In addition, to further explore whether the exciton properties are preserved during the transport process and the effect of polariton coupling on quantum efficiency, we performed reflectance spectra, I–V characteristics, and EQE spectra measurements at low temperatures (Fig. 2e, f, Supplementary Figs. 12, 13). The results show that:
i) A comparison of the I-V curves at 300 K and 80 K reveals a significant reduction in dark current, attributed to the decreased concentration of thermally excited carriers, indicating a lower density of free carriers at low temperatures.
ii) In both strong- and weak-coupling devices, we observe a pronounced steepening of the exciton absorption edges, a blue shift of the absorption maxima, and an enhancement of the EQE at low temperatures under both resonant and non-resonant excitation, reflecting the strengthened excitonic and polaritonic effects. Such enhancement may also partially arise from polariton-mediated modification of radiative rates (Purcell-like effect)54, suggesting a direction for future studies to quantitatively evaluate this effect. Correspondingly, the absorption peaks dominated by the UPB and LPB are also clearly resolved in the EQE spectra, as indicated by the black arrows in Fig. 2f. To capture the full thermal evolution of these coupled states, we carried out continuous temperature-dependent reflectance spectra from 87 K to 300 K on WS2 cavities with thicknesses ranging from ∼35 nm to ∼130 nm. Across all devices, the UPB and LPB display a smooth, monotonic redshift with increasing temperature. Importantly, the polariton branches remain spectrally resolved over the entire temperature range without collapsing into a single peak, confirming that the system persistently resides in the strong-coupling regime. The temperature-induced evolution of the spectral splitting is accompanied by consistent trends in the EQE spectra, with devices near zero detuning exhibiting distinct peak splitting even at elevated temperatures. These observations substantiate the assignment of the observed features to robust exciton-photon hybrid states (polaritons), rather than to uncoupled optical modes or interference fringes.

Based on these observations, we speculate that at low temperatures, the EQE is predominantly dominated by exciton and polariton contributions, owing to the preservation and enhancement of exciton and polariton properties of WS2 and the suppression of the free-carrier effect. This further demonstrates that polaritons contribute to photovoltaic conversion and could potentially be exploited to enhance exciton transport and improve the photonic energy conversion efficiency of excitonic semiconductors.

External and internal quantum efficiency
To quantify the effect of strong coupling on the overall photoelectric conversion efficiency and the internal processes of the WS2 devices, we calculated the EQE and the IQE of the devices as a function of thickness for wavelengths of 600 nm, 630 nm, and 650 nm (Fig. 3). To eliminate uncertainty, at each thickness, we prepared multiple devices with similar thicknesses (≥3) and performed multiple measurements (≥5), then extracted the mean and standard deviation. The EQE reflects both the absorption and internal excitation-to-charge collection processes in the device. It is known that polaritons enhance absorption on resonance and can extend the spectral responsivity of photodiodes through absorption in the LPB. Polaritons are also expected to improve carrier extraction due to improved exciton diffusion. The greatest hybridization (i.e. strongest coupling) between photons and excitons occurs when there is zero detuning between the photonic mode and the exciton energy. In our devices, this occurs at WS2 thicknesses of approximately 75 nm for the first FP cavity mode and 145 nm for the second FP cavity mode. Consequently, we see oscillatory behavior in the EQE with thickness, with peaks near the first and second zero detuning thicknesses, evidencing the beneficial effects of strong coupling. In addition to the oscillatory behavior, there is a drastic increase in EQE for all wavelengths as the thickness enters the strong coupling regime. The increase in EQE from the weak coupling regime to the strong coupling regime is a factor of about 20 near the polariton resonance (600 nm, 650 nm) and a factor of about 10 at the exciton wavelength (630 nm).

We now consider the IQE to determine the degree to which strong-coupling affects exciton transport and carrier extraction. Owing to the enhanced electric fields associated with polariton states, the electrode layers display parasitic absorption peaks (Supplementary Fig. 10). The IQE only reflects the charge collection process for photons absorbed in the WS2 active layer. IQE is calculated as the quotient of EQE and the percentage of photons absorbed in the separate WS2 layer under short-circuit conditions, and the percentage of photons dissipated in the separate WS2 layer can be extracted from layer-resolved absorption with the transfer matrix model (Supplementary Fig. 10). IQE decreases with increasing active layer thickness in conventional photovoltaic devices55–58. However, the IQE data in Fig. 3 show a strong oscillatory behavior with increasing WS2 thickness. For thin layers, both IQE and EQE exhibit lower values, partly due to (i) the limited light absorption of the thin material under weak coupling, (ii) more effects of interfacial defect recombination for thinner absorber layers, and (iii) contact-induced variability. Notably, A large increase in the IQE is observed as the thickness increases into the strong coupling regime, with a strong peak at the zero-detuning thickness for the first-order polariton. Near unity IQE is achieved for on-resonance devices. This enhancement arises from self-hybridized polariton states that concentrate optical fields within the WS2 layer, enabled by the device geometry used here.

We include in the IQE plots for Fig. 3 a phenomenological transport model (dashed line) to predict the effect of strong coupling enhanced transport as a function of WS2 thickness. Our model assumes a hopping-based exciton diffusion process in which a random walk occurs through a resonance energy transport (RET) mechanism14,59–61. Here, we expect that homogeneous RET occurs between identical but spatially separated molecules through the electric field associated with polariton states61. This concept is consistent with both the quantum electrodynamics formulation of RET and the concept of EPs62, in which the dipole-dipole interactions between light and matter can be interpreted as the rapid emission and reabsorption of cavity photons through excitons18. The resulting RET-based diffusivity is weighted by the thickness-dependent exciton fraction of the polariton branch and then multiplied by an exponentially decaying envelope function to represent free carriers having a greater chance of recombining in the bulk as thickness increases. This model is then fit to experimental data and represented as a phenomenological transport model in arbitrary units. We can use this model to qualitatively analyze the oscillatory trend of IQE-related diffusion coefficients at specific wavelengths and at which thicknesses the peaks are located. For more discussion of the transport model, please refer to the Supplementary Information (Supplementary Fig. 14). Figure 3b shows that the fitting results of the phenomenological model are in agreement with the trend of the experimental data, suggesting that the enhancement of the IQE is mainly due to improved exciton transport in the strong coupling regime.

Photovoltaic detection of polaritonic photodiodes
Finally, we evaluated the comprehensive detection performance of the exciton-polariton photodiode. Figure 4a shows the dark and optical current density curves for a representative device. The dark current density is as low as 5.33 × 10−7 A cm−2 at 0 V, indicating relatively low noise current and high sensitivity. Simultaneously, a light-switching ratio (Ion/Ioff ratio) of 7.1 × 105 is obtained. Compared with other exciton photodetectors, our photodiode exhibits lower dark current density and higher Ion/Ioff ratio (Supplementary Table 1). Fig. S15 (Supporting Information) presents the photocurrent versus light power for three representative devices operated in strong and weak coupling under 600 nm, 630 nm, and 650 nm illumination. The dependence of the photocurrent on the light intensity is fitted to a power law equation (y=bxα) from which α=1.01 to 1.04 is obtained. This confirms that the device has a good linear photovoltaic response, indicating high efficiency, as all the excited electrons/holes are captured by the electrodes before recombination63.

The responsivity (R) is closely related to strong coupling due to the benefits of increased absorption and polaritonic transport. Consequently, the trend observed in EQE should be mirrored in R. We calculated the variation of R with thickness for devices at wavelengths of 600 nm, 630 nm, and 650 nm when V=0 V is applied (Fig. 4b, Supplementary Fig. 16). This effect is confirmed by oscillatory behavior, with a significant increase in responsivity corresponding to polariton-induced transport. The maximum responsivity obtained is 0.35 A/W (600 nm, 15 μW), located near the first zero-detuned thicknesses. Additionally, there is a strong initial increase in responsivity as the strong coupling state is entered.

Noise equivalent power (NEP) is a key parameter for evaluating the ability of a photodetector to detect weak radiation and is defined as NEP=in/R (in is the dark current noise). Typically, dark current noise is caused by shot noise, thermal noise, and 1/f noise64. To fully account for frequency-dependent noise, the current noise power spectra of the device are measured at zero bias using a signal analyzer and a low-noise preamplifier (Supplementary Fig. 17). From the directly measured noise, we calculated in2 using the equation in2=B−1 ∫0B Sn(f)df where B is the bandwidth. Then we calculated the NEP at 0 V bias as shown in Fig. 4c. The NEP value is obtained to be 0.34 pW Hz−1/2 at WS2=73.5 nm under 630 nm illumination. The lower NEP indicates that the device has an excellent capability to distinguish between signal and noise, which can be achieved at high photoresponsivity and low current-noise power density.

Fast photoresponse is a key performance metric for photodetectors. We anticipate that the speed of the photodiode can be increased by utilizing enhanced transport through photonic modes. Therefore, we measured the response speed at a frequency of 10 kHz with a 520 nm laser at 0 V bias. As shown in Fig. 4d, we extracted the rise time as a function of WS2 thickness. The increase of the response speed in the strong-coupling state and the obvious peak at zero detuning thickness confirm the expected behavior. The fastest response in the strong coupling state is about 217 ns (WS2=73.5 nm). Supplementary Fig. 18 and Fig. 19 (Supporting Information) show numerous measurements of the response speed of a representative device in the strong- and weak-coupling states as well as typical response time curves, further confirming our conclusions. In addition, we obtained a −3dB cut-off frequency of 1.21 MHz (WS2=73.5 nm), which is extracted from the frequency-dependent photoresponse of the signal at a 520 nm laser at the modulation frequency as it drops to 70.7% of its initial value, as shown in Fig. 4e. The rise time τr can be estimated as 289.2 ns using the equation τr=0.35/f−3dB, which is similar to the response time measurement. Generally, the intrinsic lifetimes of excitons and polaritons fall within the picosecond-subnanosecond range, and the nanosecond-scale device photoresponse observed here was influenced by interfacial carrier trapping (Supplementary Fig. 20), suggesting that further suppression of interfacial defects can enable even faster operation. To further verify the performance enhancement of the polariton photodiode, we compared our device with other structures. Figure 4f shows the room temperature responsivity and response speed compared to other excitonic photodetectors. Our photodetectors both exhibit response times of several hundred nanoseconds at room temperature, which is faster than most excitonic photodetectors, as shown in Supplementary Table 3. Therefore, we can conclude that self-hybridized exciton polariton photodiodes are extremely attractive for high-efficiency, fast photodetection.

Methods
Device fabrication
The bottom electrical contacts are patterned using electron-beam lithography and then electron-beam evaporation deposition of 3 nm Cr and 100 nm Au on a silicon substrate with 300 nm SiO2 (University Wafer). WS2 layers are mechanically exfoliated from bulk crystals (HQ Graphene) with blue adhesive film tape and transferred onto the surface of a polydimethylsiloxane stamp. WS2 layers are transferred by an all-dry deterministic transfer procedure onto the preprepared bottom electrical contacts by using a polydimethylsiloxane stamp. The top electrical contacts are patterned with electron beam lithography and ion beam sputtering deposition of 50 nm ITO. All devices after fabrication were annealed in an N2 environment. The resulting devices are identified by optical microscopy and later characterized by atomic force microscopy (AFM, Asylum Research MFP-3D Origin) to determine the flake thickness.

Reflectance measurements
The WS2 device reflectance spectra were measured using a home-built micro-reflection system (see Supplementary Fig. 21) with a 50× objective (Olympus, NA=0.45) under normal incidence. Dispersion relations in reflection were measured using a 100× objective (NA=0.9). The illumination source used in the measurements is 10–250 W tunable Quartz Tungsten Halogen lamps (Newport 66884 Research QTH Lamp), and the reflectance signals were gathered by a grating spectrometer equipped with a CCD (SpectraPro HRS-500). The reflectance spectra are normalized by the reflectance of a silver mirror (Thorlabs).

Determination of optical constants
The optical constants of ITO, Au used in our simulations are obtained by spectroscopic ellipsometry (Supplementary Fig. 2). The optical constants of WS2 are assumed to be thickness-independent bulk optical constants of WS2 from literature65. This is due to the fact that the phenomena reported in this study are concentrated in the 17–200 nm thickness range, which represents the electron bulk in WS2, as the effect of electron quantum confinement becomes negligible at about 5 nm (6–7 layers) thickness40.

Electrical and optoelectronic measurements
The light of a supercontinuum white light laser (Fianium White Lase SC400) is coupled into a tunable, high-resolution bandpass filter (LLTF Contrast SR-VIS-HP8). The resulting monochromatic beam was focused onto the WS2 device through a fixed optical path using a 20× objective lens (NA=0.45), producing a spot with a diameter of approximately 12 µm. Under short-circuit conditions, the photocurrent was amplified by a low-noise current preamplifier (Stanford Research Systems SR570) and subsequently analyzed using a lock-in amplifier (Stanford Research Systems SR865A) (shown in Supplementary Fig. 22). The EQE is determined by dividing the photocurrent of the WS2 device by the flux of incoming photons, which is obtained with calibrated silicon (Si) and indium–gallium–arsenide (InGaAs) photodiodes. For photocurrent mapping measurements, the laser beam was focused onto the device through a fixed optical path using a 50× objective lens (NA=0.6), yielding a laser spot diameter of approximately ~2 µm. The data on response time were acquired using a Tektronix MDO34 oscilloscope. The noise current spectrum was measured by using a preamplifier (DL1211) and a signal analyzer (Keysight N9010B) in a shielding box. The electronic characteristics were measured by a Keysight B2912A source. For low temperature measurements (up to 80 K), the samples were placed in a cryostat and cooled down to the desired temperature by controlling the flow of liquid nitrogen.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors report self-hybridized exciton-polariton photodiodes based on multilayer WS2, in which transparent ITO electrodes act both as electrical contacts and anti-reflection coatings. Leveraging strong light-matter coupling, the devices exhibit broadband absorption and internal quantum efficiency (IQE) approaching unity near zero detuning. The EQE and IQE both show clear oscillatory behavior as a function of WS2 thickness, consistent with the dispersion of polaritonic modes. The photodiodes further demonstrate high responsivity, low noise-equivalent power, and fast response time (~217 ns), outperforming conventional excitonic photodetectors. These results suggest a promising design strategy for future exciton-based optoelectronics. The manuscript is technically sound and well-supported by experiments and simulations. However, the so-called “self-hybridized” concept has already appeared in many prior studies, which raises some concern about the novelty of this work. In addition, several specific issues require clarification or improvement before the manuscript can be considered for publication. 1. The IQE is derived from EQE and simulated absorption in WS2 using the transfer matrix method. The authors should more clearly state the assumptions behind isolating absorption in WS2, in particular whether parasitic absorption in ITO or Au has been considered or neglected. 2. In Figures 1b and S1, while the authors aim to demonstrate spatially uniform photocurrent response, the corresponding experimental conditions such as excitation wavelength, laser power, and beam spot size are missing. Even for qualitative measurements, all reported data should be scientifically valid and reproducible. 3. In Figure 2c, the x-axis is given in energy while all other spectra are plotted in wavelength. To ensure consistency and improve readability, the authors are strongly encouraged to present the data in wavelength units as well. 4. Although the reported response time of 217 ns is relatively fast among excitonic photodiodes, the term “ultrafast” is generally reserved for sub-nanosecond regimes. The authors should revise the manuscript to avoid overstatement of the device performance.", "reviewer_2": "Zhao et al. report their findings on exciton–polariton photodiodes constructed from ITO/multilayer WS₂/Au structures and present measurements of several key optoelectronic parameters—including EQE, IQE, responsivity, and rise time—all of which exhibit peak values at a thickness corresponding to the emergence of possible polariton modes. The characterization of the devices is thorough, and the results demonstrate a high degree of internal consistency. However, attributing the observed phenomena to polaritons does not appear to be fully convincing, as some of the reported properties are not consistent with typical polariton behavior. In my view, the manuscript would benefit from further clarification and supporting evidence before it can be considered suitable for publication. 1. In Fig 2a and 2b, the authors present the simulated and measured polariton modes for different WS₂ thicknesses. They report an increase in the vacuum Rabi splitting with increasing WS₂ thickness. However, this trend raises several concerns. First, as the thickness of the WS₂ layer increases, the mode volume V of the cavity is expected to increase. Since the light–matter coupling strength g scales inversely with the square root of the mode volume (g∝1/\\sqrt{V}), a larger mode volume should lead to a decrease in g, and thus a smaller Rabi splitting. Second, the exciton oscillator strength typically decreases in thicker flakes due to enhanced dielectric screening and a weakening of the direct K–K transition in WS₂. Therefore, one would also expect a reduction in g with increasing thickness. These considerations appear inconsistent with the observed trend and should be clarified by the authors. 2. Continuing from the previous point, the authors report a vacuum Rabi splitting of approximately 170 meV. However, even in optimized systems where monolayer WS₂ (or other TMDCs) is strongly coupled to a planar cavity, the vacuum Rabi splitting typically falls in the range of 40–60 meV (e.g. Nature Nano 17, 396-402 2022). In contrast, the authors observe a Rabi splitting that is nearly four times larger than these established values, despite using thicker WS₂ flakes, which generally exhibit reduced oscillator strength. This reported value appears unphysically large given the material system and cavity architecture, and the authors should provide further justification or supporting analysis to clarify this discrepancy. 3. In several figures (e.g., Fig. 2b, Fig. S5, Fig. S8), the authors use dashed lines as guides to the eye to indicate faint spectral features, presumably to suggest the presence of polariton modes. I recommend that the authors compute the first or second derivative of the signal with respect to energy (e.g., dI/dE or d2I/dE2), which is a widely used technique in the analysis of excitonic spectra. This approach could help reveal subtle features more clearly and provide stronger evidence for the existence of weak or overlapping polariton branches. 4. It would be helpful if the authors could provide the quality factor of their cavity. This parameter is essential for evaluating the cavity's optical performance and assessing whether the system operates in the strong coupling regime. Including this information would also facilitate comparison with previous studies. 5. In Fig. 2e, the authors present reflectance spectra at two different temperatures. Do the authors have data from a continuous temperature scan? Tracking the evolution of the polariton splitting as a function of temperature could provide additional evidence for the formation of polaritons. However, with only two temperature points shown, it is difficult to draw firm conclusions. A more comprehensive temperature-dependent analysis on a single device is encouraged. 6. In the optoelectronic measurements presented in Fig. 4, how do interfacial trap states between ITO and WS₂ affect the observed rise and response times? Given that exciton and polariton dynamics typically occur on picosecond to sub-nanosecond timescales, the reported response times on the order of hundreds of nanoseconds seem long (>> lifetime of exciton/polariton). This suggests that extrinsic effects, such as carrier trapping at the ITO/WS₂ interface, may play a significant role. The authors are encouraged to discuss the possible influence of interface quality and trap dynamics on the device performance.", "reviewer_3": "The manuscript reports a polariton photodiode in a self-hybridized TMD system and clarifies that some device performances, particularly the response time (up to hundreds of nanoseconds), could be enhanced in the strong coupling regime. The experimental data primarily support the interpretation of cavity effect of enhancement rather than one based on polaritons. Although there are some decent performance improvements, the main merit appears to lie in the fast response time, which is significantly faster than reported devices. While the performance improvements are noted, the enhancement mechanism in the so called “strong coupling regime “ remains unclear, and the demonstrated evidence is not convincing. Below are my detailed comments: 1) The novelty of polariton photodiode bring out some concerns. In the context of exciton polariton photodiodes, this idea has been tried several years ago in organic excitonic materials [e.g., ACS Photonics 5, 2921-2927 (2018)]. Though it may be an interesting attempt to realize this idea within TMDs, the experimental evidence in this manuscript is not yet sufficient to support polariton effects. Further data and clarification are required to substantiate the role of polaritons in the observed performance enhancement. 2) Insufficient evidence for the strong coupling regime. To claim the strong coupling regime, the coupling strengths were fitted under different detuning by varying the TMD thicknesses. This is problematic. The strong coupling can be verified by satisfying the criterion that the coupling strengths should be larger than the averaged linewidths of cavity modes and excitons. This means the g^2 should be greater than (gamma_c^2+gamma_x^2)/2. This criterion has not been tested in their spectroscopic measurements (Figs. 2a-b). The criterion in the SI they cited can only infer there could be spectral splitting, which does not necessarily indicate the strong coupling regime (please see many references discussing about the differences between the intermediate coupling and the strong coupling). It is also important to note that the guidelines do not exactly follow the reflectivity dips (Fig. 2b), which are misleading. 3) The coupling strengths is proportional to the sample thickness, which has been varied by more than 8 times. For a rough estimation without considering the detailed cavity mode distributions (which was also ignored in the manuscript), the coupling strengths in Fig. 2b could be varied by about 3 times. In this way, the so-called anti-crossing features in Figs. 2a-b is misleading, as they do not share similar cavity strengths. It should be appropriate to measure samples of the same thickness at different angles (angle-resolved reflectivity) and then extract the coupling strength specifically for each thickness. 4) In Fig. 2d, the authors claim that the normalized EQE data support the consistency of polariton resonances and EQE peaks. However, this is not the case in close observations. For example, some EQE peaks coincide with the exciton resonance instead of so called “LP” or “UP” states. To substantiate the polariton effect, it is strongly suggested that they should first clarify the well-separated polariton peaks by checking the criterion of the strong coupling regime, which are expected to be distant from the exciton resonances with respect to their linewidths. For instance, some convincing measurements should be at least performed in the “zero-detuning” samples to better determine the strong coupling regime. 5) In Fig. 2f, some error bars are excessively large, which are greater than the data variations. This largely undermines the reliability of the data and makes it difficult to draw convincing conclusions. The authors are strongly recommended to retake the data, and thus effectively reduce these error margins and provide more precise data to support their claims. 6) To strengthen the argument for a polariton-based photodiode, the correlation between the enhanced performances and the polariton states should be unambiguously established. In Figs 3, 4b, and 4d, the performance improvement in the device seems more likely to be due to cavity resonance rather than polariton formation. At the zero-detuning, the peak device performance coincides with the exact cavity resonance (in Fig. S9), indicating that cavity enhancement might be the optimized. This does not necessarily lead to polariton effects. To establish the polariton enhancements, demonstrating the optimized formation of polaritons and unambiguously identifying the peaks of performance parameters at the polariton states would be critical. 7) The comparisons with reported device performance seem to be suspicious. In their comparisons, the on/off ratio, dark current density, especially response time seems to be much larger than reported TMD-based photodiodes/photodetectors. However, there are some mistakes in SI that comparisons of device parameters with previously reported devices show several discrepancies. For instance, in SI Reference 20 [Adv. Electron. Mater. 7, 2000964 (2021)], the responsivity is given as 777 mA/W. But the manuscript lists it as 360 mA/W (SI Table S2). As another example, the responsivity in SI Reference 8 [Appl. Surf. Sci. 481, 1127-1132 (2019)] is reported to be about 5 orders of magnitude higher than this work. But the manuscript totally ignored it in the comparisons of responsivity (SI Table S2). The similar inconsistencies must be addressed and ensure all comparative data are correct and well-cited without such suspicious discrepancies. I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Reviewer 1\nReviewer #1 (Remarks to the Author):\nThe authors report self-hybridized exciton-polariton photodiodes based on multilayer\nWS , in which transparent ITO electrodes act both as electrical contacts and anti-\n2\nreflection coatings. Leveraging strong light-matter coupling, the devices exhibit\nbroadband absorption and internal quantum efficiency (IQE) approaching unity near\nzero detuning. The EQE and IQE both show clear oscillatory behavior as a function of\nWS thickness, consistent with the dispersion of polaritonic modes. The photodiodes\n2\nfurther demonstrate high responsivity, low noise-equivalent power, and fast response\ntime (~217 ns), outperforming conventional excitonic photodetectors. These results\nsuggest a promising design strategy for future exciton-based optoelectronics. The\nmanuscript is technically sound and well-supported by experiments and simulations.\nHowever, the so-called “self-hybridized” concept has already appeared in many prior\nstudies, which raises some concern about the novelty of this work. In addition, several\nspecific issues require clarification or improvement before the manuscript can be\nconsidered for publication.\nResponse:\nWe sincerely thank the reviewer for the constructive feedback and for recognizing the\ntechnical strength, experimental rigor, and simulation support of our manuscript. We\nappreciate the positive evaluation of our device performance, including the high\nresponsivity, low noise-equivalent power, and fast response time, as well as the\nreviewer’s recognition of the potential impact of our results on future exciton-based\noptoelectronic design strategies.\nAbout the innovations:\nWe sincerely thank the reviewer for pointing out this issue regarding novelty. While\nwe agree that the concept of self-hybridized exciton-polaritons has been reported in\nprior optical studies (Refs. [34,35,40,41] in our manuscript), we would like to\nemphasize that our work represents the first demonstration of a self-hybridized\nexciton-polariton effect for photovoltaic detection with high responsivity, low\nnoise-equivalent power, and fast response time. The key advances of our study can\nbe summarized as follows, and to further highlight these advantages, we compared our\ndevices with a recently reported work published in Nature Materials (Intrinsic strong\nlight-matter coupling with self-hybridized bound states in the continuum in van der\nWaals metasurfaces)：\nFigure R1.1. A comparison between the strategies in the previous reported work\n(Nat. Mater. 22, 970–976 (2023)) and our work.\na) Device-level realization with an open-cavity architecture:\nPrevious reports mainly focused on spectroscopic observation of self-\nhybridized polaritons in TMDC multilayers or perovskites, typically limited to\nreflectance or absorption measurements. In contrast, our work integrates this\nphenomenon into a functional photodiode architecture, directly correlating\nthe polaritonic dispersion with EQE and IQE performance metrics, which\nhave not been shown before. This closes the critical gap between spectroscopic\nphenomena and practical exciton-polariton-based optoelectronics, offering a\ncompact and fabrication-friendly architecture.\nb) Concurrent broadband absorption and transport enhancement:\nWe show that the ITO electrode simultaneously acts as a transparent contact\nand anti-reflection coating, enabling broadband absorption beyond the narrow\nexciton resonance. More importantly, the polariton states are exploited to\novercome the intrinsic exciton diffusion bottleneck, leading to near-unity\nIQE and a dramatic enhancement in responsivity. This functionality is enabled\nsolely by the intrinsic optical properties of WS and the tailored ITO electrode\n2\nintegration, without additional photonic structures such as DBRs or plasmonic\nmetal cavities.\nc) Near-unity internal quantum efficiency via tuning of coupling conditions\nSupported by both experiments and electromagnetic simulations, we observe\nF u n\nN o p ra c tic a l\no p to e le c tro n ic\nin te g ra tio n\nD e v ic e D e s ig n\nc tio n a l p h o to d io\na rc h ite c tu re\nd e\nS p e c tro s c o p ic\no b s e rv a tio n s o n\nF o c u s\nF ro m s p e c tro s c o\nto d e v ic e\nly\np y D\nC o n tro lla b le c o u p lin g\ns tre n g th ;\nN o c o rre la tio n w ith\np h o to c u rre n t\nK e y M e tric s\nire c t lin k : d is p e rs io n E\nIQ E\n200\n150 )mn(\nssen\n100 kcihT\nstrong coupling 2SW\nEQE maxima\n0550 600 650 700 750\nW avelength (nm) W avelength (nm )\nQ E / H ig h\nN o (d e v ic e\nre le v a n c e\nd is c u s s e d\nc o n c e p tu a lly )\nD e v ic e D e m o n s tra tio\nre s p o n s iv ity + F a s t s\nL o w n o is e flo o r\nn\np e e d +\noscillatory behavior in EQE and IQE as a function of WS thickness,\n2\ncorresponding to detuning-controlled polariton dispersion and Rabi-like\nsplitting. The enhanced exciton transport via polariton modes allows efficient\ncarrier collection at the contacts near zero detuning, yielding IQE values\napproaching unity that oscillate with thickness, in good agreement with\ntheoretical predictions and simulations. This reflects a direct photoelectrical\nbenefit derived from polaritonic mode engineering.\nd) Performance breakthroughs in excitonic photodiodes:\nOur devices demonstrate an unprecedented combination of high responsivity\n(0.35 A/W), low NEP (0.34 pW Hz⁻¹/²), and fast response time (217 ns). To the\nbest of our knowledge, no prior self-hybridized polariton studies have reported\nsuch device-level performance metrics.\nTherefore, while the concept of self-hybridization exists in the literature, our\nmanuscript provides the first systematic demonstration of how this effect can be\nharnessed to fundamentally overcome exciton transport limitations in excitonic\nphotodiodes. We believe this constitutes a significant step forward both in the physics\nof light-matter coupling and in practical device design for exciton-based\noptoelectronics.\n1. The IQE is derived from EQE and simulated absorption in WS using the transfer\n2\nmatrix method. The authors should more clearly state the assumptions behind isolating\nabsorption in WS , in particular whether parasitic absorption in ITO or Au has\n2\nbeen considered or neglected.\nResponse:\nWe thank the reviewer for highlighting this important point. We confirm that parasitic\nabsorption in both ITO and Au has been considered in our IQE estimation.\nSpecifically, in our optical simulations, the transfer matrix method (TMM) is used to\nanalytically compute the position-dependent electric field and layer-specific absorption,\nbased on the complex refractive indices (𝑛+𝑖𝑘) of each material. Since both ITO and\nAu have non-zero imaginary permittivity in the spectral range of interest, parasitic\nabsorption in these layers is inherently included in the simulation. Therefore, our IQE\nestimation based on WS absorption carefully accounts for losses due to these\n2\nmetallic/oxide components.\nFollowing the reviewer’s suggestion, we have clarified this assumption in the revised\nmanuscript and added layer-resolved absorption plots under the zero-detuning\ncondition (Fig. R1.2). These results show parasitic absorption in both ITO and Au at\nthe upper polariton (UP) and the lower polariton (LP) resonances, with losses at the LP\nresonance where the WS extinction coefficient decreases below the exciton energy.\nmanuscript.\nITO\n500 600 700 800\nWavelength (nm)\nFig. R1.2 | Layer resolved absorption is calculated using TMM.\nRevised:\nThe discussion on parasitic absorption has been incorporated into the main text (page\n8, paragraph 2), and Figure R1.1 has been repositioned to the Supplementary\nInformation as Figure S10.\n2. In Figures 1b and S1, while the authors aim to demonstrate spatially uniform\nphotocurrent response, the corresponding experimental conditions such as excitation\nwavelength, laser power, and beam spot size are missing. Even for qualitative\nmeasurements, all reported data should be scientifically valid and reproducible.\nResponse:\nWe thank the reviewer for this valuable suggestion.\nIn response, we have now added the missing experimental conditions, including the\nexcitation wavelength, laser power, and beam spot size, to the captions of Figure 1b and\nSupplementary Figure S1.\necnabrosbA\n73.5 nm\n152.3 nm\nWS\nTotal\n211.8 nm\nSpecifically:\n1) Figure 1b caption now states:\nExcitation wavelength: 633 nm, spot size: ~2 µm, laser power: ~40 µW.\n2) Supplementary Figure S1 caption now states:\nExcitation wavelength: 633 nm, spot size: ~2 µm, laser power: 5-10 µW.\nRevised:\nIn the revised manuscript, the excitation wavelength, laser power, and beam spot size\nhave been specified in the captions of Figures 1b (main text, page 4) and S1\n(Supplementary Information, page 2).\n3. In Figure 2c, the x-axis is given in energy while all other spectra are plotted in\nwavelength. To ensure consistency and improve readability, the authors are strongly\nencouraged to present the data in wavelength units as well.\nResponse:\nWe thank the reviewer for the helpful and constructive suggestion regarding the unit\nconsistency in the figures.\nAs Fig. R1.3 exhibits, we have displayed the x-axis in wavelength units to be\nconsistent with the other spectral plots in the manuscript. This change enhances the\nreadability and consistency of the data presentation.\n200\n150\n)\nm\nn\n(\ns\ns\ne\nn\nk100\nc\nih\nT\nW\nExperimental\nEQE maxima\n0\n550 600 650 700 750\nWWavaevleelennggtthh ((nnmm))\nFig. R1.3 | Dots are extracted from experimental EQE spectra shown in (Fig.S9),\nwhile lines are the calculated dispersions of the polariton eigenstates. The EQE\nexperimental data agree well with the calculated polariton branches of the coupled\noscillator model.\nRevised:\nIn the revised manuscript, we have updated Figure 2c to display the x-axis in\nwavelength units (i.e., as Fig. R1.3 shows), in line with the other spectral plots\nthroughout the manuscript.\n4. Although the reported response time of 217 ns is relatively fast among excitonic\nphotodiodes, the term “ultrafast” is generally reserved for sub-nanosecond regimes. The\nauthors should revise the manuscript to avoid overstatement of the device performance.\nResponse:\nWe thank the reviewer for this insightful and important comment.\nWe fully agree that the term “ultrafast” is typically reserved for response times in the\nsub-nanosecond regime. In response, we have carefully revised the manuscript to\nremove or replace all instances of “ultrafast” with more appropriate terms such as\n“fast photodetection”, to avoid any overstatement of the device performance.\nRevised:\n“Therefore, we can conclude that self-hybridized exciton polariton photodiodes are extremely\nattractive for high-efficiency, fast photodetection.” (main text, Page 10)\nReviewer #2 (Remarks to the Author):\nZhao et al. report their findings on exciton–polariton photodiodes constructed from\nITO/multilayer WS /Au structures and present measurements of several key\n2\noptoelectronic parameters—including EQE, IQE, responsivity, and rise time—all of\nwhich exhibit peak values at a thickness corresponding to the emergence of possible\npolariton modes. The characterization of the devices is thorough, and the results\ndemonstrate a high degree of internal consistency. However, attributing the observed\nphenomena to polaritons does not appear to be fully convincing, as some of the reported\nproperties are not consistent with typical polariton behavior. In my view, the manuscript\nwould benefit from further clarification and supporting evidence before it can be\nconsidered suitable for publication.\nResponse:\nWe sincerely thank the reviewer for their thoughtful and thorough evaluation of our\nmanuscript. We greatly appreciate the positive comments on the quality of device\nfabrication, the comprehensive characterization, and the internal consistency of the\nresults. Your critical feedback, particularly regarding the attribution of our results to\nexciton-polariton modes, has been extremely valuable in helping us refine our\ninterpretations and improve the clarity and rigor of the manuscript.\nIn response, we have carefully addressed each of your concerns and revised the\nmanuscript accordingly. Below, we provide detailed responses to each specific point,\nincluding additional clarifications and supporting evidence. We hope that the revised\nversion meets your expectations and more clearly conveys the significance of our\nfindings.\n1. In Fig 2a and 2b, the authors present the simulated and measured polariton modes\nfor different WS thicknesses. They report an increase in the vacuum Rabi splitting with\n2\nincreasing WS thickness. However, this trend raises several concerns. First, as the\n2\nthickness of the WS layer increases, the mode volume V of the cavity is expected to\n2\nincrease. Since the light-matter coupling strength 𝑔 scales inversely with the square\nroot of the mode volume (𝑔 ∝ 1/√𝑉), a larger mode volume should lead to a decrease\nin 𝑔, and thus a smaller Rabi splitting. Second, the exciton oscillator strength typically\ndecreases in thicker flakes due to enhanced dielectric screening and a weakening of the\ndirect K-K transition in WS . Therefore, one would also expect a reduction in 𝑔 with\n2\nincreasing thickness. These considerations appear inconsistent with the observed trend\nand should be clarified by the authors.\nResponse:\nWe thank the reviewer for raising this important and insightful question. While the\nintuitive picture is that a larger mode volume weakens the coupling strength, in our case\nmultiple cooperative effects, such as the increased number of excitons participating in\nthe interaction and the improved overlap with the optical field, work together to\nreinforce the coupling, leading to the observed enhancement in vacuum Rabi splitting.\nSpecifically:\n(1) Cooperative scaling of mode volume (V ) and exciton number (N):\nIndeed, it is correct in principle that, for a fixed number of excitons N, an increase in\nthe mode volume V leads to a reduction in the light-matter coupling strength, as\ndescribed by the relation:\n𝑁\n𝑔 ∝ √\n𝑉\nHowever, in our strongly confined self-hybridized geometry, the number of\ninteracting excitons N increases proportionally with WS thickness, and field overlap\n2\nbetween the optical mode and the excitonic medium also improves. Therefore, effective\ncoupling strength 𝑔 can increase, even as the mode volume increases slightly.\nThis cooperative scaling has been discussed in prior studies (e.g., Nat. Mater. 22,\n970-976 (2023); Nat. Commun. 13, 5597 (2022); Phys. Rev. B 107, 075415(2023)) and\nis particularly relevant in systems like ours where a bare cavity is absent and the field\nis concentrated within the absorbing medium itself.\nFurthermore, consistent physical behavior has been observed in multilayer TMDs\nsystems, as demonstrated in plasmonic systems (ACS Photonics 6, 286 (2019); Nano\nLett. 21, 605 (2021); Nanophotonics 12, 3267 (2023)), bloch surface wave platforms\n(ACS Photonics 9, 2817 (2022)). These works establish the same quantitative scaling\nlaw:\n𝑔 ∝ √𝑁 (ℏ𝛺 ∝ √𝑁 ),\n𝐿 𝑅 𝐿\ndemonstrating that increased exciton density (N) enhances field-exciton overlap\nwithout significant mode volume (V) expansion, with Rabi splitting growing from ~50\nmeV in monolayers to >100 meV in multilayers (Fig. R2.1). Crucially, this\nenhancement originates from optical mode localization within the dielectric medium\nand collective exciton participation.\na b\nFig. R2.1 | Layer-dependent Rabi splitting in TMDC. a, Rabi energy (𝛺 ) versus\n𝑅\nsquare root of layer number (√𝑁\n𝐿\n) in WS\n2\n. (ref: ACS Photonics 9(8), 2817 (2022)) b,\nRabi splitting as a function of WSe layer thickness. Dashed lines indicate the predicted\n2\n√𝑁 scaling behavior for both materials. (ref: ACS Photonics 6(2), 286 (2019))\n𝐿\n(2) Oscillator strength in thick WS flakes:\nmainly applies in the few-layer regime. Once the thickness exceeds the exciton Bohr\nradius (~1 nm (refs Nature Photon 9, 466-470 (2015)), the system approaches a quasi-\nbulk limit where oscillator strength remains relatively constant. Since our WS\n2\nthicknesses (20-200 nm) are far beyond this regime, oscillator strength reduction is\nnegligible.\n(3) Supporting experimental evidence:\nAngle-resolved reflectance measurements confirm these conclusions.\nTo directly determine the coupling strength of WS devices with different\n2\nthicknesses, we fabricated and characterized representative near-zero-detuning\nsamples and performed angle-resolved reflectance measurements across a range\nof thicknesses (see Fig. R2.2). Clear anti-crossing behavior was observed in samples\nthicker than 50 nm, signifying the formation of mixed exciton-photon modes. This\nspectral splitting reflects exciton self-hybridization under the strong coupling regime.\nBy contrast, thinner samples (<50 nm) exhibited a single dispersionless dip\ncorresponding to the bare A exciton, without any signature of coupling. We employed\na coupled oscillator model to fit the experimental data and evaluate the exciton-photon\ncoupling strength (see Table R2.1). All measured dispersion relations across the devices\nwere well reproduced by the model. Three representative devices, with WS thicknesses\n2\nof 77.9 nm, 151.2 nm, and 221 nm, correspond approximately to the zero detuning\nconditions of the first-, second-, and third-order cavity modes, respectively. The\nextracted coupling strengths are 82.5 meV, 93.2 meV, and 93.9 meV. These values\nfurther confirm that the system operates in the strong coupling regime, as both criteria\n(𝑙) 2\nfor strong coupling, 𝑔2 >\n(𝛾\n𝑐\n−𝛾𝑋)\nand 𝑔2 > (𝛾2+𝛾2)/2 are satisfied. These\n𝑙 𝐶 𝑋\n4\nresults are consistent with the cooperative mechanism described above and validate the\nrobustness of our interpretation.\nTogether, these points reconcile the reviewer’s concern: in a self-hybridized,\nabsorber-filled geometry, increases in N and field overlap, and offset the nominal V\nincrease and any few-layer oscillator-strength reduction. This explains the observed\ntrend of the vacuum Rabi splitting with WS thickness.\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\na\n)Ve(\nygrenE\nc\n)Ve(\nygrenE\nR e fle c ta n c e\nM in . M\n2 .4\nE xp . S im .\n2 .2\n2 .0\n1 .8\n4 9 n m\n1 .6\n-0 .5 0 0 .5\nS in ( ) \nR e fle c ta n c e\nM in . M\n2 .4\nE xp . S im .\n2 .2\nU P\n2 .0\nL P\n1 .8\n1 5 1 .2 n m\n1 .6\n-0 .5 0 0 .5\nS in ( ) \na x.\na x.\nb\n)Ve(\nygrenE\nd\n)Ve(\nygrenE\nM\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nM\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nR e fle c ta n c e\nM in .\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nM\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nR e fle c ta n c e\nM in .\nE xp .\nU P\nL P\n7 7 .9 n m\n-0 .5 0\nS in ( ) \nR e fle c ta n c e\nM in .\nE xp .\nU P\nL P\n2 2 1 n m\n-0 .5 0\nS in ( ) \nS im\n0 .5\nS im\n0 .5\nM\n.\nM\n.\na x.\na x.\nTable R2.1 Coupled Oscillator Model Fitting Results for Devices with Varying\nThicknesses.\n𝛾 𝛾\nThickness 𝐶 𝑋 Rabi splitting\n𝑔 (meV) (𝛾 −𝛾 )/2 √(𝛾 2+𝛾 2)/2\n𝐶 𝑋 𝐶 𝑋\n(nm) (meV)\n(meV) (meV)\n77.9 82.5 89.6 24 151.4 32.8 65.5\n151.2 93.2 76.8 24.1 178.8 26.3 56.9\n221.9 93.9 55 29.8 186.1 12.6 44.2\nRevised:\nAn expanded description of the angle-resolved reflectance spectroscopy measurements,\nalong with the corresponding dispersion data and fits obtained from the coupled\noscillator model, has been added to the revised Supplementary Information (see\nSupplementary Fig. S6 and Table S1). We have also included the following additional\nreferences in the Supplementary Information:\n4. Zhang, L., Gogna, R., Burg, W., Tutuc, E. & Deng, H. Photonic-Crystal Exciton-Polaritons in\nMonolayer Semiconductors. Nat. Commun. 9, 713 (2018).\n5. Xie, P. et al. Cavity-Assisted Boosting of Self-Hybridization Between Excitons and Photonic Bound\nStates in the Continuum in Multilayers of Transition Metal Dichalcogenides. Physical review. B\n107 (2023).\n6. Nan, L. et al. Angular Dispersion Suppression in Deeply Subwavelength Phonon Polariton Bound\nStates in the Continuum Metasurfaces. Nat. Photonics 19, 615-623 (2025).\n7. Weber, T. et al. Intrinsic Strong Light-Matter Coupling with Self-Hybridized Bound States in the\nContinuum in Van Der Waals Metasurfaces. Nat. Mater. 22, 970-976 (2023).\n8. Shen, F. et al. Transition Metal Dichalcogenide Metaphotonic and Self-Coupled Polaritonic\nPlatform Grown by Chemical Vapor Deposition. Nat. Commun. 13, 5597 (2022).\n2. Continuing from the previous point, the authors report a vacuum Rabi splitting of\napproximately 170 meV. However, even in optimized systems where monolayer WS\n2\n(or other TMDCs) is strongly coupled to a planar cavity, the vacuum Rabi splitting\ntypically falls in the range of 40-60 meV (e.g. Nature Nano 17, 396-402 2022). In\ncontrast, the authors observe a Rabi splitting that is nearly four times larger than these\nestablished values, despite using thicker WS flakes, which generally exhibit reduced\n2\noscillator strength. This reported value appears unphysically large given the material\nsystem and cavity architecture, and the authors should provide further justification or\nsupporting analysis to clarify this discrepancy.\nResponse:\nWe sincerely thank the reviewer for this important and insightful point. We confirm that\nthe large vacuum Rabi splitting of ~170 meV observed in our system is physically\nreasonable and consistent with both theory and recent literature. Although this value\nexceeds the typical 40-60 meV reported for monolayer TMDC-cavity systems, it is\nattributed to enhanced collective coupling of exciton in our multilayer self-\nhybridized geometry. Specifically:\n1) Key differences from monolayer-cavity systems:\nWe agree that monolayer WS exhibits a higher per-exciton oscillator strength\n2\ncompared to bulk flakes. However, the total light-matter coupling strength 𝑔, and\nconsequently the vacuum Rabi splitting, is determined not only by the oscillator\nstrength per exciton but also by the total number of participating excitons 𝑁, and\nthe optical mode volume 𝑉. This relationship is well established in the literature\n(e.g., Refs. [Nat. Mater. 22, 970–976 (2023); Phys. Rev. B 107, 075415; Nat.\nCommun. 13, 5597 (2022)]) following the relation:\n𝑁\n𝑔 ∝ √\n𝑉\nIn monolayer-based systems, despite the strong oscillator strength of a single\nexciton, the number of available emitters N is extremely limited due to the atomic\nthickness of the film. In contrast, in our system, the cavity is self-hybridized and\ncomposed of multilayer WS , with thicknesses >50 nm, placing the system well into\n2\nthe quasi-bulk regime. While the per-exciton oscillator strength may be lower, the\nnumber of participating excitons is significantly greater.\nIn this structure, the exciton number 𝑁 is significantly higher than that of the\nmonolayer-based system. Meanwhile, the cavity mode volume 𝑉 does not increase\nsignificantly compared with the conventional Fabry-Pérot cavity. Therefore, the\noverall coupling strength is significantly enhanced. This structure leads us to\nobserve a vacuum Rabi splitting of approximately 170 meV, which is a\nmanifestation of enhanced exciton collective coupling in thick-layer TMDC\nsystems (see Table R2.2).\n2) Literature precedent for large Rabi splitting in thick TMDC systems:\nOur observation of a ~170 meV vacuum Rabi splitting is consistent with recent\nstudies on self-hybridized polaritonic systems involving thick TMDC films.\nNotably, Munkhbat et al. (ACS Photonics 6, 139-147, 2019) and others (Phys. Rev.\nB 107, 075415, 2023; Nat. Commun. 13, 5597, 2022) have reported that strong\ncollective coupling in such systems can lead to significantly enhanced Rabi splitting,\noften exceeding those of monolayer-based systems.\nThese findings confirm that large Rabi splitting in the >100 meV range is\nphysically reasonable and achievable in TMDC systems with similar architectures.\n3) Extraction of Rabi splitting:\nTo ensure the reliability of the extracted Rabi splitting, we performed new angle-\nresolved reflectivity spectra and fitted the polariton dispersion using a standard\ncoupled oscillator model. Both the upper and lower polariton branches were well\nreproduced by the model (see Fig. R2.3). The extracted Rabi splitting is 151.4 meV,\n178.8 meV, and 186.1 meV, respectively (see Table R2.2).\nThis consistency between experimental observation and theoretical modeling\nfurther supports the physical validity of the large Rabi splitting reported in our work.\nFig. R2.3 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses (~70-220 nm). The left (right) part of the figure shows the\nexperiment (TMM calculation). The red dashed lines represent the dispersion obtained\nfor the UPB and LPB using the coupled oscillator model. The black dashed line and\nparabola represent the exciton energy and the dispersion of the cavity mode.\nTable R2.2 Coupled Oscillator Model Fitting Results for Devices with Varying\nThicknesses.\n𝛾 𝛾\nThickness 𝐶 𝑋 Rabi splitting\n𝑔 (meV) (𝛾 −𝛾 )/2 √(𝛾 2+𝛾 2)/2\n𝐶 𝑋 𝐶 𝑋\n(nm) (meV)\n(meV) (meV)\n77.9 82.5 89.6 24 151.4 32.8 65.5\n151.2 93.2 76.8 24.1 178.8 26.3 56.9\na\n)Ve(\nygrenE\nc\n)Ve(\nygrenE\nR e fle c ta n c e\nM in . M\n2 .4\nE xp . S im .\n2 .2\n2 .0\n1 .8\n4 9 n m\n1 .6\n-0 .5 0 0 .5\nS in ( ) \nR e fle c ta n c e\nM in . M\n2 .4\nE xp . S im .\n2 .2\nU P\n2 .0\nL P\n1 .8\n1 5 1 .2 n m\n1 .6\n-0 .5 0 0 .5\nS in ( ) \na x.\na x.\nb\n)Ve(\nygrenE\nd\n)Ve(\nygrenE\nM\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nM\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nR e fle c ta n c e\nM in .\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nM\n2 .4\n2 .2\n2 .0\n1 .8\n1 .6\nR e fle c ta n c e\nM in .\nE xp .\nU P\nL P\n7 7 .9 n m\n-0 .5 0\nS in ( ) \nR e fle c ta n c e\nM in .\nE xp .\nU P\nL P\n2 2 1 n m\n-0 .5 0\nS in ( ) \nS im\n0 .5\nS im\n0 .5\nM\n.\nM\n.\na x.\na x.\n221.9 93.9 55 29.8 186.1 12.6 44.2\nRevised:\nIn the revised manuscript, we have also included more explicit comparisons with\nprior literature to clarify the origin and justification of the observed coupling strength.\nThis discussion, along with supporting dispersion fits and additional data, has been\nincluded in the Supplementary Information (Supplementary Fig. S6; Table S1).\n3. In several figures (e.g., Fig. 2b, Fig. S5, Fig. S8), the authors use dashed lines as\nguides to the eye to indicate faint spectral features, presumably to suggest the presence\nof polariton modes. I recommend that the authors compute the first or second derivative\nof the signal with respect to energy (e.g., 𝑑𝐼⁄𝑑𝐸 or 𝑑2𝐼⁄𝑑𝐸2), which is a widely used\ntechnique in the analysis of excitonic spectra. This approach could help reveal subtle\nfeatures more clearly and provide stronger evidence for the existence of weak or\noverlapping polariton branches.\nResponse:\nWe sincerely thank the reviewer for this insightful and helpful suggestion. We agree\nthat calculating the first or second derivative of the optical signal with respect to energy\n(e.g., 𝑑𝐼⁄𝑑𝐸 or 𝑑2𝐼⁄𝑑𝐸2) is a well-established technique for enhancing the visibility\nof weak excitonic features or overlapping spectral branches, particularly in the context\nof polariton spectroscopy.\nFollowing the reviewer’s recommendation, we have applied derivative analysis to\nthe relevant spectral data presented in Fig. 2b, Fig. S5, and Fig. S8. The resulting\nderivative spectra are now provided as Fig.R2.4 and Fig.R2.5. These plots more clearly\nreveal the faint spectral features that were previously indicated with dashed lines and\noffer stronger evidence supporting the presence of upper and lower polariton branches,\nparticularly in regions where direct intensity analysis was inconclusive.\nWe believe this additional analysis significantly strengthens the physical\ninterpretation of our results and hope it satisfactorily addresses the reviewer’s concern.\nFig.R2.4 | Experimental reflectance spectra of WS devices with varying thickness.\n2\na, Measured reflectance spectra of WS devices. The purple dashed curves depict the\n2\ncavity polaritons dispersion (UP and LP). The yellow dashed lines indicate the A-\nexciton of WS . The black dashed line is the reflection spectrum calculated by the\n2\ntransfer matrix model. b, First derivatives of the reflectance spectra in a, highlighting\ninflection points (black arrows) that correspond to reflectance extrema and reveal weak\nor overlapping spectral features. c, c, Representative derivative spectra for devices with\nWS thicknesses of 151.2 nm, 77.3 nm, and 24.4 nm. Devices in the strong-coupling\n2\nregime (151.2 nm, 77.3 nm) exhibit two well-resolved derivative peaks straddling the\nA-exciton energy, consistent with the formation of UP and LP branches.\na\ne x\nA\nb\nevitavired\necnatcelfeR\n500 600\nW a v e\n700\nle n g th (n\n199.6 nm\n170.1\n166.8\n151.2\n126.6\n124.3\n121.8\n113.3\n102.6\n98.6\n87.3\n77.3\n73.7\n39.6\n24.4\n800 900\nm )\nc\nevitavired\necnatcelfeR\ns tro n g c o u p lin g\ns tro n g c o u p lin g\nw e a k c o u p lin g\n500 600\nW a ve le n g th\n700\n(n m )\n1 5 1 .2 n m 151.2\n7 7 .3 n m 77.3\n2 4 .4 n m 24.4\nphotocurrent features. a, Normalized EQE spectra of WS devices. Variations in\n2\nthickness (d) lead to different cavity lengths and, consequently, different resonance\nwavelengths for the cavity photons. b, First derivatives of the EQE spectra in a, with\narrows marking the inflection points corresponding to EQE maxima. c, Derivative EQE\nspectra for representative devices with WS thicknesses of 155.3 nm, 73.7 nm, and\n2\n17.8 nm. In strong-coupling devices (155.3 nm, 73.7 nm), two derivative peaks appear\non either side of the A-exciton energy, consistent with upper and lower polariton states.\nThese features are absent in weakly coupled devices (17.8 nm), confirming their\npolaritonic origin.\nRevised:\nIn the revised manuscript, we have incorporated this derivative-based analysis in the\nmain text (page 5), citing prior works that implemented similar methodologies in\nTMDC polariton systems (e.g., Phys. Rev. Lett. 113, 076802 (2014)). The\ncorresponding derivative plots (i.e., Figs. R2.4 and R2.5) are now included in the\nSupplementary Information (Supplementary Figs. S5 and S9).\nEdited References:\n52. Berkelbach, T. C. et al. Exciton Binding Energy and Nonhydrogenic Rydberg Series in Monolayer\nWS . Phys. Rev. Lett. 113, 76802 (2014).\n2\nevitavired\nEQE\n500 600\nW a v e\n700\nle n g th (n m\n1\n1\n1\n1\n1\n1\n1\n1\n9\n9\n8\n8\n7\n7\n7\n6\n5\n4\n1\n800\n)\n9\n7\n6\n5\n5\n2\n1\n0\n9\n3\n7\n3\n7\n3\n0\n5\n6\n5\n7\n9 .6\n1 .9 6\n5 .4\n5 .3\n1 .2\n6 .6\n3 .3\n2 .6\n.7\n.9\n.3\n.0\n.3\n.7\n.3\n.6\n.1 4\n.2\n.8 n m\n900\nex A\nstrong coupling\nstrong coupling\nweak coupling\n5 0 0\nW\n6\na\n0\nv\n0\ne le n g th\n7 0 0\n( n m\n1 5\n7\n1\n)\n5\n3\n7\n.3\n.7\n.8\n8 0 0\n155.3 nm\n73.7 nm\n17.8 nm\nWavelength (nm)\nevitavired\nEQE\na b\nc\n4. It would be helpful if the authors could provide the quality factor of their cavity. This\nparameter is essential for evaluating the cavity's optical performance and assessing\nwhether the system operates in the strong coupling regime. Including this information\nwould also facilitate comparison with previous studies.\nResponse:\nWe thank the reviewer for this insightful suggestion. The cavity quality factors were\nextracted from the linewidths of the cavity modes in the wavelength domain.\nSpecifically, the full width at half maximum (FWHM) of each cavity resonance, Δλ,\nwas obtained and converted into the quality factor using Q=λ /Δλ, where λ is the\n0 0\nresonance wavelength. We calculate this from TMM at a λ of 800 nm (off resonance)\n0\nfor each cavity mode. This procedure yields approximate Q values of 16, 20, and 25 for\nthe first-, second-, and third-order modes, respectively.\nThese quality factors are reasonable for self-hybridized cavity systems and are\nsufficient to support the formation of polariton states. This conclusion is further\nsupported by the linewidths analyzed in the energy domain, where the measured\nvacuum Rabi splitting significantly exceeds the combined cavity and exciton linewidths,\nthereby fulfilling the strong coupling criterion. For self-hybridized cavities, future work\nwill need to focus on strategies such as optimizing material interface quality, employing\nlower-loss active materials, or engineering photonic structures with better in-plane\nconfinement to further enhance the quality factor.\nRevised:\nWe have now included the above information regarding the cavity Q factors in the\nrevised manuscript (see Supplementary Information, Page 5).\n5. In Fig. 2e, the authors present reflectance spectra at two different temperatures. Do\nthe authors have data from a continuous temperature scan? Tracking the evolution of\nthe polariton splitting as a function of temperature could provide additional evidence\nfor the formation of polaritons. However, with only two temperature points shown, it is\ndifficult to draw firm conclusions. A more comprehensive temperature-dependent\nanalysis on a single device is encouraged.\nResponse:\nWe sincerely appreciate the reviewer’s insightful suggestion. To further investigate the\ntemperature dependence of polaritonic behavior, we conducted continuous\ntemperature-dependent reflectance measurements (from 87 K to 300 K) on several WS\n2\ncavity devices with varying thicknesses (∼35 nm, ∼49 nm, ∼70 nm, ∼108 nm, ∼120\nnm, ∼130 nm) (See Fig. R2.6 and Fig. R2.7). As shown in Fig. R2.6 and Fig. R2.7, the\nstrong coupling WS device exhibits clear reflectance spectra where both the upper and\n2\nlower polariton branches undergo consistent redshift with increasing temperature. This\ntrend is consistent with the well-known temperature dependence of exciton resonances\nin WS and confirms the excitonic contribution to the coupled states.\ntemperature range without merging into a single resonance peak, indicating that the\nsystem stays in the strong coupling regime throughout. This smooth, temperature-\ncontinuous evolution of the mode positions strongly supports the interpretation of these\nfeatures as coupled exciton-photon states (polaritons), rather than simple optical\ninterference effects or uncoupled resonances. The observed temperature dependence is\nconsistent across multiple devices with different WS thicknesses, demonstrating the\n2\nrobustness of the polaritonic features and the generality of the self-hybridized cavity\ndesign.\nFig R2.6 | Temperature-dependent reflectance spectra of devices in the weak\nand strong coupling regimes.\na b\n2\n0\n5 0 0 6 0 0 7 0 0 8 0 0\ne\nc\nn\na\ntc\ne lfe\nR\ns tro n g c o\nW\nu p lin\ng\nW\na v\ne le n g\nW\nth\nS\n(\n2\nn\n=\n1\nm\n3\n)\n0 ±\n3\n2\n2\n2\n2\n2\n1\n1 1\n1\n1\n8\n5\n0 0 K\n7 5 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\n8 0 K\n6 0 K 4 0 K\n2 0 K\n0 0 K\nn m\ne\nc\nn\na tc e\nlfe\nR\n2\n0\n5 0 0 6 0 0 7 0 0 8 0 0\nW a v e le n g th ( n m )\ne\nc\nn\na\ntc\ne lfe\nR\nw e a k c o u p lin\nW\ng\na v e le n\nW\ng\nS\nth\n2 = 3 5 ±\n3\n2\n2\n2\n2\n2\n1\n1\n1\n1\n1\n8\n5\n0 0 K\n7 5 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\n8 0 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\nn m\ne\nc\nn\na tc e\nlfe\nR\nW a v e le n g th ( n m )\nFig R2.7 | Reflectance spectra at varying temperatures for devices of different\nthicknesses.\nRevised:\nThese new results and corresponding analyses have been incorporated into the revised\nmanuscript (page 6). The updated figures are presented as follows: Fig. R2.6b now\nappears as Figs. 2e in the main text, while Fig. R2.6a and Fig. R2.7 has been included\nin the Supplementary Information (Supplementary Fig. S13).\n6. In the optoelectronic measurements presented in Fig. 4, how do interfacial trap states\nbetween ITO and WS affect the observed rise and response times? Given that exciton\n2\nand polariton dynamics typically occur on picosecond to sub-nanosecond timescales,\nthe reported response times on the order of hundreds of nanoseconds seem long (>>\nlifetime of exciton/polariton). This suggests that extrinsic effects, such as carrier\ntrapping at the ITO/WS interface, may play a significant role. The authors are\n2\nencouraged to discuss the possible influence of interface quality and trap dynamics on\nthe device performance.\nResponse:\na\necnatcelfeR\nc\necnatcelfeR\n5\n5\n0\ns\n0\nw e\n0\ntro\n0\na\nn\nk c\nW\ng c\nW\no u p lin g\n6 0 0\na v e le\no u p lin g\n6 0 0\na v e le\nn\nn\nW S = 4 9\n2\n7 0 0\ng t h ( n m\nW S = 1 0 9\n2\n7 0 0\ng t h ( n m\n±\n8\n)\n±\n8\n)\n5\n0\n5\n0\nn m\n0\nn m\n0\necnatcelfeR\necnatcelfeR\nb\nd\n5\n5\n0\ns\n0\ns tro\n0\ntro n\n0\nn g c o u p lin\n6 0 0\nW a v e le\ng c o u p lin g\n6 0 0\nW a v e le\ng\nn\nn\nW\n7\ng\nW\n7\ng\nS = 7 0 5 n m ±\n2\n0 0 8 0 0\nt h ( n m )\nS = 1 2 0 5 n m ±\n2\n0 0 8 0 0\nt h ( n m )\nWe thank the reviewer for raising this important point regarding the difference between\nthe intrinsic exciton/polariton dynamics and the device-level response time observed in\nFig. 4. To clarify this issue, we conducted a series of additional experiments to evaluate\nthe influence of interfacial trap states and extrinsic defects on both exciton lifetime and\nphotoresponse dynamics.\nIndeed, while exciton and polariton lifetimes typically lie in the picosecond–sub-\nnanosecond regime, the observed rise and response time in Fig. 4 are longer (hundreds\nof nanoseconds), indicating the presence of extrinsic limiting mechanisms. The rise and\nfall times in photodetectors are generally governed not only by the intrinsic\nexciton/polariton dynamics but also by carrier trapping and detrapping at\ninterfaces and surfaces, as well as carrier transport within the device channel.\nThese defect states are inevitably introduced during device fabrication.\nTo investigate the influence of interfacial defect states on the lifetime of\nexciton/polariton and response time of exciton-polariton devices, we prepared two\ndevices from the same nanosheet (Fig. R2.8d): pristine WS nanosheets obtained by\n2\nmechanical exfoliation, and the same WS nanosheets subjected to 23 s of oxygen\n2\nplasma treatment, emulating the defect formation typically induced during ITO\nsputtering. The surface morphology was examined using atomic force microscopy\n(AFM) (Fig. R2.8). The WS thickness was measured to be 79.9 nm (Fig. R2.8a). Root-\n2\nmean-square (r.m.s.) roughness analysis revealed that the pristine WS nanosheet\n2\nexhibited an atomically clean surface with an r.m.s. roughness of 0.31 nm (Fig. R2.8\nfb). After O plasma treatment, the r.m.s. roughness increased to 1.02 nm (Fig. R2.8\n2\nfc), indicating surface degradation and the formation of uniformly distributed defects.\nThe rise and response times of a photodetector are fundamentally governed by\nintrinsic material dynamics. Although the lifetimes of excitons and polaritons typically\nlie in the picosecond-to-sub-nanosecond range, interfacial carrier trapping at the\nITO/WS interface and the subsequent slow release of trapped carriers can prolong\n2\nthe overall photocurrent response. In our case, plasma treatment increases the density\nof interfacial trap states, which accelerates recombination (shortening the\nexciton/polariton lifetime (Fig. R2.8e)) while simultaneously slowing net carrier\nextraction and transport due to transient trapping-detrapping dynamics. These trap\nstates delay carrier collection and extend the photocurrent response into the\nnanosecond regime, consistent with the features observed in Fig. R2.8 f. Nevertheless,\nthe response times of our devices remain much faster than those of most other excitonic\nphotodetectors (main text, Fig.4f).\nAlthough the response speed is still below the theoretical limit owing to the group\nvelocity of polaritons, our additional data strengthen the validity of the results and\nhighlight the promise of this approach for future studies. Future research should focus\non reducing carrier trapping at the ITO/WS interface and optimizing the relaxation\n2\ndynamics of exciton/polaritonic states, such as suppressing non-radiative losses,\nextending lifetimes, and promoting rapid energy transfer, to further enhance the\ngeneration and transport efficiency of photogenerated carriers. Potential strategies to\nmitigate interfacial defects include adopting physical transfer of electrodes to avoid\nplasma-induced damage, introducing ultrathin interfacial passivation layers,\nemploying van der Waals contacts, or using chemical surface treatments to reduce\ntrap density. Such improvements are expected to markedly shorten the rise time of the\nphotoresponse, which is particularly critical for the development of sub-nanosecond,\nfast photodiodes for applications in optical communication and sensing. These\nimprovements are expected to fully unlock the potential of polariton-enhanced\ntransport in next-generation optoelectronic devices and shorten the rise time of the\nphotoresponse, which is particularly critical for the development of sub-nanosecond,\nfast photodiodes for optical communication and sensing applications.\nFig. R2.8 | Influence of interfacial trap states on the response time of exciton-\npolariton devices. a, Atomic force microscopy (AFM) measurement of an exfoliated\nWS flake with a thickness of 77.9 nm. b,c, Three-dimensional AFM profiles of the as-\n2\n0 2 4 6 8 1 0\n).u\n.a ( tn\ne rru\nc o to\nh P O 2 P la s m a\nP ris tin e\n3 0  m\n1\n0\n0\n0\n0 0\n.0\n.8\n.6\n.4\n.2 .0\n0 2 0 0 4 0 0 6 0 0\n263.4 ns\n477.9 ns\nT im e ( s )\n39.7 ns\n133.8 ns\n).m\nro N\n( R 0\n/R D\na\nd\n)mn(\nthgieH\n60\n0\n−600\nW S\n(7\n2\n)\n7 .9\n8\nn m )\nb\ne\nr.m\nT\nP ris tin e\n.s . = 0 .3 1 n m\nP ris tin e\nO P la s m 2 = 8 9 .5 p s\n= 4 1 1 .7 p 2\n= 5 3 .9 p s = 3 5 8 .3 p s 2\nim e d e la y (p s )\na\ns\nc\nf\nr.m\nO P la s m a 2\n.s .= 1 .0 2 n\nP ris tin e\nO P la s m a 2\nm\n1 0 n m\n-1 0 n m\nexfoliated (b) and oxygen plasma-treated (c) surfaces, showing an increase in root-\nmean-square (r.m.s.) roughness from 0.31 to 1.02 nm within 3 × 3 µm scan areas,\nindicative of surface degradation after plasma exposure. d, Optical microscopy images\nof devices fabricated on the same flake before and after oxygen plasma treatment. e,\nNormalized transient reflectance dynamics of pristine and plasma-treated WS devices.\n2\nf, Comparison of response times for pristine and plasma-treated WS devices.\nWe have added this discussion in the revised main text (page 10) and in the\nSupplementary Information (Supplementary Fig. S20) to clarify the role of interface\nquality in the observed dynamics.\nReviewer #3 (Remarks to the Author):\nThe manuscript reports a polariton photodiode in a self-hybridized TMD system and\nclarifies that some device performances, particularly the response time (up to hundreds\nof nanoseconds), could be enhanced in the strong coupling regime. The experimental\ndata primarily support the interpretation of cavity effect of enhancement rather than one\nbased on polaritons. Although there are some decent performance improvements, the\nmain merit appears to lie in the fast response time, which is significantly faster than\nreported devices. While the performance improvements are noted, the enhancement\nmechanism in the so called “strong coupling regime” remains unclear, and the\ndemonstrated evidence is not convincing. Below are my detailed comments:\nResponse:\nWe thank the reviewer for the thoughtful and critical evaluation of our manuscript. We\nsincerely appreciate the recognition of the fast response time and performance\nimprovements in our polariton photodiodes. We understand the reviewer’s concern\nregarding the attribution of device enhancements to strong light-matter coupling rather\nthan purely to cavity effects. In the revised manuscript, we have taken substantial steps\nto clarify the physical mechanisms underlying the observed enhancements and to\nfurther distinguish the role of polaritonic modes from conventional cavity-induced\neffects. We would also like to point out that while evidence of polaritons in the\noptoelectronic responses in these devices is clear, the true merit in terms of performance\n(response speed) is still far from theoretical values due to imperfect materials and\ninterfaces. Optimization of materials and related interfaces is beyond the scope of\ncurrent work and will be part of future efforts.\nWe have expanded our analysis to include:\na) Temperature-dependent reflectance spectra (87-300 K), which\ndemonstrate continuous, non-collapsing mode splitting consistent with\npolariton branches rather than Fabry-Pérot resonances (see revised Fig. 2e and\nSupplementary Fig. S13).\nb) Angle-resolved dispersion mapping and coupled oscillator model\nfitting, producing Rabi splitting values that are reproducible and consistent\nacross several independent experimental methods, further confirming the\npresence of strong coupling (see revised Supplementary Fig. S6).\nc) The relationship between reflectance spectra and EQE demonstrates\nphotoresponse and quantum efficiency enhancements that correlate well with\nthe dispersion of polariton modes (see revised Fig. 2c).\nThese revisions are described in detail in our responses to the specific comments\nbelow, and the corresponding modifications have been incorporated throughout the\nmanuscript. We hope these additions provide a clearer picture of how the strong\ncoupling between excitons and photons contributes to the observed optoelectronic\nperformance enhancements.\n1) The novelty of polariton photodiode bring out some concerns. In the context of\nexciton polariton photodiodes, this idea has been tried several years ago in organic\nexcitonic materials [e.g., ACS Photonics 5, 2921-2927 (2018)]. Though it may be an\ninteresting attempt to realize this idea within TMDs, the experimental evidence in this\nmanuscript is not yet sufficient to support polariton effects. Further data and\nclarification are required to substantiate the role of polaritons in the observed\nperformance enhancement.\nResponse:\nWe sincerely thank the reviewer for acknowledging our effort to explore polariton\nphotodiodes within the TMD material system. We appreciate the reference to prior work\non organic excitonic materials, such as ACS Photonics 5, 2921-2927 (2018), which\nindeed laid important groundwork in this area.\nWhile the concept of exciton-polariton photodetectors has been previously\ndemonstrated in organic systems, our work presents a fundamentally distinct strategy.\nIt is also worth noting that organic excitonic semiconductors cannot achieve self-\nhybridization due to low values of real part of refractive indices and therefore always\nhave to rely on closed Fabry-Perot cavity systems. Further the\nnanocrystalline/amorphous nature of organic semiconductors obfuscates the transport\nof excitons. In contrast WS has high real part of refractive index and exciton transport\n2\nis highly anisotropic in v.s. out of plane making it a much better model system for\ninvestigating polariton and strong coupling effects in photodiode and photovoltaic\ndevices.\nAs shown in Figure R3.1, unlike metal-mirror organic polariton photodiodes that rely\non a closed Fabry-Pérot cavity (ACS Photonics 5, 2921-2927 (2018)) and whose\ninternal quantum efficiency is limited by Ag electrode losses, our device realizes\ncavity-free self-hybridized (our work) polaritons in multilayer WS , where a single\narchitecture not only yields dispersive, thickness-quantized polaritons whose extrema\nalign with EQE/IQE oscillations and near-unity IQE at zero detuning but also delivers\ndevice-level advantages, high responsivity (~ 0.35 A/W), low NEP (~0.34 pW Hz-\n1/2), and sub-μs response (~217 ns), which were not accessible in previous\ndemonstrations (ACS Photonics 5, 2921-2927 (2018)).\nFigure R3.1. A comparison between the strategies in the reported work (ACS Photonics\n5, 2921–2927 (2018)) and our work.\nIn the revised manuscript we have included additional experimental evidence to\nunambiguously demonstrate the presence of polaritons and to establish their direct role\nin enhancing device performance:\n1) Temperature-dependent reflectance (87-300 K):\nIn the revised manuscript, we now provide reflectance spectra across a broad\ntemperature range (Fig. 2e and Supplementary Fig. S13), corresponding to Fig. R3.2\nand Fig. R3.3. The results show continuous, non-collapsing polariton mode\nsplitting that persists with temperature, which is inconsistent with Fabry-Pérot\nresonances but fully consistent with polariton branches.\nFig R3.2 | Temperature-dependent reflectance spectra of devices in the\nweak and strong coupling regimes.\nFig R3.3 | Reflectance spectra at varying temperatures for devices of\ndifferent thicknesses\n2) Angle-resolved dispersion mapping:\nWe further performed angle-resolved reflectance mapping and fitted the\na\necnatcelfeR\necnatcelfeR\n2\n0\n5 0\nw\n0\ne a k c o\nW\nu p lin\ng\n6 0 0\na vW\ng\ne lea nv ge\n7\nth le\nW\n0 0\n( n n g\nS\nmth\n= 2\n) 3 5 ±\n8 0\n3\n2\n2\n2\n2\n2\n1\n1\n1\n1\n1\n8\n5\n0\n0 0 K\n7 5 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\n8 0 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\nn m\nb\necneacntcaetclfeelRfeR\n2\n0\n5 0\ns\n0\ntro n g c o u\np lin g\n0 0\na v e a v e lele nn\nW S\n7 0 0\ng th ( n g th (\n= 1 2\nm ) n m\n3\n) 0 ±\n8 0\n3\n2\n2\n2\n2\n2\n1\n1\n1\n1\n1\n8\n5\n0\n0 0 K\n7 5 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\n8 0 K\n6 0 K\n4 0 K\n2 0 K\n0 0 K\nn m\ne 240 K\nc 220 K\nn 200 K\na 180 K\nt 160 K\nc 140 K\ne 120 K\nlf 100 K\ne 87 K\nR\nWS = 120 ±5 nm\n2\n500 600 700 800\nWavelength (nm)\necnatcelfeR\n5 0 0\nW\n6\na\n0 0\nv e le n\n7\ng\n0\nt h\n0\n( n m\n8\n)\n0\n0\nweak coupling strong coupling\nWS = 49 ±5 nm\n2\necnatcelfeR\n5 0 0\nW\n6\na\n0 0\nv e le n\n7\ng\n0\nt h\n0\n( n m\n8\n)\n0\n0\na b\nWS = 70 ±5 nm\n2\nc d\nstrong coupling 300 K 275 K\ne 240 K\nc 220 K\nn 200 K\na 180 K\nt 160 K\nc 140 K\ne 120 K\nlf 100 K\ne 87 K\nR\nWS = 109 ±5 nm\n2\n500 600 700 800\nWavelength (nm)\ndispersion with the coupled oscillator model (Supplementary Fig. S6; see also\nFig. R3.4). The extracted Rabi splitting values are highly reproducible and\nconsistent across independent experimental approaches, thereby fulfilling\nthe strong coupling criterion and providing unambiguous evidence for\npolariton formation and device operation in the strong coupling regime.\na Reflectance b Reflectance\nMin. Max. Min. Max.\n2.4 2.4\nExp. Sim. Exp. Sim.\n2.2 2.2\nUP\n)V )V\ne e\n(\ny g 2.0\n(\ny g 2.0\nre re\nn n\nE E\nLP\n1.8 1.8\n49 nm 77.9 nm\n1.6 1.6\n-0.5 0 0.5 -0.5 0 0.5\nc Sin () d Sin ()\nReflectance Reflectance\nMin. Max. Min. Max.\n2.4 2.4\nExp. Sim. Exp. Sim.\n2.2 2.2\n)V UP )V UP\ne e\n(\ny g 2.0\n(\ny g 2.0\nre re\nn n\nE E\nLP LP\n1.8 1.8\n151.2 nm 221 nm\n1.6 1.6\n-0.5 0 0.5 -0.5 0 0.5\nSin () Sin ()\nFig. R3.4 | Angle-resolved reflectivity spectra of WS exciton polariton\n2\ndevices with different thicknesses (~70-220 nm). The left (right) part of the\nfigure shows the experiment (TMM calculation). The red dashed lines represent\nthe dispersion obtained for the UPB and LPB using the coupled oscillator model.\nThe black dashed line and parabola represent the exciton energy and the\ndispersion of the cavity mode.\n3) Link to device performance (Reflectance ↔ EQE):\nFinally, we explicitly correlate the polariton dispersion with device-level\nmetrics by showing that the enhancements in EQE spectra occur at energies that\ncoincide with the polariton branches (Fig. 2c-d; see also Fig. R3.5). Figure\nR3.5a shows the coupled oscillator model fit, along with the experimental EQE\npeak wavelengths extracted from a range of device thicknesses. Notably, the\npeak response positions of the EQE spectra in devices with varying thicknesses\n(scattered dots) exhibit excellent agreement with the theoretical model (dashed\nlines). In particular, the evolution of their wavelength positions with thickness\nstrictly follows the characteristic dispersion relation of polaritons, with the peak\npositions corresponding to the resonance modes of the UP and LP branches,\nrespectively. This consistency between experimental observations and the\ntheoretical model provides solid evidence that the polariton state enhances light-\ntrapping efficiency by modulating photon-exciton interactions, clearly\ndemonstrating the crucial role of polariton engineering in improving photodiode\nperformance. Figure R3.5b shows the EQE spectra of three representative\ndevices, one in the weak coupling regime, one near zero detuning for the first\norder polariton, and one near zero detuning for the second order polariton. In\nthese strongly coupled devices, both the lower polariton branch (LPB) and\nupper polariton branch (UPB) exhibit spectral separations from the bare exciton\nresonance that exceed their respective linewidths. This clear spectral separation\nenables unambiguous identification of the polariton branches. The responses\nbelow the bare exciton energy of the strongly coupled devices can be observed,\nand the peaks of the responses correspond to the reflectance minima (attributed\nto UP and LP branches) reported in the Fig.R3.5c. These features are absent in\nweakly coupled devices, confirming that all these responses are dominated by\nthe E-P states. This direct correspondence between optical dispersion and\nphotoresponse provides strong evidence that the observed performance\nenhancements stem from polariton-assisted absorption and transport, rather\nthan from conventional cavity effects.\nFig. R3.5 | a, Dots are extracted from experimental EQE spectra shown in\n(Fig.S9), while lines are the calculated dispersions of the polariton eigenstates.\nThe EQE experimental data agree well with the calculated polariton branches\nof the coupled oscillator model. b, Self-normalized EQE spectra for WS\n2\nthicknesses of 151.2 nm, 77.9 nm, and 39.6 nm. Both devices in the strong\ncoupling regime (151.2 nm, 77.9 nm) show two EQE peaks on either side of the\nex energy corresponding to two well-defined polariton states. c, Reflectance\nA\nspectra of WS devices of 151.2 nm, 77.9 nm, and 39.6 nm. Consistent with the\nwell-resolved reflectance dips straddling the A-exciton energy, providing direct\nevidence of the formation of UPB and LPB.\nTaken together, these results provide mutually consistent and cross-validated\nevidence for strong exciton-photon coupling in our WS photodiodes. Unlike previous\n2\norganic polariton OPD demonstrations, where performance enhancement was limited\nby metal cavity losses, our device not only shows unambiguous polariton signatures but\nalso translates them into quantum efficiency, responsivity, noise, and speed\nadvantages. This comprehensive evidence closes the loop from fundamental polariton\nphysics to practical photodiode performance.\nWhile the response speed is still fall from theoretical maximum attributed to group\nvelocity of polaritons we hope that the additional data further expands the credibility of\nthe results and promotes more investigations in this area. We further hope these\nadditions more clearly convey the novelty and significance of our approach.\nRevised:\nThe temperature-dependent reflectance spectra and corresponding analyses have\nbeen incorporated into the revised manuscript (page 6). The updated figures are now\na\n)mn(\nssenkcihT\nSW 2\n200\n150\n100\n50\nw\n0550\ne a k co u p lin\n600W\nW a\nstro n g co u p lin g\nT M M g\nE xp e rim e n ta l\nE Q E m a xim a\n650 700\na ve le n g th (n m ) v e le n g th (n m )\n750\nb\n).u.a(\nEQE\ndezilamroN\n5 0\nstro n g co u p lin g\nstro n g co u p lin g\nw e a k co u p lin g\n0 6 0 0\nW a v e le n g th\n7 0 0\n(n m\n151.2 nm\n77.9 nm\n39.6 nm\n8\n)\n0 0\nc\necnatcelfeR\n500\n151.2 nm\n77.9 nm\n39.6 nm\nstro n g co u p lin g\nstro n g co u p lin g\nw e a k co u p lin g\n800 700\n(n m )\npresented as Fig. 2e in the main text and Supplementary Fig. S13. Additionally, explicit\ncomparisons with prior literature have been introduced to clarify the origin and\njustification of the observed coupling strength. This discussion, along with supporting\ndispersion fits and additional data, is included in the Supplementary Information\n(Supplementary Fig. S6 and Table S1). Furthermore, the content originally presented in\nFig. R3.5b has been integrated into Figure 2.\n2) Insufficient evidence for the strong coupling regime. To claim the strong coupling\nregime, the coupling strengths were fitted under different detuning by varying the TMD\nthicknesses. This is problematic. The strong coupling can be verified by satisfying the\ncriterion that the coupling strengths should be larger than the averaged linewidths of\ncavity modes and excitons. This means the 𝑔2 should be greater than (𝛾2 +𝛾2)/2.\n𝐶 𝑋\nThis criterion has not been tested in their spectroscopic measurements (Figs. 2a-b).\nThe criterion in the SI they cited can only infer there could be spectral splitting, which\ndoes not necessarily indicate the strong coupling regime (please see many references\ndiscussing about the differences between the intermediate coupling and the strong\ncoupling). It is also important to note that the guidelines do not exactly follow the\nreflectivity dips (Fig. 2b), which are misleading.\nResponse:\nWe sincerely thank the reviewer for this valuable comment and for highlighting the\nimportance of rigorously verifying the strong coupling regime using established criteria.\nWe have deeply analyzed our experimental spectra and explicitly verified the\nstrong coupling criteria as suggested.\n1) Deep analysis of reflectance spectra\nWe performed thickness-dependent reflectance calculations using the transfer\nmatrix method (TMM). In the simulated spectra, the polariton branches appear as\nreflectance minima and exhibit a pronounced anticrossing near the exciton\nresonance (~1.97 eV for WS , indicated by the yellow dashed lines in Figs. R3.6).\nmodel, explicitly incorporating exciton and cavity decay rates. The fitted\ndispersions are shown as white dashed lines in Fig. R3.1a, and the corresponding\nbare cavity modes are shown as red dashed lines. The Rabi splittings for the first\nthree cavity modes (l = 1, 2, 3) are ℏΩ = 152.62 meV, 173.38 meV, and 171.42\nmeV (Table R3.1), corresponding to coupling strengths of 86.7 meV, 89.6 meV, and\n86.7 meV.\n200\n175\n)\nm\n150\nn\n(\ns\ns125\ne\nn\nk c100\nih\nT\n75\nW\n50\n25\n0.0\n500 550 600 650 700 750\nWavelength (nm)\nFig. R3.6 | Exciton-Polariton Dispersion and thickness-dependent EQE of multi-\nlayer WS devices in strong coupling. Thickness and wavelength dependent\n2\nreflectance calculated by the transfer matrix method (TMM), showing the dispersion of\nthe exciton polaritons. White dashed lines are the coupled oscillator model fit to the\ncalculated reflectance minima, representing the polariton dispersion. The bare cavity\nmode dispersion (red dashed lines) and exciton wavelength (yellow dotted lines) are\noverlaid. White cross-hairs represent experimentally measured reflectance minima.\nTable R3.1 Coupled oscillator model fit results.\nParameter 𝒍 = 𝟏 𝒍 = 𝟐 𝒍 = 𝟑\n𝒈 (meV) 86.7 89.6 86.7\n𝐸 (eV) 1.971 1.969 1.967\n𝑋\nCavity Slope\n4.186 2.448 1.757\n(nm/nm)\nCavity Intercept\n307.5 272.3 251.5\n(nm)\n𝛾 (meV) 106.3 70.3 51.1\n𝐶,𝑙\n𝛾 (meV) 24.0 25.0 25.0\n𝑋\nRabi Splitting\n152.62 173.38 171.42\n(meV)\n(𝛾 −𝛾 )/2 41.15 22.65 13.05\n𝐶 𝑋\n√(𝜸 𝟐 +𝜸 𝟐)/𝟐 77.05 52.75 40.22\n𝑪 𝑿\nCoupling\nStrong Strong Strong\nRegime\n2) Verification using the rigorous strong-coupling criteria\nIn the revised manuscript, we clarify that a system is in the strong coupling regime\nonly when the exciton-photon energy exchange rate exceeds the combined\ndissipation rates of the constituent modes. Importantly, satisfying the condition\n2\n(𝑙)\n(𝛾 −𝛾 )\n𝑐 𝑋\n𝑔2 >\n𝑙 4\nis not sufficient, as it merely indicates the presence of a non-vanishing Rabi splitting.\nTo ensure that the Rabi splitting is experimentally resolvable, the coupling strength\nmust also satisfy\n𝑔2 > (𝛾2 +𝛾2)/2.\n𝐶 𝑋\nApplying both criteria, we confirm that our devices indeed operate in the strong\ncoupling regime. Specifically:\n1) For the first-order cavity mode ( 𝑙 =1), 𝑔 =86.7 meV, exceeding\n√(𝛾 2 +𝛾 2)/2=77.05 meV.\n𝐶 𝑋\n2) For the second-order cavity mode ( 𝑙 =2), 𝑔 =89.6 meV, exceeding\n√(𝛾 2 +𝛾 2)/2=52.72 meV;\n𝐶 𝑋\n3) For the third-order cavity mode ( 𝑙 =3), 𝑔 =86.7 meV, exceeding\n√(𝛾 2 +𝛾 2)/2=40.22 meV.\n𝐶 𝑋\nAll conditions are thus satisfied, confirming that the observed exciton-photon\nhybrid states are unambiguously in the strong coupling regime (see Table R3.1).\n3) Clarification of Fig. 2b\nAs the reviewer noted, the guidelines do not exactly follow the reflectivity dips,\nwhich could be misleading. To address this, in the revised Fig. 2b (as shown in Fig.\nR3.7) we present updated experimental reflectance spectra for representative\ndevices with different thicknesses, together with corresponding TMM calculations\n(black dashed lines) that agree well with the measurements. The dispersions of the\nupper and lower polariton branches (UPB and LPB) are indicated by purple dashed\nlines. The spectra are vertically offset for clarity.\nFig. R3.7 | Experimental reflectance spectra of WS for devices of varying WS\n2 2\nthickness. The UPB and LPB peaks are emphasized with purple dashed lines. The black\ndashed line is the reflectance spectrum calculated by TMM for each thickness.\nRevised:\nIn revised manuscript, we have systematically analyzed the reflectance spectra using\ntransfer matrix method (TMM) calculations to obtain thickness-dependent reflectance.\nThe updated results are presented in Figure 2a-b and Supporting Information Figure S3.\nFurthermore, we have demonstrated that our devices indeed operate in the strong\n(𝑙) 2\ncoupling regime by applying both criteria: 𝑔2 >\n(𝛾\n𝑐\n−𝛾𝑋)\nand 𝑔2 > (𝛾2 +𝛾2)/2.\n𝑙 𝐶 𝑋\n4\n3) The coupling strengths is proportional to the sample thickness, which has been\nvaried by more than 8 times. For a rough estimation without considering the detailed\necnatcelfeR\n9\n6\n3\n0\n.0\n.0\n.0\n.04\n2 .6 2\n1 9 9 .6 n m\n1 6 6 .8 n m\n1 5 1 .2 n m\n1 2 6 .6 n m\n1 1 3 .3 n m\n8 7 .3 n m\n7 7 .3 n m\n7 3 .7 n m\n2 4 .1 n m\n5 0 5 0 0\n.4\n2 .2\nU\n5 0W\nn\nP\na\ne\n6v\nr g y\n2 .0\ne x\nA\n0 0\ne le\n(\n6n\ne\n5g\nV\n0t\n)1\n.8\nL\n7 0\nh ( n\nP\n0m\n)\n7 5\n1\n0\n.6\n8 0 0\ncavity mode distributions (which was also ignored in the manuscript), the coupling\nstrengths in Fig. 2b could be varied by about 3 times. In this way, the so-called anti-\ncrossing features in Figs. 2a-b is misleading, as they do not share similar cavity\nstrengths. It should be appropriate to measure samples of the same thickness at\ndifferent angles (angle-resolved reflectivity) and then extract the coupling strength\nspecifically for each thickness.\nResponse:\nWe thank the reviewer for raising the important point regarding the possible ambiguity\nin interpreting the coupling strengths when varying the WS thickness over a wide range.\nthickness provide a more reliable approach for extracting the exciton-photon coupling\nstrength, as this method eliminates uncertainties arising from the variation in cavity\nmode distributions.\nTo address this, we performed systematic angle-resolved reflectivity measurements\non WS with thicknesses ranging from ~35 nm to ~255 nm (Fig. R3.8). For each\n2\nthickness, the dispersion curves of the upper and lower polariton branches were fitted\nusing the coupled oscillator model, explicitly incorporating the exciton and cavity\ndecay rates. Clear anti-crossing behavior was observed in samples thicker than 50 nm,\nsignifying the formation of mixed exciton-photon modes (Fig. R3.8 b-h). This spectral\nsplitting reflects exciton self-hybridization under the strong coupling regime. By\ncontrast, thinner samples (<50 nm) exhibited a single dispersionless dip\ncorresponding to the bare A exciton, without any signature of coupling (Fig. R3.8a). We employed\na coupled oscillator model to fit the experimental data and evaluate the exciton-photon\ncoupling strength (Table R3.2). All measured dispersion relations across the devices\nwere well reproduced by the model. Three representative devices, with WS thicknesses\n2\nof 75 nm, 150 nm, and 211 nm, correspond approximately to the zero detuning\nconditions of the first-, second-, and third-order cavity modes, respectively. The\nextracted coupling strengths are 82.5 meV (Fig. R3.8b), 93.2 meV (Fig. R3.8d), and\n93.9 meV (Fig. R3.8h). These values are comparable across different thicknesses\n(𝒍) 𝟐\nand satisfy both strong coupling criteria, 𝒈𝟐 >\n(𝜸\n𝒄\n−𝜸𝑿)\nand 𝒈𝟐 > (𝜸𝟐 +𝜸𝟐)/𝟐,\n𝒍 𝑪 𝑿\n𝟒\nthereby confirming that the observed anticrossings arise from genuine strong\nexciton-photon coupling rather than mere Fabry-Pérot cavity effects associated\nwith thickness-dependent variations.\nFig. R3.8 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\na\n)Ve(\nygrenE\n)Ve(\nygrenE\ne\nM\n2.4\n2.2\n2.0\n1.8\n1.6\nM\n2.4\n2.2\n2.0\n1.8\n1.6\nR e fle cta n ce\nin .\nE xp . S im\n4 9 n m\n-0.5 0 0.5\nS in ( ) \nR e fle cta n ce\nin .\nE xp . S im\nU P\nL P\n1 6 9 .7 n m\n-0.5 0 0.5\nS in ( ) \nM\n.\nM\n.\na x.\na x.\n)Ve(\nygrenE\n)Ve(\nygrenE\nb R e fle cta n ce\nM in .\n2.4\nE xp .\n2.2\nU P\n2.0\nL P 1.8\n7 7 .9 n m\n1.6\n-0.5 0\nS in ( ) \nf\nR e fle cta n ce\nM in .\n2.4\nE xp .\n2.2\nU P\n2.0\n1.8\nL P\n1 7 1 .4 n m\n1.6\n-0.5 0\nS in ( ) \nS im\n0.5\nS im\n0.5\nM\n.\nM\n.\na x.\na x.\nc\n)Ve(\nygrenE\n)Ve(\nygrenE\ng\nM\n2.4\n2.2\n2.0\n1.8\n1.6\nM\n2.4\n2.2\n2.0\n1.8\n1.6\nR\nin .\nE xp .\nU P\nL P\n8 9 n m\n-0.5\nR\nin .\nE xp .\nU P\nL P\n1 9 6 n m\n-0.5\ne\ne\nfle cta n\nfle cta n\nce\n)\nce\n)\nS im\n0.5\nS im\n0.5\nM\n.\nM\n.\na\na\nx.\nx.\nd\n)Ve(\nygrenE\n)Ve(\nygrenE\nh\nR e fle cta n ce\nM in .\n2.4\nE xp .\n2.2\nU P\n2.0\nL P 1.8\n1 5 1 .2 n m\n1.6\n-0.5 0\nS in ( ) \nR e fle cta n ce\nM in .\n2.4\nE xp .\n2.2\nU P\n2.0\nL P\n1.8\n2 2 1 n m\n1.6\n-0.5 0\nS in ( ) \nS im\n0.5\nS im\n0.5\nM\n.\nM\n.\na x.\na x.\nTable R3.2 | Coupled Oscillator Model Fitting Results for Devices with Varying\nThicknesses.\n𝛾 𝛾\nThickness 𝐶 𝑋 Rabi splitting\n𝑔 (meV) (𝛾 −𝛾 )/2 √(𝛾 2+𝛾 2)/2\n𝐶 𝑋 𝐶 𝑋\n(nm) (meV)\n(meV) (meV)\n77.9 82.5 89.6 24 151.4 32.8 65.5\n89.0 92.7 132.9 24 150.2 54.5 95.5\n151.2 93.2 76.87 24.1 178.8 26.3 56.9\n169.7 88.3 87.2 24.3 165.0 31.45 55.75\n171.4 84.94 70.5 24.75 163.60 22.875 52.83\n196.0 101.57 61.8 24.2 199.6 18.8 46.9\n196.0 98.1 40.3 29 195.9 18.8 46.9\n221.9 93.9 55 29.8 186.1 12.6 44.2\nRevised:\nIn the revised manuscript, we have performed angle-resolved reflectance spectroscopy\nmeasurements. The corresponding dispersion data and coupled oscillator model fits are\nnow included in the Supplementary Information (Supplementary Figs. S6 and Table S1)\nto more accurately characterize the thickness-dependent variation in coupling strength.\n4) In Fig. 2d, the authors claim that the normalized EQE data support the consistency\nof polariton resonances and EQE peaks. However, this is not the case in close\nobservations. For example, some EQE peaks coincide with the exciton resonance\ninstead of so called “LP” or “UP” states. To substantiate the polariton effect, it is\nstrongly suggested that they should first clarify the well-separated polariton peaks by\nchecking the criterion of the strong coupling regime, which are expected to be distant\nfrom the exciton resonances with respect to their linewidths. For instance, some\nconvincing measurements should be at least performed in the “zero-detuning” samples\nto better determine the strong coupling regime.\nResponse:\nWe appreciate the reviewer’s observation and agree that a rigorous assignment of EQE\npeaks to polariton modes requires verification of both the strong coupling regime and\nsufficient spectral separation between the polariton branches and the bare exciton.\nWe confirm that the observed EQE peaks indeed originate from polariton resonances rather\nthan bare excitonic absorption. To rigorously establish this assignment, we have\nreanalyzed the data with stricter strong coupling criteria and carried out additional\nmeasurements on zero-detuning devices.\n1) Verification of strong coupling regime:\nTo strengthen our analysis, we performed further measurements on additional\ndevices and collected angle-resolved reflectivity spectra, which were then fitted\nusing a coupled oscillator model to precisely identify the LP and UP states. We\napplied established strong coupling criteria to all measured devices, retaining only\n(𝑙) 2\nthose that satisfy 𝑔2 >\n(𝛾\n𝑐\n−𝛾𝑋)\nand 𝑔2 > (𝛾2 +𝛾2)/2, for the EQE-polariton\n𝑙 𝐶 𝑋\n4\ncomparison. In these devices, the LP and UP branches are spectrally distant\nfrom the bare exciton by more than their respective linewidths, ensuring\nunambiguous identification (Fig. R3.9).\n2) Zero-detuning devices as direct evidence:\nIn detuned cavities, one polariton branch may naturally converge toward the\nexciton resonance, which explains the occasional coincidence of EQE peaks with\nthe exciton energy.\nTo directly confirm the strong coupling regime, we fabricated and characterized\nrepresentative zero-detuning samples (77.9 nm, 151.2 nm). For these devices, we\nperformed angle-resolved reflectance and reflectance intensity spectra together\nwith the corresponding EQE data (Fig. R3.9).\nWS devices with thicknesses of 77.9 nm and 151.2 nm correspond closely to\n2\nthe zero-detuning conditions for the first- and second-order cavity modes,\nrespectively. In their angle-resolved reflectance spectra, we observe clear\nanticrossing behavior (Fig. R3.9a, b), indicative of hybrid exciton-photon states\nformed under strong coupling. These spectral splittings reflect the self-\nhybridization of excitons with cavity modes, and the measured dispersions are well\nreproduced by the coupled oscillator model. The extracted coupling strengths (82.5\nmeV and 93.2 meV) satisfy both established strong coupling criteria, confirming\nthe robustness of the exciton-photon interaction.\nReflectance intensity spectra further reveal two distinct absorption peaks on\neither side of the A-exciton resonance for the strongly coupled devices (151.2 nm\nand 77.9 nm), in excellent agreement with the predicted lower and upper polariton\nbranches (Fig. R3.9c). By contrast, weakly coupled devices (39.6 nm) exhibit only\na single excitonic feature (Fig. R3.9c).\nConsistently, the EQE spectra of zero-detuning devices display well-separated\nresponses from the LPB and UPB, with splitting that exceed their respective\nlinewidths (Fig. R3.9d). These EQE peaks correspond directly to the reflectance\nminima associated with the polariton branches (Fig. R3.9c). The responses below\nthe bare exciton energy of the strongly coupled devices can be observed. Such\nfeatures are absent in weakly coupled devices (Fig. R3.9c, d; device of 39.6 nm),\nproviding direct evidence that the observed photocurrent originates from exciton-\nphoton hybrid (E-P) states.\nReflectance\nMin. Max.\n2.4\nExp. Sim.\n2.2\n)V\ne\n(\ny 2.0\ng\nre\nn\nE\n1.8\n1.6\n-0.5 0 0.5\nSin ()\n2.4\n2.2\n2.0\n1.8\n1.6\n151.2nm\nUpper\npolariton\nLower\npolariton\n151.2 nm\nstrong coupling\ne c 77.9 nm\nn\na\ntc\ne lfe\nR\nstrong coupling\n39.6 nm\nweak coupling\n500 600 700 800\nWavelength (nm)\na Reflectance b\nMin. Max.\nExp. Sim.\n)V\ne\n(\ny\ng\nre\nn\nE\n-0.5 0 0.5\nSin ()\nc d\n151.2 nm\nu\n).\n.a strong coupling\n( E 77.9 nm\nQ\nE\nd e z\nila\nm strong coupling\nro\nN 39.6 nm\nweak coupling\n500 600 700 800\nWavelength (nm)\n2.4\n2.2\n2.0\n1.8\n2.4\n77.9 nm\n2.2\nUpper\npolariton\n2.0\n1.8\nLower\npolariton\n1.6\nFig. R3.9 | Angle-resolved reflectivity and polariton characterization of WS\n2\nexciton-polariton devices. a,b, Angle-resolved reflectivity spectra of WS exciton-\n2\npolariton devices with varying thicknesses, corresponding to zero detuning of the l = 1,\n2 cavity modes. The left (right) panels display experimental results (TMM calculations).\nRed dashed lines indicate the upper (UPB) and lower polariton branch (LPB)\ndispersions derived from the coupled oscillator model. The black dashed line and\nparabola denote the exciton energy and cavity mode dispersion, respectively. The\nreflectance spectrum extracted at sin(θ) = 0 reveals a clear splitting at the corresponding\nexciton resonance. c, Experimental reflectance spectra. d, Corresponding self-\nnormalized external quantum efficiency (EQE) spectra. Both devices (151.2 nm and\n77.9 nm thick) exhibit two EQE peaks flanking the exciton (ex ) energy, confirming\nA\nthe formation of well-defined polariton states in the strong coupling regime.\n3) Derivative analysis for feature visibility:\nTo strengthen this identification, we further analyzed the first derivatives of both\nreflectance and EQE spectra (Fig. R3.10). In strongly coupled devices (151.2 nm\nand 77.9 nm), two distinct derivative peaks appear on either side of the A-exciton\nresonance, precisely matching the coupled oscillator model prediction for LP and\nUP states. In contrast, weakly coupled devices (39.6 nm) display only a single\nexcitonic feature. The alignment of derivative peaks with reflectance minima and\nEQE maxima provides direct cross-modal evidence of polariton origin.\na b\n151.2nm 115511.2.2nm\ne\nv strong coupling strong coupling\nita\ne\nv\nir\ne\n77.9nm v\nita\n7777.9.9nm\nd v\ne ir\nc e\nn d\na E\ntc Q\ne strong coupling E strong coupling\nlfe\nR 39.6 nm 3399.6.6 nm\nweak coupling weak coupling\n500 600 700 800 500 600 700 800\nWWaavveelleennggtthh ( n(nmm) ) WWaavveelleennggthth (n (mn)m)\nFig. R3.10 | Derivative analysis of reflectance and EQE spectra in WS exciton-\n2\npolariton devices. a, First derivatives of the reflectance spectra. b, Corresponding first\nderivatives of the EQE spectra. Black arrows mark the inflection points, which align\nwith the reflectance minima and EQE maxima in Fig. R3.4c,d. Strongly coupled\ndevices (151.2 nm and 77.9 nm) exhibit dual derivative peaks flanking the A-exciton\nenergy, corresponding to upper (UP) and lower polariton (LP) states. These features are\nabsent in the weakly coupled device (39.6 nm), confirming their polaritonic origin.\nIn short, angle-resolved reflectance measurements on both strong- and weak-\ncoupling devices, together with coupled oscillator fitting and derivative analysis,\nunambiguously confirm that the observed EQE peaks arise from well-defined polariton\nstates, fully addressing the reviewer’s concern.\nRevised:\nIn the revised manuscript, Fig. R3.9 d have been incorporated into Figure 2d. The above\ndiscussions have been included in the revised manuscript (see Page 5 in the main text).\n5) In Fig. 2f, some error bars are excessively large, which are greater than the data\nvariations. This largely undermines the reliability of the data and makes it difficult to\ndraw convincing conclusions. The authors are strongly recommended to retake the data,\nand thus effectively reduce these error margins and provide more precise data to support\ntheir claims.\nResponse:\nWe appreciate the reviewer’s careful observation. Following the suggestion, we have\nretaken the measurements in Fig. 2f (as shown in Fig. R3.11) using improved\nexperimental stability and averaging over more repeated measurements to reduce\nrandom fluctuations. Correspondingly, we have also retaken the measurements of the\nEQE response spectra in Fig. S13 (now presented in Fig. R3.12). The updated data now\nexhibits reduced error bars, enhancing the reliability of the results and supporting the\nobserved trends more convincingly. The revised figure has been updated accordingly in\nthe manuscript.\nFig R3.11 | EQE spectra measured at 300 K and 80 K for devices in the weak\nand strong coupling regimes. The error bands represent the experimental value\nvariation (standard deviation) observed from multiple measurements (≥5) of multiple\ndevices (≥3).\nFig R3.12 | EQE spectra measured at 300 K and 80 K for devices with\ndifferent thicknesses in the weak and strong coupling regimes.\nRevised:\nIn the revised manuscript, Fig. R3.11 has been included in Figure 2f. Fig. R3.12 has\nbeen included in Supplementary Information (see Supplementary Fig. S13).\na\nc\n1\n0\n0\n0\n0\n0\n.0\n.8\n.6\n.4\n.2\n.0\n5 0 0\n).u.a(\nesnopser\ndezila)m.ur.oaN(\nesnopser\ndezilamroN\n).u.a(\nEQE\ndezilamroN\n1\n0\n0\n1\n0\n0\n. 0 1 .0\n. 5 0 .5\n. 0 0 .0\n5 0 . 0 1 .0\n. 5\n0 .5\n. 0\n0 .05\n0 5 0\n7 0\nw\n0\ns\n00\n0\ne a\ntro\nk c o u p lin g\n6 0 0\nW a v e\nn g c o u p lin g\n6 0 0 6 0 0\nW a v e W a\n8 0 0 9 0 0\n7 0 0\nle n g\n7 0 0 7 0 0\nle n g v e le n\nb\nd\nt h\nW\nt hg\nW\nS\nt h\n3 0 0 K\n8 0 K\nW S = 3 5 5 n ±\n2\n8 0 0\n3 0 0 K\n( n m )8 0 K\n= 1 3 0 5 n ±\n2\n8 0 0 8 0 0\n( n m ) ( n m )\nm\n9\nm\n9\n0\n09 0\n0\n00\n).u\n.a\n(\nE\nQ\nE\nd\ne\nz ila\nm\nro N\n).u.a(\nEQE\ndezilamroN\n1\n0\n0\n0\n0\n0\n.0\n.8\n.6\n.4\n.2\n.0\n5 0\nW\nS = 4 9 ±\n2\n6 0 0\nW a\nS = 1 0 8 2\nW a\nT = 8 0 K\nw e a k c o u p lin g\nT = 3 0 0 K\n5 n m\n7 0 0 8 0 0\nv e le n g th (n m )\nT = 8 0 K\ns tro n g c o u p lin g\nT = 3 0 0\n± 5 n m\nv e le n g th (n m )\n0 0\n).u.a(\nEQE\ndezilamroN\n1 .0\n0 .8\n0 .6\n0 .4\n0 .2\n0 .0 W\n5 0 0\n1 .0\n0 .8\n0 .6\n0 .4\n0 .2\n0 .0\n5 0 0\nS =\n2\n7 0 ± 5\n6 0 0\nW a v\n6 0 0\nT =\nn m\ne le\n8 0 K\ns tro n g c o u p lin g\nT = 3 0 0 K\n7 0 0 8 0 0\nn g th (n m )\ns tro n g c o u p lin g\n7 0 0 8 0 0\n9 0 0\n9 0 0\n).u\n.a\n(\nE\nQ\nE\nd\ne\nz ila\nm\nro N W S = 2 1\nW\n2 0\na\nv\nT = 8\n5 n m\ne le\nT\n= 3 0 0 K\nn g\nth (n m )\n6) To strengthen the argument for a polariton-based photodiode, the correlation\nbetween the enhanced performances and the polariton states should be unambiguously\nestablished. In Figs 3, 4b, and 4d, the performance improvement in the device seems\nmore likely to be due to cavity resonance rather than polariton formation. At the zero-\ndetuning, the peak device performance coincides with the exact cavity resonance (in\nFig. S9), indicating that cavity enhancement might be the optimized. This does not\nnecessarily lead to polariton effects. To establish the polariton enhancements,\ndemonstrating the optimized formation of polaritons and unambiguously identifying\nthe peaks of performance parameters at the polariton states would be critical.\nResponse:\nWe sincerely thank the reviewer for the insightful comment regarding the need to\nunambiguously establish the correlation between enhanced device performance and\npolariton states. We fully agree that distinguishing between cavity resonance effects and\npolariton formation is critical to strengthen the argument for a polariton photodiode.\nWhile cavity resonances can indeed enhance the local optical field, our data\ndemonstrate that strong exciton-photon coupling leads to polariton states whose\ninfluence extends beyond what is expected from bare cavity effects alone. Finally, we\nwould also like to point out that while evidence of polaritons in the optoelectronic\nresponses in these devices is clear, the true merit in terms of performance (response\nspeed) is still far from theoretical values due to imperfect materials and interfaces.\nOptimization of materials and related interfaces is beyond the scope of current work\nand will be part of future efforts.\nBelow we address the reviewer’s concerns with additional evidence supporting the\npolaritonic origin of the enhancements.\n1) Zero-detuning devices provide unambiguous evidence.\nIn the absence of strong coupling, cavity resonance enhances only a single optical\nmode without splitting, leading merely to a strengthened resonance peak (Fig.\nR3.13 c,d; device of 39.6 nm). By contrast, exciton-polariton formation results\nin the emergence of distinct lower and upper polariton branches (LP/UP)\nseparated by a Rabi splitting (Fig. R3.13 c,d; device of 77.9 nm and 151.2 nm).\nIn representative zero-detuning devices (e.g. WS thicknesses of 77.9 nm and\n2\n151.2 nm, corresponding approximately to the first- and second-order maxima ),\nthe LP/UP are well resolved (see Fig.R3.13a,b) and the reflectance minima\n(polariton modes, see Fig.R3.13c) show a one-to-one correspondence with EQE\nmaxima (see Fig.R3.4 d) , confirming that the photoresponse in these devices is\ndominated by exciton-polariton states. The extracted couplings (≈92 and 93.5\nmeV) satisfy the strong coupling criteria. Owing to exciton-polariton formation,\nstrongly coupled devices can harvest more photons in the spectral region near the\nbandgap. Compared with conventional cavity resonance, this approach avoids the\nnarrowband absorption inherent to cavity enhancement and the limitation of high\nabsorption efficiency only at specific wavelengths. As a result, both the cut-off\nwavelength is extended, and near-unity internal quantum efficiency is achieved\nfor on-resonance devices (see Fig.R3.13d). The observed performance\nenhancement in these devices cannot be explained by a single uncoupled cavity\nmode.\nReflectance\nMin. Max.\n2.4\nExp. Sim.\n2.2\n)V\ne\n(\ny 2.0\ng\nre\nn\nE\n1.8\n1.6\n-0.5 0 0.5\nSin ()\nFig. R3.13 | Angle-resolved reflectivity and polariton characterization of WS\n2\nexciton-polariton devices. a,b, Angle-resolved reflectivity spectra of WS exciton-\n2\npolariton devices with varying thicknesses, corresponding to near-zero detuning of\nthe l = 1, 2 cavity modes. The left (right) panels display experimental results (TMM\ncalculations). Red dashed lines indicate the upper (UPB) and lower polariton branch\n(LPB) dispersions derived from the coupled oscillator model. The black dashed line and\nparabola denote the exciton energy and cavity mode dispersion, respectively. The\nreflectance spectrum extracted at sin(θ) = 0 reveals a clear splitting at the corresponding\nexciton resonance. c, Experimental reflectance spectra. d, Corresponding self-\nnormalized external quantum efficiency (EQE) spectra. Both devices (151.2 nm and\n77.9 nm thick) exhibit two EQE peaks flanking the exciton (ex ) energy, confirming\nA\nthe formation of well-defined polariton states in the strong coupling regime.\n2.4\n2.2\n2.0\n1.8\n1.6\n151.2nm\nUpper\npolariton\nLower\npolariton\na Reflectance b\nMin. Max.\nExp. Sim.\n)V\ne\n(\ny\ng\nre\nn\nE\n-0.5 0 0.5\nSin ()\nc d\n151.2 nm\nstrong coupling\ne c 77.9 nm\nn\na\ntc\ne lfe\nR\nstrong coupling\n39.6 nm\nweak coupling\n500 600 700 800\nWavelength (nm)\n2.4\n2.2\n2.0\n1.8\n2.4\n77.9 nm\n2.2\nUpper\npolariton\n2.0\n1.8\nLower\npolariton\n1.6\n151.2 nm\nu\n).\n.a strong coupling\n( E 77.9 nm\nQ\nE\nd e z\nila\nm strong coupling\nro\nN 39.6 nm\nweak coupling\n500 600 700 800\nWavelength (nm)\n2) Device performance peaks follow polariton energies under detuning.\nIf the enhancement were due solely to cavity resonance, the performance\nmaximum would remain fixed at the bare cavity energy regardless of detuning.\nHowever, we find that the positions of EQE shift in accordance with the polariton\ndispersion when the cavity detuning is varied (see Fig.R3.14). For zero-detuning\ndevices the LP and UP are maximally split and the EQE performance maxima\ncorrespond to the polariton energies rather than the bare cavity resonance.\n200\n150\n100\nExperimental\nEQE maxima\n0\n550 600 650 700 750\nFig. R3.14 | Dots are extracted from experimental EQE spectra shown in (Fig.S10),\nwhile lines are the calculated dispersions of the polariton eigenstates. The EQE\nexperimental data agree well with the calculated polariton branches of the coupled\noscillator model.\n3) Direct mapping of polariton dispersion.\nWe have performed angle-resolved reflectance mapping and fitted the results using a\ncoupled-oscillator model. From these dispersions we extracted LP/UP energies and\ncoupling strengths (see Fig. R3.15,and Table R3.3), and verified that the devices in\nwhich the performance peaks occur satisfy the rigorous strong-coupling criteria (both\n(𝑙) 2\n𝑔2 >\n(𝛾\n𝑐\n−𝛾𝑋)\nand 𝑔2 > (𝛾2 +𝛾2)/2). Thus, in the samples considered, the optical\n𝑙 𝐶 𝑋\n4\nmodes are genuinely hybridized exciton-photon states rather than bare cavity modes.\n)mn(\nssenkcihT\nSW\n2 strong coupling\nweak coupling\nWWavaevleelennggtthh ((nnmm))\na Reflectance b Reflectance c Reflectance d Reflectance\nMin. Max. Min. Max. Min. Max. Min. Max.\n2.4 2.4 2.4 2.4\nExp. Sim. Exp. Sim. Exp. Sim. Exp. Sim.\n2.2 2.2 2.2 2.2\nUP UP UP\n)V )V )V )V\ne ( y g re\nn E\n2.0 e ( y g re\nn E\n2.0 e ( y g re\nn E\n2.0 e ( y g re\nn E\n2.0\n1.8 1.8 LP 1.8 1.8 LP\nLP\n49 nm 77.9 nm 89 nm 151.2 nm\n1.6 1.6 1.6 1.6\n-0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5\nSin () Sin () Sin () Sin ()\ne f g h\nReflectance Reflectance Reflectance Reflectance\nMin. Max. Min. Max. Min. Max. Min. Max.\n2.4 2.4 2.4 2.4\nExp. Sim. Exp. Sim. Exp. Sim. Exp. Sim.\n2.2 2.2 2.2 2.2\nUP UP\n)V )V )V\nUP\n)V UP\ne e e e\n(\ny g 2.0\n(\ny g 2.0\n(\ny g 2.0\n(\ny g 2.0\nre re re re\nn n n n\nE E E E\nLP LP\n1.8 1.8 1.8 1.8\nLP LP\n169.7 nm 171.4 nm 196 nm 221 nm\n1.6 1.6 1.6 1.6\n-0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5\nSin () Sin () Sin () Sin ()\nFig. R3.15 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\nTable R3.3 | Coupled Oscillator Model Fitting Results for Devices with Varying\nThicknesses.\n𝛾 𝛾\nThickness 𝐶 𝑋 Rabi splitting\n𝑔 (meV) (𝛾 −𝛾 )/2 √(𝛾 2+𝛾 2)/2\n𝐶 𝑋 𝐶 𝑋\n(nm) (meV)\n(meV) (meV)\n77.9 82.5 89.6 24 151.4 32.8 65.5\n89.0 92.7 132.9 24 150.2 54.5 95.5\n151.2 93.2 76.87 24.1 178.8 26.3 56.9\n169.7 88.3 87.2 24.3 165.0 31.45 55.75\n171.4 84.94 70.5 24.75 163.60 22.875 52.83\n196.0 101.57 61.8 24.2 199.6 18.8 46.9\n196.0 98.1 40.3 29 195.9 18.8 46.9\n221.9 93.9 55 29.8 186.1 12.6 44.2\n4) Persistence of strong coupling across temperature.\nTo capture the full thermal evolution of these coupled states, we carried out\ncontinuous temperature-dependent reflectance spectroscopy from 87 K to 300 K\non WS cavities with thicknesses ranging from ∼35 nm to ∼130 nm (see Fig.\ndisplay smooth, monotonic redshift with increasing temperature. Importantly, the\npolariton branches remain spectrally resolved over the entire temperature range\nwithout collapsing into a single peak, confirming that the system persistently\nresides in the strong-coupling regime. The temperature-induced evolution of the\nspectral splitting is accompanied by consistent trends in the EQE spectra\n(Fig.R3.16 c, Fig.R3.17) with devices near zero detuning exhibiting distinct peak\nsplitting even at elevated temperatures. In both strong- and weak-coupling\ndevices, the EQE spectra exhibits a pronounced steepening of the exciton\nabsorption edges, a blue shift of the absorption maxima, and an enhancement of\nthe EQE at low temperatures under both resonant and non-resonant excitation,\nreflecting strengthened excitonic and polaritonic effects. Correspondingly, the\nabsorption peaks dominated by the upper and lower polariton branches (UP and\nLP) are also clearly resolved in the EQE spectra, as indicated by the black arrows\nin Fig. R3.16c and Fig. R3.17. These observations substantiate the assignment of\nthe observed features to robust exciton-photon hybrid states (polaritons), rather\nthan to uncoupled optical modes or interference fringes. The observed\ntemperature dependence is consistent across multiple devices with different WS\n2\nthicknesses, demonstrating the robustness of the polaritonic features and the\ngenerality of the self-hybridized cavity design.\nFig R3.16 | a,b, Temperature-dependent reflectance spectra of devices in weak\nand strong coupling regimes. The polaritonic branches remain clearly resolved\nacross the entire temperature range without merging into a single resonance,\nconfirming the persistence of strong coupling. c, Corresponding relative external\nquantum efficiency (EQE) response spectra for weakly and strongly coupled\ndevices at 300 K and 80 K. The error bands represent the experimental value\nvariation (standard deviation) observed from multiple measurements (≥5) of\nmultiple devices (≥3).\necnatcelfeR\na\necnatcelfeR 2\n0\n5\nw\n0 0\ne a k co\nW\nu p lin g\n6 0 0\na v eW lea nv ge\n7\nth le\nW\n0 0\n( n n g\nS\nmth\n= 35 ± 2\n8 0\n) 300 K 275 K\n5 nm\n0\nb\necneacntcaetlcfeelRfeR 2\n0\n5\nstro\n0 0\nn g co u p lin g\nW S = 130 ± 2\n6 0 0 7 0 0 8 0\nW a v e le n g th (n m ) W a v e le n g th (n m )\n5 nm\n0\n).u.a(\nesnopser\ndezila)m.ur.oaN(\nesnopser\ndezilamroN\nc\n).u.a(\nEQE\ndezilamroN\n1 .01.0\n0 .50.5\n0 .0 0.0\n5 0 1 .0 1.0\n0 .5 0.5\n0 .0 0.05\n05 0\nw e a k co u p lin g\n0 6 0 0 W a v e\nstro n g co u p lin g\n0 6 0 0 0 6 0 0\nW a v e W a\n7 0 0 le n g\n7 0 0 7 0 0\nle n g v e le n\nW S = 35 5 nm ± 2\n8 0 0 9 0 0 300 K th (n m )80 K\nW S = 130 5 nm ± 2\n8 0 0 9 0 0 8 0 0 9 0 0\nth (n m ) g th (n m )\nFig R3.17 | Temperature-dependent relative EQE response spectra and\nreflectance spectra of devices in weak and strong coupling regimes.\nThe additional experiments and analyses substantiate that the observed device\nenhancements stem from polariton states rather than cavity resonances, thereby\nreinforcing the validity of our conclusions and comprehensively addressing the\nreviewer’s concerns.\nRevised:\nIn the revised manuscript, Fig. R3.13d have been incorporated into Figure 2, and the\ncorresponding discussions are included on page 5 of the main text. We have also\nperformed angle-resolved reflectance spectroscopy measurements, with the\ncorresponding dispersion data and coupled oscillator model fits provided in the\nSupplementary Information (Supplementary Fig. S6 and Table S1) to more accurately\ncapture the thickness-dependent variation in coupling strength. In addition, Figure\nR3.16 has been updated as Fig. 1e-f in the main text, and Fig. R3.17 has been updated\nas Supplementary Fig. S13.\n7) The comparisons with reported device performance seem to be suspicious. In their\ncomparisons, the on/off ratio, dark current density, especially response time seems to\nbe much larger than reported TMD-based photodiodes/photodetectors. However, there\nare some mistakes in SI that comparisons of device parameters with previously reported\ndevices show several discrepancies. For instance, in SI Reference 20 [Adv. Electron.\nMater. 7, 2000964 (2021)], the responsivity is given as 777 mA/W. But the manuscript\nlists it as 360 mA/W (SI Table S2). As another example, the responsivity in SI Reference\na\nc\n).u.a( EQE dezilamroN\n).u.a( EQE dezilamroN\n1.0 0.8 0.6 0.4\n0.2\n0.0 W\n500\n1.0 0.8 0.6 0.4 0.2\n0.0 W\nT=300 K\nS = 49 ±5 nm 2\n600 700 800 900\nW a v e le n g th (n m )\nT=80 K strong coupling T=300 K\nS = 108 ±5 nm 2\n600 700 800 900\nW a v e le n g th (n m )\necnatcelfeR\necnatcelfeR\nw eak coupling\n500 600\nW a v e le\nstrong coupling\n500 600\nW a v e le\nW S = 49 5 nm ± 2\n700 800\nn g th (n m )\nW S = 109 5 nm ± 2\n700 800\nn g th (n m )\nb\nd\n).u.a( EQE dezilamroN\n).u.a( EQE dezilamroN\n1.0 0.8 0.6 0.4\n0.2\n0.0 W\n500\n1.0 0.8 0.6 0.4 0.2\n0.0 W\nT=300 K\nS = 70 ±5 nm 2\n600 700 800\nW a v e le n g th (n m )\nT=80 K strong coupling T=300 K\nS = 120 ±5 nm 2\n600 700 800\nW a v e le n g th (n m )\n900\n900\necnatcelfeR\necnatcelfeR\nW S = 70 5 nm ± 2\n500 600 700 800\nW a v e le n g th (n m )\nW S = 120 5 nm ± 2\n500 600 700 800\nW a v e le n g th (n m )\n8 [Appl. Surf. Sci. 481, 1127-1132 (2019)] is reported to be about 5 orders of magnitude\nhigher than this work. But the manuscript totally ignored it in the comparisons of\nresponsivity (SI Table S2). The similar inconsistencies must be addressed and ensure\nall comparative data are correct and well-cited without such suspicious discrepancies.\nResponse:\nWe sincerely thank the reviewer for the careful reading and pointing out the\ninconsistencies in the supplementary comparisons. We have carefully reviewed and\ncross-checked all cited references and comparative data in SI Table S2. The following\nmodifications have been made:\na) Correction of Ref. 20 [Adv. Electron. Mater. 7, 2000964 (2021)]:\nThe responsivity value for this reference was mistakenly listed as 360 mA/W in\nSI Table S2 (corrected in the revised SI Table S3). We sincerely apologize for this\nmistake. Notably, in the previous version of the manuscript, the correct value of\n777 mA/W was already cited in Fig. 4f of the main text (shown as the cyan circle\nin the lower right corner). We have now corrected the supplementary table to\nensure full consistency.\nb) Discussion of Ref. 8 [Appl. Surf. Sci. 481, 1127-1132 (2019)]:\nThe high responsivity reported in this work (>1000 A/W) arises mainly from\nAu-based localized surface plasmon resonance (LSPR) enhancement combined\nwith photoconductive gain under high external bias, rather than from intrinsic\nexcitonic processes of the active material. Since our devices, and those included in\nSI Table S2 (corrected in the revised SI Table S3), focus on exciton-driven\nphotocurrent mechanisms, direct comparison with plasmonic-enhanced\nresponsivity values and photoconductive gain under high external bias would not\nprovide a fair or physically meaningful benchmark of intrinsic device performance.\nTherefore, to maintain a consistent and relevant comparison framework, this\nreference has been excluded from the revised supplementary tables (SI Table S2\nand S3), with the rationale explicitly noted.\nc) Verification of all other references:\nWe have carefully re-checked all remaining references and ensured that each\nparameter is now accurately quoted and consistently cited without discrepancy.\nThese revisions are now clearly marked in the updated supplementary\ninformation (Table S3). We apologize for the oversight and have taken extra care\nto ensure that all comparisons are now consistent, fair, and transparent. These\ncorrections do not affect the main conclusions of our work but reinforce the rigor\nand reliability of our analysis.\nRevised:\nIn the revised manuscript, the responsivity (R) value for Ref. 20 [Adv. Electron.\nMater. 7, 2000964 (2021)] in SI Table S3 has been corrected to 777 mA W⁻¹.\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse:\nWe sincerely thank you for the time and effort you dedicated to reviewing our\nmanuscript, as well as for your constructive comments. We have carefully addressed all\nthe issues raised by you and the co-reviewer in our detailed responses and revised\nmanuscript. Regarding your concerns about the presentation of novelty and the\nevidence for strong coupling, we have added additional experimental data and\ntheoretical analyses (see our detailed replies to Reviewer #3’s comments). We greatly\nappreciate your valuable suggestions, which have helped us further enhance the clarity\nand rigor of our work.\nResponse to Reviewer 1\nReviewer #1 (Comments to the Author):\nThe authors have addressed all of the concerns raised, and I believe the paper is now\nsuitable for acceptance in its current form.\nResponse:\nWe thank the reviewer for this positive and final assessment of our manuscript. We are\npleased that our previous responses and revisions were found to be satisfactory. We\ngreatly appreciate your professional review of our manuscript.\nReviewer #2 (Comments to the Author):\nThe authors have adequately addressed my previous concerns regarding the size of the\nvacuum Rabi splitting, the Q value of the cavity mode, and the clarity of the observed\nanti-crossing. They have also provided a detailed discussion on the influence of trap\nstates on device performance. Overall, I find that the revised manuscript meets the\nstandard for publication.\nI do have one remaining question regarding the temperature-dependent polariton\nbranches. The splitting appears to persist up to room temperature. Could the authors\nclarify the physical origin of this robustness?\nResponse:\nWe sincerely appreciate your review of our response letter and revised manuscript, as\nwell as your positive evaluation of our work. We are delighted to learn that the revisions\nand supplementary discussions addressing all your earlier concerns, including the\nmagnitude of the vacuum Rabi splitting, the Q factor of the cavity mode, the clarity of\nthe anti-crossing behavior, and the influence of trap states on device performance, have\nbeen met with your approval. Your insightful comments have been invaluable in\nenhancing the overall quality of our manuscript. We are also very pleased that you\nconsider the current version of our manuscript to meet the standard for publication.\nRegarding your question on the temperature-dependent polariton branches and the\npersistence of the splitting up to room temperature, the robustness originates primarily\nfrom the large exciton binding energy in our material system. Two-dimensional (2D)\ntransition metal dichalcogenides (TMDCs) such as WS exhibit reduced dielectric\n2\nscreening and strong quantum confinement effects, leading to exceptionally large\nexciton binding energies (>300 meV) in monolayer materials, while the interlayer van\nder Waals interactions also ensure binding energies greater than the thermal energy at\nroom temperature (~25 meV) even in bulk or multilayer forms. [ref: Phys. Rev. Lett.,\n2014, 113(7), 76802；Nat. Commun., 2016, 7, 10643.] This high binding energy\nprevents thermal dissociation of excitons, maintaining coupling with the cavity photons\neven as temperature increases. Additionally, the vacuum Rabi splitting (Ω ≈ 180 meV)\nis much larger than both the thermal broadening and the homogeneous linewidths of\nthe excitons and cavity mode, ensuring that the anti-crossing remains resolvable.\nTherefore, the large exciton binding energy, strong exciton-photon coupling, and\nsubstantial Rabi splitting collectively ensure that the polariton branches remain clearly\nresolved and the splitting persists up to room temperature.\nReviewer #3 (Comments to the Author):\nWe appreciate the authors’ efforts to address our previous concerns in the last report.\nThe manuscript has been indeed improved from the earlier version. The authors have\nseriously looked into the polaritonic effects and carefully examined the criterion of the\nstrong coupling regime. More importantly, the authors have paid substantial attentions\nto the correlations between the polaritonic resonances and maxima of the device\nperformance measures. These points are critical to check the technical correctness of\ntheir claim. However, we still have several concerns for these technical points as well\nas the novelty of this work before it can be considered for publication. Here are our\ndetailed comments as below.\nResponse:\nWe sincerely appreciate the reviewer’s thoughtful and constructive feedback. We are\npleased to know that the improvements in the revised manuscript have been recognized,\nespecially regarding the identification of polaritonic effects, the verification of the\nstrong coupling regime, and the correlation between the polariton resonances and the\ndevice performance. We would like to once again express our gratitude for these\ninsightful comments and suggestions, which have not only helped us produce a\nsubstantially improved manuscript but also motivated us to conduct a deeper\ninvestigation into the underlying physics.\nWe have carefully addressed all remaining concerns regarding both the technical\ncorrectness and the novelty of our work. In this revised version, we have further\nclarified the physical mechanisms, added new supporting analyses, and strengthened\nthe presentation of experimental evidence to ensure that our claims are technically\nsound and well justified. Detailed point-by-point responses are provided below.\n1) In comparison with previous similar work [e.g., ACS Photonics 5, 2921-2927 (2018)],\nthe authors claimed the novelty lies on the self-hybridized polaritons for the\nphotodiodes, broadband absorption, polariton-enabled transport and improved device\nperformance. The concept of self-hybridized exciton polaritons is not new, and does not\nseem to play a significant role in the device performance (actually it might introduce\nsome intrinsic limits for the device performance), which has also been pointed out by\nthe other reviewer. However, the polariton-enabled transport seems to be a quite\ninteresting point, which is actually an important topic for the polaritonics [e.g., Nat\nMater 2015, 14, 1123-1129]. Moreover, the polariton-mediated Purcell effect could be\nalso some interesting point for boosting optoelectronic device performance, which\nmight be related with this work [e.g., Nature 2024, 626, 300-306]. The authors are\nsuggested to find some more significant points to support the claimed novelty.\nResponse:\nWe thank the reviewer for their constructive feedback and for highlighting the\ninteresting aspects of polariton-enabled transport in our work. We appreciate the\nsuggestion to further emphasize the novelty by identifying additional significant points.\nBelow, we address the reviewer’s concerns point by point, clarifying how our study\nadvances beyond prior work while building on established concepts in polaritonics.\nFirst, regarding the comparison to previous similar work (e.g., ACS Photonics 5,\n2921-2927 (2018), hereafter Ref. 31 in our manuscript), we agree that strong light-\nmatter coupling has been explored in organic photodiodes to extend spectral\nresponsivity. However, that study relied on external optical cavities with partially\ntransparent metal reflectors, which introduced energy losses and limited device\nperformance. In contrast, our work demonstrates the integration of self-hybridized\nexciton-polaritons into an inorganic excitonic photodiode (using WS ) without\n2\nexternal cavities, leveraging the material’s intrinsic high refractive index for self-\ntrapping and hybridization of light. This eliminates the need for lossy reflectors,\nenabling broadband absorption across 500-800 nm while achieving near-unity\ninternal quantum efficiency (IQE > 90%) near zero detuning, performance metrics\nthat outperform those in Ref. 31(EQE <15%; IQE < 25%) and related organic systems.\nWe have revised the introduction and discussion sections to more explicitly contrast\nthese architectural differences, emphasizing how our open-cavity approach (inspired by\nrecent advances, e.g., Refs. 34-35) avoids the trade-offs in prior designs.\nWhile the concept of self-hybridized exciton-polaritons is indeed not new (as\ndemonstrated before in multilayer TMDCs), its application here plays an important role\nin device performance. In our devices, self-hybridization enables strong photon-\nexciton coupling directly within the WS absorber, resulting in observable anti-\n2\ncrossing in thickness-dependent reflectance and external quantum efficiency (EQE)\nspectra, with Rabi splittings of 152-173 meV. This directly contributes to enhanced\nsub-bandgap absorption via the lower polariton branch (LPB), extending the cut-off\nwavelength (Supplementary Fig. S11) without compromising broadband response.\nCritically, it facilitates polariton-enhanced transport, leading to oscillatory IQE\nbehavior with thickness and peaks at zero-detuning points (≈75 nm and 145 nm for\nl=1,2 modes), approaching unity IQE, suggesting improved exciton diffusion lengths\nthat mitigate recombination losses in conventional excitonic photodiodes. The self-\nhybridized state appears to alleviate the narrow-band absorption and short diffusion\nlength typically associated with excitonic semiconductors such as WS . To address the\n2\nreviewer’s point, we have added a discussion in the main text (Device structure and\nworking principle section) on potential limitations, such as imperfect interfaces\naffecting response speed (e.g., interfacial carrier trapping, Supplementary Fig. S20),\nand how material optimizations could further elevate performance toward theoretical\npolaritonic limits.\nWe fully concur that polariton-enabled transport is a key novelty and an important\ntopic in polaritonics, as noted in the reference (Nat. Mater. 2015, 14, 1123–1129; now\ncited as Ref. 47 in the revised manuscript). Our study provides direct experimental\nevidence of this in a functional photodiode, where strong coupling enhances exciton\ntransport via photonic modes, yielding a minimum response time of 217 ns—faster than\nreportedroom-temperature excitonic photodetectors (Supplementary Table 3). This is\nsupported by our phenomenological transport model (Supplementary Fig. S14), which\naligns with experimental IQE trends and attributes oscillations to resonance energy\ntransfer weighted by the polariton’s excitonic fraction. Unlike prior demonstrations\nfocused on fundamental transport (e.g., long-range propagation in monolayers, Refs.\n23-24), our work integrates this into a device architecture, demonstrating a 20-fold EQE\nincrease entering the strong-coupling regime and a -3 dB cutoff frequency of 1.21 MHz.\nAdditionally, we appreciate the suggestion regarding the polariton-mediated Purcell\neffect (e.g., Nature 2024, 626, 300-306; now cited as a new reference 54 in the\ndiscussion), which can facilitate more efficient charge separation and suppressed non-\nradiative recombination. While our primary focus is on transport enhancement,\npolaritonic effects may indeed contribute to modified radiative rates, potentially\nboosting charge separation at interfaces. Our low-temperature EQE data (Fig. 2e-f)\nshow preserved polaritonic features and enhanced efficiency, consistent with\nsuppressed non-radiative recombination, aligning with Purcell-like modifications.\nWe have added a brief discussion in the EQE section to explore this connection, noting a\ndirection for future studies to quantitatively evaluate this effect.\nWe hope these revisions incorporate the reviewer’s suggested references and enhance\nthe manuscript’s clarity on novelty. We believe they address the concerns and strengthen\nthe paper’s impact.\nRevised:\nIn the revised manuscript, we have expanded the Introduction and Discussion\nsections to better clarify the novelty of our cavity-free, self-hybridized polariton\nphotodiode compared with previously reported cavity-based systems (see pages 2 and\n8 of the main text). Specifically, we emphasize that the transparent top electrode\nsimultaneously serves as an antireflective layer, enabling strong light–matter\ncoupling in an open-cavity architecture with broadband absorption and polariton-\nenhanced transport.\nIn the Device structure and working principle section (page 3), we have incorporated\na discussion on extrinsic performance limitations—including interfacial trap states,\nnonideal contact geometry, and optical confinement issues—to clarify that the current\nresponse speed is constrained by technological, rather than fundamental, factors.\nIn the External and internal quantum efficiency analysis (pages 6-7), we further\ndiscuss how self-hybridized polariton states concentrate optical fields within the WS₂\nlayer, providing an inherent advantage of the open-cavity configuration over externally\nmirrored systems. Additionally, we have introduced a brief note on the polariton-\nmediated Purcell-like effect (Ref. 54), highlighting its potential role in modifying\nradiative rates and suggesting it as a promising avenue for future quantitative\ninvestigation.\nRelevant references have been updated accordingly.\n2) For the tests of the strong coupling regime, there are still some points that should be\ncarefully examined. First, the thickness-dependent coupling strength seems to be quite\ncomplicated. For the same cavity conditions, the coupling strength scales with the\nsquare-root of the thickness. For example, the thicknesses between 150-180 nm seem\nto host the similar l=2 cavity mode (Fig. R3.5a). In Table R3.2, the coupling strength\nfor the thicknesses between 150 nm and 180 nm does not really follow this scaling law.\nThis causes some confusion. Second, how did the authors determine the cavity\nlinewidths? The linewidths are so broad and even broader than the exciton linewidths.\nFor higher-order cavity modes, the linewidths generally should become narrower. The\nfitted cavity linewidths do not really follow this trend. Third, how did the authors\ndetermine the exciton linewidths. As the authors mentioned in the response, the\noscillator strength of the excitons should become constant for the bulk thickness (the\nreference of Nature Photon 2015 is irrelevant with their argument there). For this\nscenario, the exciton linewidths should be also taken as a constant and independent of\nthe bulk thickness. In Table R3.2, this does not seem to be the case.\nResponse:\nWe appreciate the reviewer’s thoughtful comments on the strong coupling analysis.\nBelow we clarify the physical reasoning and methodological details regarding the\ncoupling strength, linewidth determination, and their thickness dependence.\n(1) Thickness dependence of coupling strength.\nIn principle, when the optical mode volume 𝑉 remains approximately constant,\nan increase in the number of excitons 𝑁 enhances the overall exciton-photon\ninteraction strength, following the relation\n𝑁\n𝑔 ∝ √\n𝑉\nHowever, in our self-hybridized system, the number of emitters (𝑵) exhibits an\napproximately linear increase with thickness, while the optical mode volume (𝑽)\nalso expands with increasing thickness. As a result, the overall coupling strength\n(𝑔) varies only slightly over a wide thickness range, which is consistent with our\nexperimental observations. Therefore, the observed Rabi splitting in our structures\nreflects a balance between the increased exciton density and the enlarged mode\nvolume. This interplay between 𝑁 and 𝑉 has been well established in the literature\non self-hybridized and collective light-matter coupling systems [Nat. Mater. 22,\n970–976 (2023); Phys. Rev. B 107, 075415 (2023); Nat. Commun. 13, 5597 (2022)].\n(2) Determination of cavity linewidths.\nThe cavity linewidths are extracted by fitting the reflectance of each cavity\nmode at large detuning, where exciton-photon interaction is minimal. These\nlinewidths serve as the initial inputs for the thickness-dependent fits and are allowed\nto vary slightly to achieve the best fit. The relatively broad linewidths arise from\nthe intrinsic optical dissipation and imperfect optical confinement in the WS layers,\n2\nwhich is typical for self-hybridized cavity systems and still sufficient to sustain\npolaritonic states. Moreover, the measured vacuum Rabi splitting exceeds the\ncombined linewidths of the exciton and cavity modes, satisfying the strong-\ncoupling criterion. Future improvements in interface quality and in-plane\nconfinement are expected to further enhance the cavity Q-factor.\n(3) Determination of exciton linewidths.\nThe exciton linewidths are obtained by fitting a Lorentz oscillator to the\nimaginary part of the dielectric function around the A-exciton resonance. The\nfitting is performed over a narrow spectral range on the high-energy side to avoid\nthe influence of overlapping absorption tails. The extracted linewidths show only\nsmall fluctuations across different thicknesses, mainly due to experimental and\nfitting uncertainties, while the intrinsic exciton linewidth remains essentially\nconstant, consistent with the expected bulk-like behavior.\n(4) Clarifications in angle-dependent fitting.\nFollowing the reviewer’s suggestion, we have refined the angle-dependent fitting\nprocedure (Fig.R3.1 and Table R3.1): (i) the exciton linewidths are now fixed to\nthe constant values extracted from the dielectric function, and (ii) the cavity\nlinewidths are fixed to those obtained from thickness-dependent fitting. Only the\ncoupling strength and parameters related to cavity dispersion are allowed to vary.\nThis ensures consistency and reduces the number of fitting parameters.\n(5) Regarding the reference Nature Photon. 9, 466-470 (2015):\nWe thank the reviewer for pointing this out. The reference to Nature Photonics 9,\n466-470 (2015) was not intended to directly support the thickness dependence of\nthe oscillator strength, but rather to provide quantitative information on the exciton\nBohr radius (~1 nm) in WS . This small Bohr radius indicates that the exciton\n2\nwavefunction is highly localized within a single layer, implying that the oscillator\nstrength becomes nearly independent of thickness once the film exceeds a few\nlayers. We apologize for the confusion regarding the relevance of this reference.\nFig. R3.1 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\nTable R3.1 | Coupled Oscillator Model Fitting Results for Devices with Varying\nThicknesses.\n𝛾 𝛾\nThickness 𝐶 𝑋 Rabi splitting\n𝑔 (meV) (𝛾 −𝛾 )/2 √(𝛾 2+𝛾 2)/2\n𝐶 𝑋 𝐶 𝑋\n(nm) (meV)\n(meV) (meV)\n77.9 86.5 106.3 26.5 153.5 66.4 77.3\n89.0 87.5 106.3 26.5 155.7 66.4 77.3\na\n)Ve(\nygrenE\n)mn(\nhtgnelevaW\nR\nM in .\n2.4\nE xp .\n2.2\n2.0\n1.8\n1.6\n-0.5\ne\nR\nM in .\nE xp .\n550\n650\n750\n-0.5\ne fle cta n ce\nM\nS im .\n4 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 6 9 .7 n m\n0 0.5\nS in ( ) \na x.\na x.\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nb\nM\n550\n600\n650\n700\n750\nf\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n7 7 .9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 7 1 .4 n m\n0 0.5\nS in ( ) \na x.\na x.\nc\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nM\n550\n600\n650\n700\n750\ng\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n8 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 9 6 n m\n0 0.5\nS in ( ) \na x.\na x.\nd\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n1 5 1 .2 n m\n-0.5 0 0.5\nS in ( ) \nh\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n2 2 1 n m\n-0.5 0 0.5\nS in ( ) \n151.2 91.7 70.2 26.5 178.1 48.3 53.1\n169.7 89.1 70.2 26.5 172.8 48.3 53.1\n171.4 89.3 70.2 26.5 173.2 48.3 53.1\n196.0 96.5 51.1 26.5 191.4 38.8 40.7\n221.9 93.9 51.1 26.5 186.2 38.8 40.7\nRevised:\nIn the revised manuscript, we have carried out a revised dispersion data fitting\nanalysis. The Supplementary Information has been updated accordingly with the new\nFigure S6 and Table S1.\n3) In Fig. R3.8, some angle-resolved polariton branches of the experimental data are\nhard to be resolved for some thicknesses. For example, the upper branch for the\nthicknesses of 196 nm and 221 nm, and the lower branch for the thickness of 196 nm\nneed to be resolved with better resolutions.\nResponse:\nWe appreciate the reviewer’s careful observation. We have re-examined the angle-\nresolved reflectance spectra for the 196 nm and 221 nm WS devices and found that the\n2\nupper and lower polariton branches are indeed present but were previously less\ndistinguishable due to the limited color scale range. As shown in Fig. R3.2, by\noptimizing the colorbar contrast and dynamic range, both branches now appear clearly\nresolved and exhibit excellent agreement with the fitted coupled-oscillator model. The\ncorresponding normal-incidence (sin θ = 0) spectra also shows pronounced mode\nsplitting at the exciton resonance. In addition, we have applied the same adjustment to\nthe angle-resolved reflectance spectra of other device thicknesses (Fig. R3.3), which\nsimilarly improves the visibility of both upper and lower polariton branches. These\nrevisions enhance the clarity of the features without any data manipulation, confirming\nthat the observed branches are genuine polaritonic modes.\nFig. R3.2 | Angle-resolved reflectivity spectra of WS exciton-polariton devices\n2\nwith thicknesses of 196 nm and 221 nm. The reflectance spectrum extracted at sin(θ)\n= 0 reveals a clear splitting at the corresponding exciton resonance.\nFig. R3.3 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses.\na\n)mn(\nhtgnelevaW\n5\n6\n6\n7\n7\n5\n0\n5\n0\n0\n0\n0\n0\nin .\nE x p .\nU P\nL P\n-0 .5\ne fle c ta n\nc e\nS\n1 9\n)\nM\nim .550\n600\n650\n700\n750\n6 n m\n0 .5\na x .\np\nUo p p e r\nla rito n\nL o w e r\np o la rito n\nb\n)mn(\nhtgnelevaW\n5\n6\n6\n7\n7\n5\n0\n5\n0\n0\n0\n0\n0\nin .\nE x p .\nU P\nL P\n-0 .5\ne fle c ta n c e\nS\n2 2\nM\nim .\n1 n m\n0 .5\na x .\n5\n5\n0\n6\n0\n0\n6\n5\n0\n7\n0\n0\n7\n5\n0\np\nUo pla p e r\nrito n\nL o w e r\np o la rito n\na Reflectance b Reflectance c Reflectance d Reflectance\nMin. Max. Min. Max. Min. Max. Min. Max.\n2.4\nExp. Sim. Exp. Sim. Exp. Sim. Exp. Sim.\n550 550 550\n2.2 UP\nUP UP\n)m\nn600\n)m\nn\n600\n)m\nn600\n)V\ne\n(\ny g re\nn E\n2.0\n( h\ntg\nn\ne le v\na W\n650\n( h\ntg\nn\ne le v\na W\n650\n( h\ntg\nn\ne le v\na W\nLP LP\n700 700 700\n1.8\n750 750 750\n49 nm 77.9 nm 89 nm 151.2 nm\n1.6\n-0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5\nSin () Sin () Sin () Sin ()\ne f g h\nReflectance Reflectance Reflectance Reflectance\nMin. Max. Min. Max. Min. Max. Min. Max.\nExp. Sim. Exp. Sim. Exp. Sim. Exp. Sim.\n550 550 550 550\nUP\nUP\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n700 700 700 700\nLP LP\n750 750 750 750\n169.7 nm 171.4 nm 196 nm 221 nm\n-0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5\nSin () Sin () Sin () Sin ()\nRevised:\nFigure S6 in the Supporting Information has been updated (corresponding to Fig.\nR3.3 in the response) with optimized colorbar contrast and dynamic range, allowing\nclearer visualization of the upper and lower polariton branches.\n4) In Fig. R3.5c (the same in R3.9c, R3.13c), there are another extra reflectance dip at\nthe exciton resonance. Does it correspond to the uncoupled exciton resonances?\nHow does it affect the determination of the strong coupling regime here?\nResponse:\nWe thank the reviewer for the careful evaluation and valuable feedback. Regarding the\nadditional reflectivity dip near the exciton resonance, it can be reasonably understood\nas arising from multi-mode exciton-photon coupling in relatively thicker WS layers,\n2\nwhere photonic modes of different orders give rise to additional, slightly detuned\npolariton branches.\na) Experimental consistency: As shown in both the experimental reflectance\nspectra (Fig. R3.4a) and the simulations (Fig. R3.4b), the dip around ~630 nm\nfor the 77.9 nm and 151.2 nm samples corresponds to contributions from the\nlower polariton LP(l = 2) and upper polariton UP(l = 1), respectively, instead of\nan independent exciton absorption feature.\nb) Multi-mode strong coupling: In relatively thicker WS\n2\nsamples, the cavity can\nsupport multiple Fabry-Pérot modes. When the excitons couple to these\ndifferent-order optical modes, additional polariton branches emerge, such as\nlower polaritons of higher-order cavity modes or upper polaritons of lower-\norder ones, resulting in weak reflectance dips slightly detuned from the main\nbranches. Although these dips also originate from exciton-photon coupling, the\nstrong orthogonality in spatial distribution and symmetry between different\ncavity modes ensures that their influence on the primary exciton-cavity photon\ncoupling (i.e., the main Rabi splitting used to identify strong coupling) is\nminimal. Therefore, these secondary polariton branches are treated as\nindependent spectral features that have negligible interference with the main\ncoupling and do not alter our conclusion regarding the strong-coupling regime,\nincluding the observed anti-crossing and splitting magnitude.\nc) Robustness of strong-coupling identification: The identification of strong\ncoupling is based on the Rabi splitting of the primary mode (𝑔² > (𝛾 ² + 𝛾 ²)/2,\n𝐶 𝑋\nwhere γ denotes the linewidths) and the clear anti-crossing behavior, both of\nwhich are evident in the dispersion map (Fig. R3.4b, gray dashed fits).\nFurthermore, temperature-dependent measurements (Fig. R3.4c) provide\nadditional confirmation: the main UP and LP branches redshift from 87 K to\naddition to the main branches, a weaker dip associated with higher-order modes\nalso appears; its energy agrees well with the TMM calculation, further\nsupporting its coupled origin.\nd) Exclusion of uncoupled exciton absorption: If a significant uncoupled exciton\nstate were present, a distinct, thickness-independent dip (corresponding to pure\nexciton absorption) would be expected. However, no such feature is observed\nexperimentally. Instead, all observed dips shift systematically with thickness\nand are consistent with the multi-mode coupling model.\nTaken together, these results indicate that the extra reflectance dip originates from a\nsecondary photonic mode. Importantly, this feature does not undermine the strong-\ncoupling conclusion; on the contrary, it aligns well with and further supports the multi-\nmode strong-coupling picture described in our manuscript.\na b c\nstrong coupling\ne 140 K\nlfe 120 K R\nWS2= 70 ±5 nm\n500 600 700 800\nWavelength (nm)\nFig. R3.4 | Experimental and simulated reflectance spectra in WS cavities. a,\n2\npronounced anticrossing behavior near the A-exciton resonance. b, Thickness and\nwavelength dependent reflectance calculated by the transfer matrix method (TMM),\nshowing the dispersion of exciton polaritons. c, Temperature-dependent reflectance\nspectra (87 K-300 K).\n151.2 nm\nstrong coupling\nc n 77.9 nm a tc e lfe\nRe\nstrong coupling\n39.6 nm\nTMM\nExperimental\nweak coupling EQE maxima\n500 600 700 800\nWavelength (nm)\n200\n150\n100\n50\n0550\n600 650 700 750\n)m\nn ( s s e n k c\nih\nT S2\nW\nW a v e le n g th (n m )\n151.2 nm 77.9 nm\nWavelength (nm)\n5) Why are there some thickness error bars for the samples in temperature-dependent\nmeasurements, while the thicknesses in the angle-resolved measurements seem to be\nvery accurate without error bars?\nResponse:\nWe thank the reviewer for this question. In the temperature-dependent measurements,\nwe measured multiple devices within a ±5 nm thickness range and repeated the\nmeasurements several times. The error bars in thickness therefore reflect the standard\ndeviation across these multiple devices and measurements. In contrast, for the angle-\nresolved measurements, we selected representative devices of well-defined thicknesses\nto clearly resolve the polariton dispersion.\n6) We noted that in some figures of the angle-resolved spectra, the y-axis is labeled in\n‘Energy’. while it is also labeled in ‘Wavelength’ in other spectra in the manuscript and\nsupplementary materials. For consistency and easier comparison across all the\npresented data, we recommend unifying the units throughout the manuscript.\nResponse:\nWe thank the reviewer for this suggestion. We agree that consistent units improve clarity.\nIn the revised manuscript and supplementary materials, we have unified the units in all\nangle-resolved spectra to Wavelength (nm) to facilitate direct comparison across\ndifferent datasets (Fig. R3.5).\nFig. R3.5 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\nRevised:\nIn the revised manuscript, we have updated the angle-resolved spectra in Figure S6 to\ndisplay the y-axis in wavelength units (as shown in Fig. R3.5), in line with the other\nspectral plots throughout the manuscript.\na\n)Ve(\nygrenE\n)mn(\nhtgnelevaW\nR\nM in .\n2.4\nE xp .\n2.2\n2.0\n1.8\n1.6\n-0.5\ne\nR\nM in .\nE xp .\n550\n650\n750\n-0.5\ne fle cta n ce\nM\nS im .\n4 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 6 9 .7 n m\n0 0.5\nS in ( ) \na x.\na x.\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nb\nM\n550\n600\n650\n700\n750\nf\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n7 7 .9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 7 1 .4 n m\n0 0.5\nS in ( ) \na x.\na x.\nc\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nM\n550\n600\n650\n700\n750\ng\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n8 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 9 6 n m\n0 0.5\nS in ( ) \na x.\na x.\nd\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n1 5 1 .2 n m\n-0.5 0 0.5\nS in ( ) \nh\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n2 2 1 n m\n-0.5 0 0.5\nS in ( ) \nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse:\nWe sincerely thank you for your time and effort in co-reviewing our manuscript as part\nof the Nature Communications initiative. We appreciate your constructive comments\nand the thoughtful evaluation provided alongside the primary reviewer. In the revised\nmanuscript, we have carefully addressed all the points raised, including clarifications\non the technical correctness of the strong-coupling analysis, detailed discussion on\npolariton-assisted transport and possible Purcell effects, and enhanced presentation of\nthe angle-resolved reflectance data to better resolve upper and lower polariton\nbranches(see our detailed replies to Reviewer #3’s comments).\nWe have further emphasized the novelty of our cavity-free, self-hybridized polariton\nphotodiode, contrasting it with prior cavity-based systems, and provided additional\nsupporting analyses demonstrating improved device performance metrics. All updates\nand new discussions, including references to relevant prior work, are incorporated in\nthe revised Introduction, Discussion, and Supplementary Information (e.g.,\nSupplementary Fig. S6 and Tables S1).\nWe greatly appreciate your valuable suggestions, which have helped us strengthen\nthe clarity, rigor, and overall impact of our work.\nReviewer #3 (Comments to the Author):\nThe authors have adequately addressed most of my previous comments.\nBefore recommending publication, I have one final suggestion regarding the framing\nof the work. The use of self-hybridized polaritons in an open cavity is presented as a\npractical approach for device applications. However, the relatively high refractive index\nof TMDs results in an intrinsically moderate quality factor for such open cavities. While\nthis configuration demonstrates polariton effects, applications generally require cavities\nwith higher quality factors and smaller mode volumes for optimal performance. If\npossible, I would like to suggest the authors tone down the claims of novelty regarding\nthe open cavity configuration in the final version and instead acknowledge that future\nperformance could be significantly improved through the careful design of an integrated\nexternal cavity.\nResponse:\nWe sincerely thank the reviewer for the positive assessment of our revised manuscript\nand for acknowledging that most of the previous comments have been satisfactorily\naddressed. We appreciate the reviewer’s careful reading and the constructive final\nsuggestion regarding the framing of our work.\nRegarding the open cavity:\nIn the revised manuscript, we have carefully softened the claims concerning the\nnovelty and application relevance of the open-cavity configuration. We also explicitly\nstate that practical device implementations would benefit from optimized cavity designs\nthat provide higher quality factors and smaller mode volumes, thereby enabling device\nperformance closer to theoretical limits. We are grateful for this constructive suggestion,\nwhich has helped us present a more balanced and realistic discussion of the potential\napplications of our work.\nRevised:\nIn the revised manuscript, we have updated the Introduction, Results, and Discussion\nsections to further softened the emphasis on the open-cavity configuration (see pages\n2 ,9 and 12 of the main text).\nAdditionally, in the Device structure and working principle section (page 4), we now\ninclude a discussion highlighting that, although our open-cavity platform is suitable for\ndemonstrating self-hybridized polariton formation, future device-level performance\ncould be significantly enhanced by integrating carefully engineered dielectric\nenvironments or high-Q cavities."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all of the concerns raised, and I believe the paper is now suitable for acceptance in its current form.", "reviewer_2": "The authors have adequately addressed my previous concerns regarding the size of the vacuum Rabi splitting, the Q value of the cavity mode, and the clarity of the observed anti-crossing. They have also provided a detailed discussion on the influence of trap states on device performance. Overall, I find that the revised manuscript meets the standard for publication. I do have one remaining question regarding the temperature-dependent polariton branches. The splitting appears to persist up to room temperature. Could the authors clarify the physical origin of this robustness?", "reviewer_3": "We appreciate the authors’ efforts to address our previous concerns in the last report. The manuscript has been indeed improved from the earlier version. The authors have seriously looked into the polaritonic effects and carefully examined the criterion of the strong coupling regime. More importantly, the authors have paid substantial attentions to the correlations between the polaritonic resonances and maxima of the device performance measures. These points are critical to check the technical correctness of their claim. However, we still have several concerns for these technical points as well as the novelty of this work before it can be considered for publication. Here are our detailed comments as below. 1) In comparison with previous similar work [e.g., ACS Photonics 5, 2921-2927 (2018)], the authors claimed the novelty lies on the self-hybridized polaritons for the photodiodes, broadband absorption, polariton-enabled transport and improved device performance. The concept of self-hybridized exciton polaritons is not new, and does not seem to play a significant role in the device performance (actually it might introduce some intrinsic limits for the device performance), which has also been pointed out by the other reviewer. However, the polariton-enabled transport seems to be a quite interesting point, which is actually an important topic for the polaritonics [e.g., Nat Mater 2015, 14, 1123-1129]. Moreover, the polariton-mediated Purcell effect could be also some interesting point for boosting optoelectronic device performance, which might be related with this work [e.g., Nature 2024, 626, 300-306]. The authors are suggested to find some more significant points to support the claimed novelty. 2) For the tests of the strong coupling regime, there are still some points that should be carefully examined. First, the thickness-dependent coupling strength seems to be quite complicated. For the same cavity conditions, the coupling strength scales with the square-root of the thickness. For example, the thicknesses between 150-180 nm seem to host the similar l=2 cavity mode (Fig. R3.5a). In Table R3.2, the coupling strength for the thicknesses between 150 nm and 180 nm does not really follow this scaling law. This causes some confusion. Second, how did the authors determine the cavity linewidths? The linewidths are so broad and even broader than the exciton linewidths. For higher-order cavity modes, the linewidths generally should become narrower. The fitted cavity linewidths do not really follow this trend. Third, how did the authors determine the exciton linewidths. As the authors mentioned in the response, the oscillator strength of the excitons should become constant for the bulk thickness (the reference of Nature Photon 2015 is irrelevant with their argument there). For this scenario, the exciton linewidths should be also taken as a constant and independent of the bulk thickness. In Table R3.2, this does not seem to be the case. 3) In Fig. R3.8, some angle-resolved polariton branches of the experimental data are hard to be resolved for some thicknesses. For example, the upper branch for the thicknesses of 196 nm and 221 nm, and the lower branch for the thickness of 196 nm need to be resolved with better resolutions. 4) In Fig. R3.5c (the same in R3.9c, R3.13c), there are another extra reflectance dip at the exciton resonance. Does it correspond to the uncoupled exciton resonances? How does it affect the determination of the strong coupling regime here? 5) Why are there some thickness error bars for the samples in temperature-dependent measurements, while the thicknesses in the angle-resolved measurements seem to be very accurate without error bars? 6) We noted that in some figures of the angle-resolved spectra, the y-axis is labeled in \"Energy\". while it is also labeled in \"Wavelength\" in other spectra in the manuscript and supplementary materials. For consistency and easier comparison across all the presented data, we recommend unifying the units throughout the manuscript. I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Reviewer 3\nReviewer #3 (Comments to the Author):\nWe appreciate the authors’ efforts to address our previous concerns in the last report.\nThe manuscript has been indeed improved from the earlier version. The authors have\nseriously looked into the polaritonic effects and carefully examined the criterion of the\nstrong coupling regime. More importantly, the authors have paid substantial attentions\nto the correlations between the polaritonic resonances and maxima of the device\nperformance measures. These points are critical to check the technical correctness of\ntheir claim. However, we still have several concerns for these technical points as well\nas the novelty of this work before it can be considered for publication. Here are our\ndetailed comments as below.\nResponse:\nWe sincerely appreciate the reviewer’s thoughtful and constructive feedback. We are\npleased to know that the improvements in the revised manuscript have been recognized,\nespecially regarding the identification of polaritonic effects, the verification of the\nstrong coupling regime, and the correlation between the polariton resonances and the\ndevice performance. We would like to once again express our gratitude for these\ninsightful comments and suggestions, which have not only helped us produce a\nsubstantially improved manuscript but also motivated us to conduct a deeper\ninvestigation into the underlying physics.\nWe have carefully addressed all remaining concerns regarding both the technical\ncorrectness and the novelty of our work. In this revised version, we have further\nclarified the physical mechanisms, added new supporting analyses, and strengthened\nthe presentation of experimental evidence to ensure that our claims are technically\nsound and well justified. Detailed point-by-point responses are provided below.\n1) In comparison with previous similar work [e.g., ACS Photonics 5, 2921-2927 (2018)],\nthe authors claimed the novelty lies on the self-hybridized polaritons for the\nphotodiodes, broadband absorption, polariton-enabled transport and improved device\nperformance. The concept of self-hybridized exciton polaritons is not new, and does not\nseem to play a significant role in the device performance (actually it might introduce\nsome intrinsic limits for the device performance), which has also been pointed out by\nthe other reviewer. However, the polariton-enabled transport seems to be a quite\ninteresting point, which is actually an important topic for the polaritonics [e.g., Nat\nMater 2015, 14, 1123-1129]. Moreover, the polariton-mediated Purcell effect could be\nalso some interesting point for boosting optoelectronic device performance, which\nmight be related with this work [e.g., Nature 2024, 626, 300-306]. The authors are\nsuggested to find some more significant points to support the claimed novelty.\nResponse:\nWe thank the reviewer for their constructive feedback and for highlighting the\ninteresting aspects of polariton-enabled transport in our work. We appreciate the\nsuggestion to further emphasize the novelty by identifying additional significant points.\nBelow, we address the reviewer’s concerns point by point, clarifying how our study\nadvances beyond prior work while building on established concepts in polaritonics.\nFirst, regarding the comparison to previous similar work (e.g., ACS Photonics 5,\n2921-2927 (2018), hereafter Ref. 31 in our manuscript), we agree that strong light-\nmatter coupling has been explored in organic photodiodes to extend spectral\nresponsivity. However, that study relied on external optical cavities with partially\ntransparent metal reflectors, which introduced energy losses and limited device\nperformance. In contrast, our work demonstrates the integration of self-hybridized\nexciton-polaritons into an inorganic excitonic photodiode (using WS ) without\n2\nexternal cavities, leveraging the material’s intrinsic high refractive index for self-\ntrapping and hybridization of light. This eliminates the need for lossy reflectors,\nenabling broadband absorption across 500-800 nm while achieving near-unity\ninternal quantum efficiency (IQE > 90%) near zero detuning, performance metrics\nthat outperform those in Ref. 31(EQE <15%; IQE < 25%) and related organic systems.\nWe have revised the introduction and discussion sections to more explicitly contrast\nthese architectural differences, emphasizing how our open-cavity approach (inspired by\nrecent advances, e.g., Refs. 34-35) avoids the trade-offs in prior designs.\nWhile the concept of self-hybridized exciton-polaritons is indeed not new (as\ndemonstrated before in multilayer TMDCs), its application here plays an important role\nin device performance. In our devices, self-hybridization enables strong photon-\nexciton coupling directly within the WS absorber, resulting in observable anti-\n2\ncrossing in thickness-dependent reflectance and external quantum efficiency (EQE)\nspectra, with Rabi splittings of 152-173 meV. This directly contributes to enhanced\nsub-bandgap absorption via the lower polariton branch (LPB), extending the cut-off\nwavelength (Supplementary Fig. S11) without compromising broadband response.\nCritically, it facilitates polariton-enhanced transport, leading to oscillatory IQE\nbehavior with thickness and peaks at zero-detuning points (≈75 nm and 145 nm for\nl=1,2 modes), approaching unity IQE, suggesting improved exciton diffusion lengths\nthat mitigate recombination losses in conventional excitonic photodiodes. The self-\nhybridized state appears to alleviate the narrow-band absorption and short diffusion\nlength typically associated with excitonic semiconductors such as WS . To address the\n2\nreviewer’s point, we have added a discussion in the main text (Device structure and\nworking principle section) on potential limitations, such as imperfect interfaces\naffecting response speed (e.g., interfacial carrier trapping, Supplementary Fig. S20),\nand how material optimizations could further elevate performance toward theoretical\npolaritonic limits.\nWe fully concur that polariton-enabled transport is a key novelty and an important\ntopic in polaritonics, as noted in the reference (Nat. Mater. 2015, 14, 1123–1129; now\ncited as Ref. 47 in the revised manuscript). Our study provides direct experimental\nevidence of this in a functional photodiode, where strong coupling enhances exciton\ntransport via photonic modes, yielding a minimum response time of 217 ns—faster than\nreportedroom-temperature excitonic photodetectors (Supplementary Table 3). This is\nsupported by our phenomenological transport model (Supplementary Fig. S14), which\naligns with experimental IQE trends and attributes oscillations to resonance energy\ntransfer weighted by the polariton’s excitonic fraction. Unlike prior demonstrations\nfocused on fundamental transport (e.g., long-range propagation in monolayers, Refs.\n23-24), our work integrates this into a device architecture, demonstrating a 20-fold EQE\nincrease entering the strong-coupling regime and a -3 dB cutoff frequency of 1.21 MHz.\nAdditionally, we appreciate the suggestion regarding the polariton-mediated Purcell\neffect (e.g., Nature 2024, 626, 300-306; now cited as a new reference 54 in the\ndiscussion), which can facilitate more efficient charge separation and suppressed non-\nradiative recombination. While our primary focus is on transport enhancement,\npolaritonic effects may indeed contribute to modified radiative rates, potentially\nboosting charge separation at interfaces. Our low-temperature EQE data (Fig. 2e-f)\nshow preserved polaritonic features and enhanced efficiency, consistent with\nsuppressed non-radiative recombination, aligning with Purcell-like modifications.\nWe have added a brief discussion in the EQE section to explore this connection, noting a\ndirection for future studies to quantitatively evaluate this effect.\nWe hope these revisions incorporate the reviewer’s suggested references and enhance\nthe manuscript’s clarity on novelty. We believe they address the concerns and strengthen\nthe paper’s impact.\nRevised:\nIn the revised manuscript, we have expanded the Introduction and Discussion\nsections to better clarify the novelty of our cavity-free, self-hybridized polariton\nphotodiode compared with previously reported cavity-based systems (see pages 2 and\n8 of the main text). Specifically, we emphasize that the transparent top electrode\nsimultaneously serves as an antireflective layer, enabling strong light–matter\ncoupling in an open-cavity architecture with broadband absorption and polariton-\nenhanced transport.\nIn the Device structure and working principle section (page 3), we have incorporated\na discussion on extrinsic performance limitations—including interfacial trap states,\nnonideal contact geometry, and optical confinement issues—to clarify that the current\nresponse speed is constrained by technological, rather than fundamental, factors.\nIn the External and internal quantum efficiency analysis (pages 6-7), we further\ndiscuss how self-hybridized polariton states concentrate optical fields within the WS₂\nlayer, providing an inherent advantage of the open-cavity configuration over externally\nmirrored systems. Additionally, we have introduced a brief note on the polariton-\nmediated Purcell-like effect (Ref. 54), highlighting its potential role in modifying\nradiative rates and suggesting it as a promising avenue for future quantitative\ninvestigation.\nRelevant references have been updated accordingly.\n2) For the tests of the strong coupling regime, there are still some points that should be\ncarefully examined. First, the thickness-dependent coupling strength seems to be quite\ncomplicated. For the same cavity conditions, the coupling strength scales with the\nsquare-root of the thickness. For example, the thicknesses between 150-180 nm seem\nto host the similar l=2 cavity mode (Fig. R3.5a). In Table R3.2, the coupling strength\nfor the thicknesses between 150 nm and 180 nm does not really follow this scaling law.\nThis causes some confusion. Second, how did the authors determine the cavity\nlinewidths? The linewidths are so broad and even broader than the exciton linewidths.\nFor higher-order cavity modes, the linewidths generally should become narrower. The\nfitted cavity linewidths do not really follow this trend. Third, how did the authors\ndetermine the exciton linewidths. As the authors mentioned in the response, the\noscillator strength of the excitons should become constant for the bulk thickness (the\nreference of Nature Photon 2015 is irrelevant with their argument there). For this\nscenario, the exciton linewidths should be also taken as a constant and independent of\nthe bulk thickness. In Table R3.2, this does not seem to be the case.\nResponse:\nWe appreciate the reviewer’s thoughtful comments on the strong coupling analysis.\nBelow we clarify the physical reasoning and methodological details regarding the\ncoupling strength, linewidth determination, and their thickness dependence.\n(1) Thickness dependence of coupling strength.\nIn principle, when the optical mode volume 𝑉 remains approximately constant,\nan increase in the number of excitons 𝑁 enhances the overall exciton-photon\ninteraction strength, following the relation\n𝑁\n𝑔 ∝ √\n𝑉\nHowever, in our self-hybridized system, the number of emitters (𝑵) exhibits an\napproximately linear increase with thickness, while the optical mode volume (𝑽)\nalso expands with increasing thickness. As a result, the overall coupling strength\n(𝑔) varies only slightly over a wide thickness range, which is consistent with our\nexperimental observations. Therefore, the observed Rabi splitting in our structures\nreflects a balance between the increased exciton density and the enlarged mode\nvolume. This interplay between 𝑁 and 𝑉 has been well established in the literature\non self-hybridized and collective light-matter coupling systems [Nat. Mater. 22,\n970–976 (2023); Phys. Rev. B 107, 075415 (2023); Nat. Commun. 13, 5597 (2022)].\n(2) Determination of cavity linewidths.\nThe cavity linewidths are extracted by fitting the reflectance of each cavity\nmode at large detuning, where exciton-photon interaction is minimal. These\nlinewidths serve as the initial inputs for the thickness-dependent fits and are allowed\nto vary slightly to achieve the best fit. The relatively broad linewidths arise from\nthe intrinsic optical dissipation and imperfect optical confinement in the WS layers,\n2\nwhich is typical for self-hybridized cavity systems and still sufficient to sustain\npolaritonic states. Moreover, the measured vacuum Rabi splitting exceeds the\ncombined linewidths of the exciton and cavity modes, satisfying the strong-\ncoupling criterion. Future improvements in interface quality and in-plane\nconfinement are expected to further enhance the cavity Q-factor.\n(3) Determination of exciton linewidths.\nThe exciton linewidths are obtained by fitting a Lorentz oscillator to the\nimaginary part of the dielectric function around the A-exciton resonance. The\nfitting is performed over a narrow spectral range on the high-energy side to avoid\nthe influence of overlapping absorption tails. The extracted linewidths show only\nsmall fluctuations across different thicknesses, mainly due to experimental and\nfitting uncertainties, while the intrinsic exciton linewidth remains essentially\nconstant, consistent with the expected bulk-like behavior.\n(4) Clarifications in angle-dependent fitting.\nFollowing the reviewer’s suggestion, we have refined the angle-dependent fitting\nprocedure (Fig.R3.1 and Table R3.1): (i) the exciton linewidths are now fixed to\nthe constant values extracted from the dielectric function, and (ii) the cavity\nlinewidths are fixed to those obtained from thickness-dependent fitting. Only the\ncoupling strength and parameters related to cavity dispersion are allowed to vary.\nThis ensures consistency and reduces the number of fitting parameters.\n(5) Regarding the reference Nature Photon. 9, 466-470 (2015):\nWe thank the reviewer for pointing this out. The reference to Nature Photonics 9,\n466-470 (2015) was not intended to directly support the thickness dependence of\nthe oscillator strength, but rather to provide quantitative information on the exciton\nBohr radius (~1 nm) in WS . This small Bohr radius indicates that the exciton\n2\nwavefunction is highly localized within a single layer, implying that the oscillator\nstrength becomes nearly independent of thickness once the film exceeds a few\nlayers. We apologize for the confusion regarding the relevance of this reference.\nFig. R3.1 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\nTable R3.1 | Coupled Oscillator Model Fitting Results for Devices with Varying\nThicknesses.\n𝛾 𝛾\nThickness 𝐶 𝑋 Rabi splitting\n𝑔 (meV) (𝛾 −𝛾 )/2 √(𝛾 2+𝛾 2)/2\n𝐶 𝑋 𝐶 𝑋\n(nm) (meV)\n(meV) (meV)\n77.9 86.5 106.3 26.5 153.5 66.4 77.3\n89.0 87.5 106.3 26.5 155.7 66.4 77.3\na\n)Ve(\nygrenE\n)mn(\nhtgnelevaW\nR\nM in .\n2.4\nE xp .\n2.2\n2.0\n1.8\n1.6\n-0.5\ne\nR\nM in .\nE xp .\n550\n650\n750\n-0.5\ne fle cta n ce\nM\nS im .\n4 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 6 9 .7 n m\n0 0.5\nS in ( ) \na x.\na x.\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nb\nM\n550\n600\n650\n700\n750\nf\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n7 7 .9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 7 1 .4 n m\n0 0.5\nS in ( ) \na x.\na x.\nc\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nM\n550\n600\n650\n700\n750\ng\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n8 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 9 6 n m\n0 0.5\nS in ( ) \na x.\na x.\nd\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n1 5 1 .2 n m\n-0.5 0 0.5\nS in ( ) \nh\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n2 2 1 n m\n-0.5 0 0.5\nS in ( ) \n151.2 91.7 70.2 26.5 178.1 48.3 53.1\n169.7 89.1 70.2 26.5 172.8 48.3 53.1\n171.4 89.3 70.2 26.5 173.2 48.3 53.1\n196.0 96.5 51.1 26.5 191.4 38.8 40.7\n221.9 93.9 51.1 26.5 186.2 38.8 40.7\nRevised:\nIn the revised manuscript, we have carried out a revised dispersion data fitting\nanalysis. The Supplementary Information has been updated accordingly with the new\nFigure S6 and Table S1.\n3) In Fig. R3.8, some angle-resolved polariton branches of the experimental data are\nhard to be resolved for some thicknesses. For example, the upper branch for the\nthicknesses of 196 nm and 221 nm, and the lower branch for the thickness of 196 nm\nneed to be resolved with better resolutions.\nResponse:\nWe appreciate the reviewer’s careful observation. We have re-examined the angle-\nresolved reflectance spectra for the 196 nm and 221 nm WS devices and found that the\n2\nupper and lower polariton branches are indeed present but were previously less\ndistinguishable due to the limited color scale range. As shown in Fig. R3.2, by\noptimizing the colorbar contrast and dynamic range, both branches now appear clearly\nresolved and exhibit excellent agreement with the fitted coupled-oscillator model. The\ncorresponding normal-incidence (sin θ = 0) spectra also shows pronounced mode\nsplitting at the exciton resonance. In addition, we have applied the same adjustment to\nthe angle-resolved reflectance spectra of other device thicknesses (Fig. R3.3), which\nsimilarly improves the visibility of both upper and lower polariton branches. These\nrevisions enhance the clarity of the features without any data manipulation, confirming\nthat the observed branches are genuine polaritonic modes.\nFig. R3.2 | Angle-resolved reflectivity spectra of WS exciton-polariton devices\n2\nwith thicknesses of 196 nm and 221 nm. The reflectance spectrum extracted at sin(θ)\n= 0 reveals a clear splitting at the corresponding exciton resonance.\nFig. R3.3 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses.\na\n)mn(\nhtgnelevaW\n5\n6\n6\n7\n7\n5\n0\n5\n0\n0\n0\n0\n0\nin .\nE x p .\nU P\nL P\n-0 .5\ne fle c ta n\nc e\nS\n1 9\n)\nM\nim .550\n600\n650\n700\n750\n6 n m\n0 .5\na x .\np\nUo p p e r\nla rito n\nL o w e r\np o la rito n\nb\n)mn(\nhtgnelevaW\n5\n6\n6\n7\n7\n5\n0\n5\n0\n0\n0\n0\n0\nin .\nE x p .\nU P\nL P\n-0 .5\ne fle c ta n c e\nS\n2 2\nM\nim .\n1 n m\n0 .5\na x .\n5\n5\n0\n6\n0\n0\n6\n5\n0\n7\n0\n0\n7\n5\n0\np\nUo pla p e r\nrito n\nL o w e r\np o la rito n\na Reflectance b Reflectance c Reflectance d Reflectance\nMin. Max. Min. Max. Min. Max. Min. Max.\n2.4\nExp. Sim. Exp. Sim. Exp. Sim. Exp. Sim.\n550 550 550\n2.2 UP\nUP UP\n)m\nn600\n)m\nn\n600\n)m\nn600\n)V\ne\n(\ny g re\nn E\n2.0\n( h\ntg\nn\ne le v\na W\n650\n( h\ntg\nn\ne le v\na W\n650\n( h\ntg\nn\ne le v\na W\nLP LP\n700 700 700\n1.8\n750 750 750\n49 nm 77.9 nm 89 nm 151.2 nm\n1.6\n-0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5\nSin () Sin () Sin () Sin ()\ne f g h\nReflectance Reflectance Reflectance Reflectance\nMin. Max. Min. Max. Min. Max. Min. Max.\nExp. Sim. Exp. Sim. Exp. Sim. Exp. Sim.\n550 550 550 550\nUP\nUP\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n)m\nn ( h tg\nn e le\nv a W\n6\n6\n0\n5\n0\n700 700 700 700\nLP LP\n750 750 750 750\n169.7 nm 171.4 nm 196 nm 221 nm\n-0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5 -0.5 0 0.5\nSin () Sin () Sin () Sin ()\nRevised:\nFigure S6 in the Supporting Information has been updated (corresponding to Fig.\nR3.3 in the response) with optimized colorbar contrast and dynamic range, allowing\nclearer visualization of the upper and lower polariton branches.\n4) In Fig. R3.5c (the same in R3.9c, R3.13c), there are another extra reflectance dip at\nthe exciton resonance. Does it correspond to the uncoupled exciton resonances?\nHow does it affect the determination of the strong coupling regime here?\nResponse:\nWe thank the reviewer for the careful evaluation and valuable feedback. Regarding the\nadditional reflectivity dip near the exciton resonance, it can be reasonably understood\nas arising from multi-mode exciton-photon coupling in relatively thicker WS layers,\n2\nwhere photonic modes of different orders give rise to additional, slightly detuned\npolariton branches.\na) Experimental consistency: As shown in both the experimental reflectance\nspectra (Fig. R3.4a) and the simulations (Fig. R3.4b), the dip around ~630 nm\nfor the 77.9 nm and 151.2 nm samples corresponds to contributions from the\nlower polariton LP(l = 2) and upper polariton UP(l = 1), respectively, instead of\nan independent exciton absorption feature.\nb) Multi-mode strong coupling: In relatively thicker WS\n2\nsamples, the cavity can\nsupport multiple Fabry-Pérot modes. When the excitons couple to these\ndifferent-order optical modes, additional polariton branches emerge, such as\nlower polaritons of higher-order cavity modes or upper polaritons of lower-\norder ones, resulting in weak reflectance dips slightly detuned from the main\nbranches. Although these dips also originate from exciton-photon coupling, the\nstrong orthogonality in spatial distribution and symmetry between different\ncavity modes ensures that their influence on the primary exciton-cavity photon\ncoupling (i.e., the main Rabi splitting used to identify strong coupling) is\nminimal. Therefore, these secondary polariton branches are treated as\nindependent spectral features that have negligible interference with the main\ncoupling and do not alter our conclusion regarding the strong-coupling regime,\nincluding the observed anti-crossing and splitting magnitude.\nc) Robustness of strong-coupling identification: The identification of strong\ncoupling is based on the Rabi splitting of the primary mode (𝑔² > (𝛾 ² + 𝛾 ²)/2,\n𝐶 𝑋\nwhere γ denotes the linewidths) and the clear anti-crossing behavior, both of\nwhich are evident in the dispersion map (Fig. R3.4b, gray dashed fits).\nFurthermore, temperature-dependent measurements (Fig. R3.4c) provide\nadditional confirmation: the main UP and LP branches redshift from 87 K to\naddition to the main branches, a weaker dip associated with higher-order modes\nalso appears; its energy agrees well with the TMM calculation, further\nsupporting its coupled origin.\nd) Exclusion of uncoupled exciton absorption: If a significant uncoupled exciton\nstate were present, a distinct, thickness-independent dip (corresponding to pure\nexciton absorption) would be expected. However, no such feature is observed\nexperimentally. Instead, all observed dips shift systematically with thickness\nand are consistent with the multi-mode coupling model.\nTaken together, these results indicate that the extra reflectance dip originates from a\nsecondary photonic mode. Importantly, this feature does not undermine the strong-\ncoupling conclusion; on the contrary, it aligns well with and further supports the multi-\nmode strong-coupling picture described in our manuscript.\na b c\nstrong coupling\ne 140 K\nlfe 120 K R\nWS2= 70 ±5 nm\n500 600 700 800\nWavelength (nm)\nFig. R3.4 | Experimental and simulated reflectance spectra in WS cavities. a,\n2\npronounced anticrossing behavior near the A-exciton resonance. b, Thickness and\nwavelength dependent reflectance calculated by the transfer matrix method (TMM),\nshowing the dispersion of exciton polaritons. c, Temperature-dependent reflectance\nspectra (87 K-300 K).\n151.2 nm\nstrong coupling\nc n 77.9 nm a tc e lfe\nRe\nstrong coupling\n39.6 nm\nTMM\nExperimental\nweak coupling EQE maxima\n500 600 700 800\nWavelength (nm)\n200\n150\n100\n50\n0550\n600 650 700 750\n)m\nn ( s s e n k c\nih\nT S2\nW\nW a v e le n g th (n m )\n151.2 nm 77.9 nm\nWavelength (nm)\n5) Why are there some thickness error bars for the samples in temperature-dependent\nmeasurements, while the thicknesses in the angle-resolved measurements seem to be\nvery accurate without error bars?\nResponse:\nWe thank the reviewer for this question. In the temperature-dependent measurements,\nwe measured multiple devices within a ±5 nm thickness range and repeated the\nmeasurements several times. The error bars in thickness therefore reflect the standard\ndeviation across these multiple devices and measurements. In contrast, for the angle-\nresolved measurements, we selected representative devices of well-defined thicknesses\nto clearly resolve the polariton dispersion.\n6) We noted that in some figures of the angle-resolved spectra, the y-axis is labeled in\n‘Energy’. while it is also labeled in ‘Wavelength’ in other spectra in the manuscript and\nsupplementary materials. For consistency and easier comparison across all the\npresented data, we recommend unifying the units throughout the manuscript.\nResponse:\nWe thank the reviewer for this suggestion. We agree that consistent units improve clarity.\nIn the revised manuscript and supplementary materials, we have unified the units in all\nangle-resolved spectra to Wavelength (nm) to facilitate direct comparison across\ndifferent datasets (Fig. R3.5).\nFig. R3.5 | Angle-resolved reflectivity spectra of WS exciton polariton devices\n2\nwith different thicknesses. The left (right) part of the figure shows the experiment\n(TMM calculation). The red dashed lines represent the dispersion obtained for the UPB\nand LPB using the coupled oscillator model. The black dashed line and parabola\nrepresent the exciton energy and the dispersion of the cavity mode.\nRevised:\nIn the revised manuscript, we have updated the angle-resolved spectra in Figure S6 to\ndisplay the y-axis in wavelength units (as shown in Fig. R3.5), in line with the other\nspectral plots throughout the manuscript.\na\n)Ve(\nygrenE\n)mn(\nhtgnelevaW\nR\nM in .\n2.4\nE xp .\n2.2\n2.0\n1.8\n1.6\n-0.5\ne\nR\nM in .\nE xp .\n550\n650\n750\n-0.5\ne fle cta n ce\nM\nS im .\n4 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 6 9 .7 n m\n0 0.5\nS in ( ) \na x.\na x.\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nb\nM\n550\n600\n650\n700\n750\nf\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n7 7 .9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 7 1 .4 n m\n0 0.5\nS in ( ) \na x.\na x.\nc\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nM\n550\n600\n650\n700\n750\ng\nM\n550\n600\n650\n700\nin .\nE xp .\nU P\nL P\n-0.5\nR\nin .\nE xp .\nU P\nL P\n-0.5\ne fle cta n ce\nM\nS im .\n8 9 n m\n0 0.5\nS in ( ) \ne fle cta n ce\nM\nS im .\n1 9 6 n m\n0 0.5\nS in ( ) \na x.\na x.\nd\n)mn(\nhtgnelevaW\n)mn(\nhtgnelevaW\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n1 5 1 .2 n m\n-0.5 0 0.5\nS in ( ) \nh\nR e fle cta n ce\nM in . M a x.\nE xp . S im .\n600\n700\n750\n2 2 1 n m\n-0.5 0 0.5\nS in ( ) \nReviewer #4 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports.\nThis is part of the Nature Communications initiative to facilitate training in peer review\nand to provide appropriate recognition for Early Career Researchers who co-review\nmanuscripts.\nResponse:\nWe sincerely thank you for your time and effort in co-reviewing our manuscript as part\nof the Nature Communications initiative. We appreciate your constructive comments\nand the thoughtful evaluation provided alongside the primary reviewer. In the revised\nmanuscript, we have carefully addressed all the points raised, including clarifications\non the technical correctness of the strong-coupling analysis, detailed discussion on\npolariton-assisted transport and possible Purcell effects, and enhanced presentation of\nthe angle-resolved reflectance data to better resolve upper and lower polariton\nbranches(see our detailed replies to Reviewer #3’s comments).\nWe have further emphasized the novelty of our cavity-free, self-hybridized polariton\nphotodiode, contrasting it with prior cavity-based systems, and provided additional\nsupporting analyses demonstrating improved device performance metrics. All updates\nand new discussions, including references to relevant prior work, are incorporated in\nthe revised Introduction, Discussion, and Supplementary Information (e.g.,\nSupplementary Fig. S6 and Tables S1).\nWe greatly appreciate your valuable suggestions, which have helped us strengthen\nthe clarity, rigor, and overall impact of our work.\nReviewer #3 (Comments to the Author):\nThe authors have adequately addressed most of my previous comments.\nBefore recommending publication, I have one final suggestion regarding the framing\nof the work. The use of self-hybridized polaritons in an open cavity is presented as a\npractical approach for device applications. However, the relatively high refractive index\nof TMDs results in an intrinsically moderate quality factor for such open cavities. While\nthis configuration demonstrates polariton effects, applications generally require cavities\nwith higher quality factors and smaller mode volumes for optimal performance. If\npossible, I would like to suggest the authors tone down the claims of novelty regarding\nthe open cavity configuration in the final version and instead acknowledge that future\nperformance could be significantly improved through the careful design of an integrated\nexternal cavity.\nResponse:\nWe sincerely thank the reviewer for the positive assessment of our revised manuscript\nand for acknowledging that most of the previous comments have been satisfactorily\naddressed. We appreciate the reviewer’s careful reading and the constructive final\nsuggestion regarding the framing of our work.\nRegarding the open cavity:\nIn the revised manuscript, we have carefully softened the claims concerning the\nnovelty and application relevance of the open-cavity configuration. We also explicitly\nstate that practical device implementations would benefit from optimized cavity designs\nthat provide higher quality factors and smaller mode volumes, thereby enabling device\nperformance closer to theoretical limits. We are grateful for this constructive suggestion,\nwhich has helped us present a more balanced and realistic discussion of the potential\napplications of our work.\nRevised:\nIn the revised manuscript, we have updated the Introduction, Results, and Discussion\nsections to further softened the emphasis on the open-cavity configuration (see pages\n2 ,9 and 12 of the main text).\nAdditionally, in the Device structure and working principle section (page 4), we now\ninclude a discussion highlighting that, although our open-cavity platform is suitable for\ndemonstrating self-hybridized polariton formation, future device-level performance\ncould be significantly enhanced by integrating carefully engineered dielectric\nenvironments or high-Q cavities."}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors have adequately addressed most of my previous comments. Before recommending publication, I have one final suggestion regarding the framing of the work. The use of self-hybridized polaritons in an open cavity is presented as a practical approach for device applications. However, the relatively high refractive index of TMDs results in an intrinsically moderate quality factor for such open cavities. While this configuration demonstrates polariton effects, applications generally require cavities with higher quality factors and smaller mode volumes for optimal performance. If possible, I would like to suggest the authors tone down the claims of novelty regarding the open cavity configuration in the final version and instead acknowledge that future performance could be significantly improved through the careful design of an integrated external cavity."}, "author_response": "Response to Reviewer 3\nReviewer #3 (Comments to the Author):\nThe authors have adequately addressed most of my previous comments.\nBefore recommending publication, I have one final suggestion regarding the framing\nof the work. The use of self-hybridized polaritons in an open cavity is presented as a\npractical approach for device applications. However, the relatively high refractive index\nof TMDs results in an intrinsically moderate quality factor for such open cavities. While\nthis configuration demonstrates polariton effects, applications generally require cavities\nwith higher quality factors and smaller mode volumes for optimal performance. If\npossible, I would like to suggest the authors tone down the claims of novelty regarding\nthe open cavity configuration in the final version and instead acknowledge that future\nperformance could be significantly improved through the careful design of an integrated\nexternal cavity.\nResponse:\nWe sincerely thank the reviewer for the positive assessment of our revised manuscript\nand for acknowledging that most of the previous comments have been satisfactorily\naddressed. We appreciate the reviewer’s careful reading and the constructive final\nsuggestion regarding the framing of our work.\nRegarding the open cavity:\nIn the revised manuscript, we have carefully softened the claims concerning the\nnovelty and application relevance of the open-cavity configuration. We also explicitly\nstate that practical device implementations would benefit from optimized cavity designs\nthat provide higher quality factors and smaller mode volumes, thereby enabling device\nperformance closer to theoretical limits. We are grateful for this constructive suggestion,\nwhich has helped us present a more balanced and realistic discussion of the potential\napplications of our work.\nRevised:\nIn the revised manuscript, we have updated the Introduction, Results, and Discussion\nsections to further softened the emphasis on the open-cavity configuration (see pages\n2 ,9 and 12 of the main text).\nAdditionally, in the Device structure and working principle section (page 4), we now\ninclude a discussion highlighting that, although our open-cavity platform is suitable for\ndemonstrating self-hybridized polariton formation, future device-level performance\ncould be significantly enhanced by integrating carefully engineered dielectric\nenvironments or high-Q cavities."}]

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