Helicity-selective and spectrally tunable chiral thermal emissions

Domain: biology
Published: 2026-03-18
Editorial outcome: minor_revision

Abstract
Integrating nonlocal metasurfaces with thermal photonics has endowed thermal emissions with temporal and spatial coherence, as well as chirality. However, the static response of current thermal emitters hinders their broader applications, especially in high-precision sensing. Here, we present an anisotropic thermal metasurface that harnesses exclusively elevated temperatures inherent in thermal photonics to achieve helicity-switchable and wavelength-tunable circularly polarized coherent thermal emissions in the mid-infrared. Through a meticulous design to achieve a pair of high-Q quasi-guided mode resonances with opposite chirality and significant emission circular dichroism, we experimentally demonstrate helicity-switchable circularly polarized thermal emissions with high temporal coherence (Q > 150) and strong emission circular dichroism (> 0.8) over a ~100 nm wavelength range through a temperature change of 250 K. We further reveal that simple geometric design enables full polarization tailoring. Our platform offers a compact and scalable pathway toward on-chip applications including circular dichroism spectroscopy for enantiomer identification.

Introduction
Thermal emission (ThE), the intrinsic generation of electromagnetic radiation from thermally induced charge fluctuations within matter, is a universal phenomenon in all objects at temperatures above absolute zero. However, classical blackbody radiation is inherently broadband, incoherent, and unpolarized, thus fundamentally limiting its utility in advanced photonic applications such as infrared camouflage1–3, thermos-photovoltaics4,5, and imaging6–9. Recent advances in nanophotonics10–14, particularly in metasurfaces and photonic crystals (PhCs), have enabled the manipulation of phase, amplitude, and polarization of ThE at subwavelength scales15–22. This critical transition from disordered to programmable ThE has significantly enhanced the potential of thermal emitters in functional light sources23,24, on-chip thermophotonic control23,25, and quantum thermo-photonics8. Furthermore, the introduction of high-Q quasi-bound states, including quasi-guided modes (QGMs) induced by band-folding engineering26–28 and quasi-bound states in the continuum (QBICs) supported by conventional metasurface29,30, has recently propelled the development of coherent ThE. These resonances, characterized by high-radiative-Q (Qrad), can be efficiently excited by thermal fluctuations to produce nearly monochromatic, highly directional, and coherent ThE, thus providing a powerful mechanism to overcome the limitations of classical blackbody radiation.

The pursuit of spectrally coherent thermal sources traces back to the pioneering concepts of photonic band engineering. Seminal works by Yablonovitch31 and John32 first proposed that periodic dielectric structures (PhCs) could inhibit spontaneous emission within a frequency band gap, thereby suppressing broadband ThE. Building on this foundation, researchers demonstrated that introducing structural defects or exploiting band-edge modes could effectively channel the thermal energy into narrow spectral bands. For instance, Lin et al.33 experimentally realized highly coherent ThE from the band edge of 3D tungsten PhCs. Similarly, theoretical and experimental studies on defect modes in photonic lattices, such as those by Sigalas et al.34 and Sang et al.35, confirmed that breaking the periodicity can induce sharp transmission peaks within the forbidden gap. Additionally, Ben-Abdallah et al.36 demonstrated that single-defect Bragg stacks could enable high-power and narrow-band emission attributed to the coupling between defect states and surface waves. These findings collectively highlight the potential of band-gap-engineered structures in achieving dynamic and spectrally selective ThE, providing a strong foundation for high-coherence thermal sources in advanced applications.

Precise control of the polarization state of ThE, especially achieving circular polarization (CP), remains a major challenge in thermo-photonics. This difficulty mainly arises from the need to maintain a stable and controllable phase difference between two orthogonal polarization components. However, CP thermal emitters are crucial for applications such as circular dichroism (CD) spectroscopy15,37, polarization-encoded communication6, and high-resolution thermal imaging38, where distinguishing left-handed CP (LCP) and right-handed CP (RCP) states boost device performance and information fidelity. While various approaches, including chiral metamaterials17,39–42, plasmonic structures37,38,43, and nano-antennas44, have been explored to generate CP-ThE, most existing designs are static, emitting only one fixed helicity at a preset wavelength without dynamic tuning capability. Consequently, practical systems often require multiple devices customized for specific wavelengths and helicities, increasing system complexity, footprint, and fabrication demands. These challenges hinder the development of compact, low-power, and reconfigurable photonic systems45.

To date, achieving dynamic helicity switching of CP-ThE at a desired wavelength with a single device remains a significant challenge39,45,46. Conventional polarization control techniques, such as modulating structural chirality using active materials like phase-change materials (PCM)47, liquid crystals48, and DNA origami49,50, have mostly been explored outside the realm of ThE. Yet, these designs are typically limited to static, wavelength-locked responses. Moreover, the substantial optical losses inherent to these materials—for example, the large absorption in one phase of PCM—impose fundamental constraints on achievable resonance linewidths, posing major obstacles to their deployment as efficient and coherent thermal emitters.

In this work, we propose and experimentally demonstrate an anisotropic nonlocal thermal metasurface that enables helicity-switchable and wavelength-tunable coherent CP-ThE in the mid-infrared (MIR) regime using a single device. Our structure starts with a regular array of Ge bricks, where the Ge layer is vertically separated from the metallic substrate by a low-refractive-index Al2O3 spacer, forming a standard metal–dielectric–semiconductor (MDS) multilayer architecture. By introducing lattice perturbations and mirror symmetry breaking, the original dispersion band of GMs lying below the light cone are folded into the radiation continuum51,52, giving rise to two high-Q QGMs at distinct frequencies with opposite chiral responses. Different types of perturbations within the structures serve as effective control knobs to tailor both the Q-factors of the resonant modes and the polarization states of the emitted radiations. This enables continuous tunability from LCP to linear polarization (LP), and then to RCP. The key enabler of our thermal tunability lies in the high-Q, helicity-selective QGMs, which leverage the strong thermo-optic effect exhibited by high-quality Ge films deposited by electron-beam evaporation. This provides a robust thermal tuning mechanism without any structural reconfiguration. Experimentally, we showcase helicity-switchable, highly coherent (Q > 150), and strongly chiral (emission circular dichroism, ECD > 0.8) CP-ThE, achieving a helicity-reversal spectral tuning range approaching 100 nm through temperature modulation alone. It’s also predicted that the helicity-switching coverage bandwidth can be extended to ~500 nm over a broader temperature range. This platform surpasses the limitations of single-emitter systems in polarization and spectral control, delivering a compact, thermally tunable, and chirality-reconfigurable mid-IR coherent source with strong potential for chiral sensing53, enantiomer identification54, information encoding38, and polarization imaging7.

Results
Working principle and numerical simulations
For MIR ThE applications, Fig. 1a, b illustrates and compares the real part (n) and the imaginary part (k) of the refractive index of two different materials as a function of temperature. Taking the traditional PCM of VO2 as an example, as the temperature increases, it undergoes a transition from an insulating state at low temperatures to a metallic state at high temperatures, leading to a significant increase in the material’s absorption loss (γabs)55. This change not only substantially reduces the temporal coherence of the ThE from a metasurface made from VO2, but also causes a rapid mismatch between the structural radiation loss (γrad) and γabs, resulting in a dramatic decrease in emissivity56. Furthermore, the refractive index of PCM typically undergoes rapid changes within a very narrow temperature range, which presents significant challenges: nonlinear spectral tuning of the emission with temperature makes accurate spectral tuning of ThE require precise temperature control, complicating the whole system and yielding only a narrow operational window. In contrast, Ge, with its high refractive index and extremely low extinction coefficient (crystalline: k = 0; amorphous: k < 10-3)57, remains transparent in the mid-IR range, as shown in Fig. 1b. In particular, its significant thermo-optic effect manifests as a nearly linear relationship between the real part of the refractive index and temperature. This characteristic not only ensures the temporal coherence of the ThE but also provides excellent freedom for spectral tuning, greatly enhancing the flexibility and stability of temperature control.

Figure 1c delineates the underlying principle of CP-ThE and helicity switching within our system. The device architecture features an optically thick gold layer at the bottom to prevent light transmission58 and ensure unidirectional emission. The topmost layer consists of a symmetry-breaking, partially etched Ge double-brick metasurface supporting nonlocal high-Q resonances. These two layers are separated by a low-refractive-index Al2O3 (n ≈ 1.55) spacer, which exhibits negligible absorption in the targeted spectral range59. The thickness of the spacer is optimized to facilitate impedance matching of the entire structure to vacuum and minimize the influence of metal intrinsic losses on the upper nonlocal modes, thereby enhancing the temporal coherence of ThE27. When an electric current is applied to the bottom metal layer to heat the entire device, thermal fluctuations couple into the nonlocal high-Q chiral modes of the upper Ge structure, leading to a significant enhancement of ThE at the target wavelength. Importantly, the emitted CP-ThE can repeatedly switch between LCP and RCP at the target wavelength in response to temperature changes, as indicated by the cyclic arrows in Fig. 1c. The inset on the right shows a top-view of the geometric structure. To further elucidate the thermal tuning mechanism, Fig. 1d presents the temperature-dependent refractive index of the amorphous Ge film due to the thermo-optic effect. The significant thermo-optic coefficient (dnGe/dT ≈ 7.6 × 10-4 K-1) establishes temperature as a key active tuning parameter in our system. This result is obtained by fitting the refractive index based on spectral measurements of ThE at different temperatures. Building on this effect, Fig. 1e outlines the helicity-switching mechanism: at a given temperature T0, the device supports CP-ThE with opposite helicities at different wavelengths. As the temperature increases, the resonance wavelength experiences a redshift induced by refractive index modulation, leading to helicity reversal at the target wavelength λ0. This indicates that only a small temperature difference ΔT suffices to dynamically switch the emission helicity. Moreover, Fig. 1f shows that such a thermally induced helicity reversal can occur over a broad spectral range, enabling dynamic and wavelength-tunable control of CP-ThE. Collectively, these findings demonstrate the feasibility of our single-device platform for achieving highly emission circular dichroism (ECD = (IR - IL)/(IR + IL), where IR and IL represent the intensities of RCP and LCP-ThE, respectively) and coherent ThE with dynamically switchable helicity and wide wavelength tunability in the MIR regime. It is worth noting that the choice of Ge represents a strategic trade-off between optical performance, thermal sensitivity, and material stability. Although silicon is a mature alternative material with a stable thermo-optic coefficient60, it has a lower refractive index and a narrower transparent range (i.e., the range where optical absorption is negligible) in the MIR region. On the other hand, alternative materials such as chalcogenide glasses or lead salts (e.g., PbTe) exhibit remarkably high refractive indices and large thermo-optic responses. However, their application is often constrained by lower thermal stability (e.g., low glass transition temperatures or phase-change risks) and complex fabrication requirements incompatible with standard processes61. In contrast, Ge offers an optimal balance: it combines a high refractive index for strong mode confinement and a large thermo-optic coefficient for efficient tuning within a broader transparent range, all while maintaining excellent thermal stability and fabrication maturity.

Figure 2a schematically depicts the design process of the metasurface architecture that enables the aforementioned functionalities. It starts with a regular array of partially etched Ge bricks, with periodicities a and b along the x and y directions, respectively, as shown in the left panel of Fig. 2a. The geometric parameters are defined as follows: a = 1,645 nm, b = 2,838 nm, w = 980 nm, L = 1,822.5 nm, h = 1,070 nm, h1 = 220 nm, and t = 860 nm. The regular Ge brick array with a period of a supports a series of bound GMs below the light cone along the ΓX direction, as seen in the shaded region of Fig. 2b (left). These bands exhibit infinite radiation Q factors, indicating ideal mode confinement51,52. Among these, two modes of interest are highlighted with colored solid lines. By introducing structural perturbations such as a lateral shift dx between adjacent blocks or a modulation in block length δ, the unit cell’s periodicity along the x direction is doubled from a to 2a. This causes the first Brillouin zone (BZ) to shrink, as shown in the middle panel of Fig. 2a, resulting in the folding of modes from the X point to the Γ point. Consequently, two QGMs (QGM1 and QGM2) emerge, as shown in the right panel of Fig. 2b, both of which maintain high but finite Q factors. Importantly, the Q factors of QGM1 and QGM2 at the Γ point (Fig. 2c) can be independently and continuously tuned by adjusting dx and δ, respectively. Additionally, under fixed perturbations, stable high-Q factors are maintained across a wide wavevector range in the band (Fig. S1). This demonstrates that the temporal coherence of the ThE can be effectively controlled through structural geometry. To further introduce optical chirality, we incorporated an additional structural perturbation dy along the y direction to disrupt the mirror symmetry of the Ge block array. With fixed parameters dx = 100 nm and δ = 258 nm, we calculated the degree of circular polarization40 (DoCP = S3/S0) for the two QGMs as a function of dy and the residual Ge film thickness h1, as shown in Fig. 2d. The results reveal that the two modes consistently exhibit opposite helicities across all tested parameters, with corresponding far-field polarization patterns throughout the design process presented in Fig. S2. Notably, within a broad parameter space marked by the white dashed box, both modes achieve DoCP values exceeding 0.98 in absolute magnitudes, indicating the device’s robustness to fabrication-induced geometric variations while achieving strong chiral ThE. This high DoCP is realized using a simple structural design, avoiding complex fabrication techniques such as oblique-angle etching47,62,63 or multilayer alignment64, highlighting the practicality and scalability of the approach.

Notably, by simply adjusting the magnitude of the structural perturbation dy, the ThE can be continuously tuned with the polarization states across a full circle on the Poincaré sphere, as shown in Fig. 2e. This enables a smooth transition from LCP to LP, and then to RCP. This highlights the capability to access the full-Stokes parameter space through simple geometric design. In contrast, helicity switching in a single device is solely induced by temperature changes, leveraging the inherent temperature dependence of ThE. This approach eliminates the need for external electrical or mechanical actuation, offering flexible control over the polarization state of emitted radiation.

For the final device design, we selected the geometric parameters corresponding to the white pentagram on the right in Fig. 2d (h1 = 220 nm, dy = 180 nm). Initially, we calculated the emissivity spectra of the two CP modes at different temperatures, as shown in Fig. 3a, b. At a temperature of 458.15 K, the device exhibits LCP and RCP ThE at 5.22 μm and 5.32 μm, respectively, each dominated by distinct dipolar resonances (Fig. S3). The pronounced contrast between opposite helicities reflects the high purity of either LCP or RCP, consistent with the calculated DoCP presented in Fig. 2d. When the temperature increases to 578.15 K, thermo-optic effects induce a refractive index change, resulting in an overall redshift of the emission spectrum. Consequently, the LCP emission originally located at 5.22 μm redshifts to 5.32 μm, enabling helicity switch of the CP-ThE at this target wavelength. The insets and electric-field profiles in Fig. S3 further confirm the distinct modal characteristics associated with the emitted radiations at different temperatures. These field distributions highlight the helicity-dependent field enhancements and demonstrate that each mode is decoupled from the opposite CP (Fig. S4). In addition, the corresponding spectra exhibit ultra-narrow linewidths and a highly directional emission profile (Fig. S5), indicative of their high temporal and spatial coherence. We further performed numerical simulations to evaluate the variation of the emission wavelength as a function of the temperature, as shown in Fig. 3c, where the color amplitude represents the value of the ECD. The results indicate that, within a temperature range from 323.15 K to 1023.15 K (safely distant from the melting point of Ge), helicity switching with high ECD can be achieved across a broadband tuning window of 485 nm. These results clearly confirm all the key functional features introduced in Fig. 1. Such broadband, thermally reconfigurable chiral emission demonstrates the potential of this platform for the dynamic control of spin-dependent thermal photonics and next-generation polarization-selective infrared technologies.

Experimental demonstration
Based on the above design, we successfully fabricated the thermal emitter using standard nanofabrication techniques (see Fig. S7 for more details). We note that all the films in the fabricated sample were deposited via evaporation, resulting in an amorphous state. This introduces more dissipation than the crystalline materials used in simulations. Figure 4a presents top- and cross-sectional scanning electron microscope (SEM) images of the fabricated device, revealing excellent structural integrity and high fabrication precision. The experimental setup for characterizing the ThE signals is schematically presented in Fig. 4b. The sample was mounted on a precision three-axis rotational stage (JCOPTIX, China) equipped with an integrated heating system, enabling angle-resolved ThE measurements with high accuracy. Two coaxial apertures were positioned along the emission path to effectively suppress background radiation, thereby improving the measurement precision. A quarter-wave plate and a rotatable linear polarizer (JCOPTIX, China) were used to analyze the polarization states of the emitted radiation, which was finally collected and analyzed using a Fourier-transform infrared (FTIR) spectrometer equipped with a mercury cadmium telluride (MCT) detector.

At a temperature of 458.15 K, the angle-resolved ThE spectra for both RCP and LCP components were measured, as shown in Fig. 4c, d. The experimental results align quite well with the numerical simulations (Supplementary Fig. S6), further confirming the robustness of the device design, the reliability of the fabrication process, and the compatibility of the structure with standard nanofabrication workflows. The measured spectra clearly reveal distinct CP-ThE peaks at 5.243 μm and 5.321 μm, corresponding to LCP and RCP outputs, respectively. To further analyze the coherence properties and polarization selectivity, the normal-direction ThE spectra for both helicities were extracted and fitted using Lorentzian functions. As illustrated in Fig. 4e, the results confirm ultrahigh temporal coherence (Q > 150) along with strong helicity contrast (ECD > 0.8) for the emitted radiations. To experimentally demonstrate dynamic helicity switching at the target wavelength of 5.321 μm, the device temperature was increased to 578.15 K. As a result of the thermo-optic effect, the emission spectrum undergoes a noticeable redshift. Consequently, the LCP emission mode originally located at 5.243 μm shifts precisely to 5.321 μm, enabling a helicity inversion at the target wavelength of 5.321 μm. Figure 4h displays the extracted normal-direction emissivity spectra and corresponding fits at 578.15 K for both helicities, further confirming that both high temporal coherence and strong helicity contrast are retained throughout the switching process.

More importantly, the helicity switching of the thermal emitter is highly reversible. By simply heating or cooling the device, the CP-ThE at a certain target wavelength can be reversibly switched between the RCP and LCP states. This behavior is in excellent agreement with the numerical predictions shown in Fig. 3a, b, further confirming the effectiveness of our structural design in enabling tunable chiral emission in the MIR regime.

We further investigated the temperature-dependent spectral response of CP-ThE within the operational range of our heating stage. Figure 5a visually illustrates the broadband tunability of RCP and LCP ThE supported by two modes in a single device. Additionally, the shaded gray regions highlight how helicity switching can be achieved across a broad wavelength range (Δλ) by tuning the temperature. These features were experimentally verified, with the measured results presented in Fig. 5b, c. The experimental findings indicate that, in the temperature range from 398.15 K to 648.15 K, the center wavelengths of the LCP and RCP ThEs exhibited significant redshifts of approximately 169 nm and 182.5 nm, respectively. By extracting the emission peak wavelengths at various discrete temperatures and performing linear fitting (Fig. 5d, e), we found that the RCP and LCP ThEs exhibit temperature dependencies as high as 0.675 nm/K and 0.73 nm/K, respectively. The shaded gray regions in Fig. 5d, e highlight the spectral window within which helicity switching occurs, covering a broad range of approximately 95.6 nm through a temperature increase of 250 K, thereby enabling reversible switching between RCP and LCP-ThE states over such a substantial bandwidth. To further elucidate the polarization performance across this temperature-tunable range, Fig. 5f provides a detailed analysis of the ECD values under various thermal conditions. Notably, due to thermal background noise, the ECD slightly degrades at lower temperatures. However, even in the presence of such thermal fluctuations, both RCP and LCP states maintain high ECD values within the helicity-switchable spectral window. The onset and endpoint of these solid lines correspond to the spectral boundaries within which helicity switching of the CP-ThE has been experimentally realized. It is important to underscore that, despite the temperature limitations of our experimental setup, the device successfully demonstrates reversible switching between two high-ECD helicity states across a nearly 100-nm-wide MIR band. More significantly, by employing a heating apparatus capable of a broader temperature range of operation, the spectral bandwidth over which helicity switching occurs is expected to expand further—potentially reaching nearly 500 nm, as suggested by the trend illustrated in Fig. 3c.

Collectively, these results validate the capability of our single metasurface to achieve reversible helicity switching of ThE across a wide MIR spectral window, governed solely by thermal excitation. The observed large and tunable spectral shifts, combined with robust helicity contrast, highlight the device’s highly responsive thermal behavior with strong potential for application in reconfigurable infrared photonic systems, such as polarization-selective thermal sources, infrared displays, or security tagging, aligning well with the performance metrics expected for integration into practical thermal photonic platforms.

It is important to emphasize that switching speed in our current experimental setup was limited by the thermal inertia of the external heating stage (Bruker, A540). However, in practical applications, rapid temperature modulation rates of 104~105 K/s can be achieved by integrating micro-heaters for Joule heating65, which would significantly facilitate fast spectral adjustment and helicity switching speed. Regarding the comparison with electrically reconfigurable metasurfaces66, a trade-off exists between speed and structural complexity. While electrical modulation schemes typically offer shorter response times, they require complex electrode networks and biasing circuits. In contrast, although the response speed of thermally tunable devices is constrained by thermal inertia, a key advantage lies in their simplicity and scalability. Since our approach does not require pixel-level wiring, it holds great potential for cost-effective, large-scale deployment. Crucially, for primary target applications such as MIR chiral sensing and molecular spectroscopy, the modulation speed offered by thermal tuning is sufficient. In these scenarios, spectral coverage and high emission quality are often more critical than nanosecond-level switching speeds.

Additionally, all results presented above focus on the high-temperature regime. As the temperature decreases, particularly approaching the zero-temperature limit, the proposed tuning mechanism faces two fundamental constraints. First, the thermo-optic coefficient of Ge diminishes significantly as phonon populations freeze out and lattice anharmonicity weakens, rendering the refractive index insensitive to temperature changes. Second, and more critically, governed by Planck’s law, the peak of the blackbody radiation shifts toward the far-infrared and millimeter-wave regions. Consequently, the ThE intensity within the target MIR window vanishes exponentially, making the device effectively inactive at cryogenic temperatures.

Methods
Numerical simulations
All numerical simulations were performed using the finite element method in the commercial software COMSOL Multiphysics. Floquet periodic boundary conditions were implemented in the lateral directions of a unit cell to define the wave vectors, while perfectly matched layers were employed in the z direction to mimic a non-reflecting infinite domain.

Fabrication
First, a 5-nm-thick adhesion layer of Ti, a 150-nm-thick Au film, and another 3-nm-thick Ti were successively deposited onto a Si substrate using electron beam evaporation. After that, two layers of 860-nm-thick Al2O3 and 1070-nm-thick Ge were deposited onto the sample using the same equipment. Then, the 180-nm-thick PMMA resist layer was spin-coated onto the sample, and the designed structure pattern was exposed onto the PMMA layer by an electron beam lithography system. After developing, a 30-nm-thick Al2O3 was evaporated onto the sample and the lift-off process was used to transfer the pattern from PMMA to Al2O3 layer. The Al2O3 pattern worked as a mask to obtain the Ge structure in a subsequent inductive coupled plasma-enhanced reaction ion etching process, and finally the residual Al2O3 was completely removed by soaking in NaOH solution for 60 s and cleaned with deionized water.

Optical measurements
The emissivity of the fabricated samples was measured using a Fourier transform infrared spectrometer (FTIR) (Bruker Vertex 80V) equipped with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector. The emissivity measurements were carried out by loading the sample onto an emission adapter (Bruker, A540), which was employed to heat the sample at different temperatures to induce thermal emissions. Steel plates of the same size, blackened by the black soot of candles, were used as the standard blackbody reference. Both the blackbody reference and the fabricated sample were mounted onto a temperature-controlled heating stage. When the blackbody and the sample were heated to above 100 °C, the emitted power was directed into the FTIR system and detected by an MCT detector. This comparative measurement setup ensures high accuracy in the emission measurements. All spectra were acquired using the same FTIR spectrometer with a spectral resolution setting of 2 cm−1, which is sufficient to resolve the high-Q resonant features. To balance the signal-to-noise ratio and acquisition time, 15 scans were averaged for each measurement. For clarity, the data are presented in wavelength units (μm) after conversion from wavenumber (cm−1).

Data availability
The data supporting the findings of this study are available within the paper and its supplementary information.

Code availability
The numerical simulations in this work were performed using the commercial software COMSOL Multiphysics. All relevant setup and configuration details are provided in the Methods section and the main text. Additional information regarding the simulation models is available from the corresponding author (Z.H.) upon reasonable request.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In the present manuscript, the authors theoretically propose and experimentally demonstrate a nanophotonic platform (an anisotropic and nonlocal metasurface) capable to perform reconfigurable circularly polarized thermal emission in the mid-infrared regime. The proposed system enables dynamic and fine control over several key features of thermal emission, including temporal (wavelength-tunable), spatial (highly directional), and circular polarization (helicity-switchable) coherence properties. The main results of the paper can be summarized as follows:\n- Fundamental aspect: realization and demonstration of an insightful mechanism, based on the thermo-optic effect, enabling the emission and switching of circularly polarized thermal radiation with dynamic control of coherence properties, without requiring structural reconfiguration;\n- Experimental aspect: experimental validation of the proposed concept, showing excellent agreement between numerical simulations and practical measurements over a broad wavelength window and temperature range;\n- Applied aspect: compact and simple structural design, together with robustness against fabrication imperfections, thereby suggesting feasible and scalable realistic implementation of the proposed system in a single device.\nIn my opinion, these results represent an original and significant contribution to the fields of nanophotonics, thermal emission, and metamaterials, both from fundamental and applied perspectives, paving the way toward new avenues for controlling thermal emission properties and developing spin-dependent thermal functionalities. Overall, the manuscript is very well written, and despite its technical depth, the main ideas are clearly conveyed and easy to follow. I found it a pleasure to read and therefore strongly recommend this work for publication in Nature Communications in its current form.\nNevertheless, to further improve the manuscript, I suggest the authors address the following minor points:\n1. For completeness, the reference list in the Introduction section could be slightly expanded and updated by considering some recent and relevant works in the area of thermal photonics and circularly polarized thermal emission, for example:\n- C. Khandekar and Z. Jacob, Phys. Rev. Appl. 12, 014053 (2019);\n- T. Liu et al., eLight 2, 25 (2022);\n- X. Wang and Z. Jacob, Light Sci. Appl. 11, 342 (2022);\n- X. Wang et al., Sci. Adv. 9, eade4203 (2023);\n- M. F. Picardi et al., J. Appl. Phys. 133, 111101 (2023).\nOf course, authors are free to considering whether including (or not) some of the above-mentioned references to improve the quality and clarity of their explanations.\n2. The choice of material appears to play a crucial role in enabling the thermal tuning mechanism. Since the analysis only focuses on Ge due to its large thermo-optic coefficient, a brief discussion of alternative material platforms, including their potential advantages and limitations, would be valuable.\n3. The authors state that the results can be extrapolated to higher temperatures. Conversely, it would be interesting to comment on the behavior at lower temperatures, particularly approaching the zero-temperature limit, where the thermo-optic effect may become negligible. A brief discussion on this issue could provide additional physical insight.\n4. Finally, a few minor typographical errors should be corrected:\n- Line 129: \"[...] cyclic arrows in Fig. 1a [...]\", should read as \"[...] cyclic arrows in Fig. 1c [...]\";\n- Line 149: \"[...] refractive index based on based on spectral [...]\", should read as \"[...] refractive index based on spectral [...]\";\n- Line 283: \"[...] regions in Figs. 5b and c highlight [...]\", should read as \"[...] regions in Figs. 5e and h highlight [...]\".\nRespectfully, I hope these comments may help the authors further improve the manuscript.", "reviewer_2": "This work introduce a single thermal metasurface capable of generating coherent mid-infrared circularly polarized thermal emission with temperature-controlled wavelength tunability and reversible helicity switching. The proposed device is based on a Ge anisotropic metasurface supporting two high-Q quasi-guided modes of opposite chirality, whose resonance wavelengths are tuned via the strong thermo-optic effect of Ge. This device enables LCP–RCP switching at a fixed wavelength without the use of active materials or external actuation. Experimental results demonstrate high temporal coherence, strong circular dichroism and tunable helicity switching, in good agreement with numerical simulations. Overall, the manuscript presents an original and promising approach toward compact, passive and dynamically reconfigurable chiral thermal emitters.\nThe work is of clear fundamental interest and shows potential for applications across several technological domains. Therefore, it could be suitable for publication in Nature Communications. However, I recommend that the following points be addressed before final consideration\n1) The bibliography concerning earlier work on spectrally coherent thermal sources (for example those based on edge modes, defect modes, photonic bandgaps and related concepts) should be expanded. In particular, the following seminal works should be discussed and properly cited:\nE. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987).\nS. John, Phys. Rev. Lett. 58, 2486 (1987).\nM. M. Sigalas et al., Microwave Opt. Technol. Lett. 15, 153 (1997).\nS. Y. Lin, J. G. Fleming, and I. El-Kady, Appl. Phys. Lett. 83, 593 (2003).\nH. Sang, Z. Y. Li, and B. Y. Gu, Phys. Lett. A 331, 414 (2004).\nP. Ben-Abdallah and B. Ni, J. Appl. Phys. 97, 104910 (2005).\n2) The authors should provide a quantitative comparison of the degree of temporal coherence achieved in this work with that reported for previously demonstrated spectrally coherent thermal emitters.\n3) The assumption of infrared transparency of the Al₂O₃ spacer (n ≈ 1.55) should be explicitly justified in the relevant mid-infrared wavelength range.\n4) The geometry and material composition of the thermal source should be described more clearly in the main text. In particular, Fig. 1, which illustrates the helicity-switching principle, should be revised to provide a clearer and more explicit representation of the structure. This point is important and requires clarification.\n5) The authors should discuss the dynamic response of the device, including estimates or direct measurements of the thermal inertia and helicity switching time, which are critical parameters for assessing practical applicability. In addition, the performance of the proposed thermal switching approach should be quantitatively compared with that of electrically reconfigurable metasurfaces, particularly in terms of response speed and modulation capability.\n6) The manuscript should clarify the experimental uncertainties, including temperature stability, spectral resolution, polarization analysis accuracy and emissivity extraction errors.", "reviewer_3": "Dear Editor and Authors:\nI have carefully read the manuscript by Sun et al. titled “Helicity-selective and spectrally tunable chiral thermal emissions”.\nThe authors report results on a smartly designed metasurface based on a multilayer of gold, alumina and germanium that allows switching the helicity of the thermal emission at a given wavelength by increasing or decreasing the device temperature. They start by describing the working principle of their device, based on the combination of the thermo-optical effect in Ge and the excitation of quasi guided modes with different helicity and different resonant wavelengths. They perform exhaustive simulations to characterize their device and how the different geometrical parameters of the unit cell imping on the thermal emission properties, specially its degree of circular polarization, and how the change in T shifts the different emission bands. Finally, they fabricate their optimal device, and experimentally proved that their metasurface effectively allows the switching of the helicity over a broad region of ~100 nm.\nThe reported results are highly interesting for the nanophotonics community, specially because they provide solid experimental evidence, back up by extensive simulations, of a reconfigurable device. Their results are sound, technically correct, and most of their conclusions are back-up by experimental/simulation evidence (see comments below for aspects to improve). The manuscript is well written and structured, its length is adequate for its content, and all the figures, which are mostly clear (see comment below for improvement), are necessary. All in all, I could recommend publication of this manuscript provided the author address the following minor issues:\n-In the abstract, the acronym CP is not defined, and it is not used again. So I would recommend writing directly circularly polarized, or whatever the authors intended to convey.\n-In the abstract (lines 28-30) and in the text (lines 180-188), it is suggested, or could be understood, that they have achieved dynamically switchable full polarization control. This is not true, as the change from CP to LP is done with the unit cell design (no reconfigurability), while the helicity switching is done with temperature changes. The authors should change the text so that this is clear and avoid confusions.\n-In the abstract (last line) and in some other parts of the text, they mention isomer identification, but, given that they are reporting a CP emitting device, they should better use enantiomer identification.\n-Figure 1f can be confusing. When I saw it first I could not understand it quite well, and I had to read the whole paper and, later, go back and understand it. It seems that at the same T and wavelength, one can obtain both LCP and RCP. The figure is very nice, but they should separate in wavelengths the contribution of LCP and RCP, as they do in Figure 3c. This is, each ECD spectra should be in a different line.\n-Regarding the figures in the supplementary material, I would suggest the authors to order them in the same way they are mentioned. I mean, Figure s1 is not mentioned until the end of the manuscript. This makes their identification confusing (see next comment).\n-In Line 166, when Figure S5 is mentioned. Is it really figure S5? In this figure there is not tuning of dx or delta whatsoever. That tuning is shown in Figure S2, but there is no Q factors there. Please, clarify. In addition, in line 209, is it figure 3 or S3?\n-Regarding the multipole decomposition of the quasi guided modes. Why is the LCP mode primary of magnetic dipole nature while the RCP is a mixture of toroidal and magnetic dipole? Please, elaborate on this point."}, "author_response": "Response to Reviewer Comments\nIn this document the reviewers' comments are pasted in blue, our response in black and the parts newly\nadded to the manuscript are in red.\nReply to Reviewer #1\nReviewer #1:\nIn the present manuscript, the authors theoretically propose and experimentally demonstrate a\nnanophotonic platform (an anisotropic and nonlocal metasurface) capable to perform reconfigurable\ncircularly polarized thermal emission in the mid-infrared regime. The proposed system enables dynamic\nand fine control over several key features of thermal emission, including temporal (wavelength-tunable),\nspatial (highly directional), and circular polarization (helicity-switchable) coherence properties. The\nmain results of the paper can be summarized as follows:\n- Fundamental aspect: realization and demonstration of an insightful mechanism, based on the thermo-\noptic effect, enabling the emission and switching of circularly polarized thermal radiation with dynamic\ncontrol of coherence properties, without requiring structural reconfiguration;\n- Experimental aspect: experimental validation of the proposed concept, showing excellent agreement\nbetween numerical simulations and practical measurements over a broad wavelength window and\ntemperature range;\n- Applied aspect: compact and simple structural design, together with robustness against fabrication\nimperfections, thereby suggesting feasible and scalable realistic implementation of the proposed system\nin a single device.\nIn my opinion, these results represent an original and significant contribution to the fields of\nnanophotonics, thermal emission, and metamaterials, both from fundamental and applied perspectives,\npaving the way toward new avenues for controlling thermal emission properties and developing spin-\ndependent thermal functionalities. Overall, the manuscript is very well written, and despite its technical\ndepth, the main ideas are clearly conveyed and easy to follow. I found it a pleasure to read and therefore\nstrongly recommend this work for publication in Nature Communications in its current form.\nNevertheless, to further improve the manuscript, I suggest the authors address the following minor\npoints:\nOur response: We sincerely thank reviewer #1 for the positive assessment and careful evaluation of\nour manuscript. We appreciate that the reviewer has accurately highlighted the fundamental,\nexperimental, and applied contributions of our work, particularly regarding the thermo-optic\nmechanism and the robustness of the proposed design. We are also encouraged by the positive remarks\non the manuscript's readability. Following the reviewer's suggestions, we have addressed the minor\npoints listed below to further improve the paper's clarity and completeness.\n1. For completeness, the reference list in the Introduction section could be slightly expanded and\nupdated by considering some recent and relevant works in the area of thermal photonics and circularly\npolarized thermal emission, for example:\n- C. Khandekar and Z. Jacob, Phys. Rev. Appl. 12, 014053 (2019);\n- T. Liu et al., eLight 2, 25 (2022);\n- X. Wang and Z. Jacob, Light Sci. Appl. 11, 342 (2022);\n- X. Wang et al., Sci. Adv. 9, eade4203 (2023);\n- M. F. Picardi et al., J. Appl. Phys. 133, 111101 (2023).\nOf course, authors are free to considering whether including (or not) some of the above-mentioned\nreferences to improve the quality and clarity of their explanations.\nOur response: We thank the reviewer for bringing these important recent works to out attention. We\nagree that these references are highly relevant and provide a broader context for our study regarding\nthermal photonics and circularly polarized thermal emission. Accordingly, we have updated the\nIntroduction to include these references, which has improved the completeness and clarity of our\ndiscussion.\nUpdated main text:\n''…have enabled the manipulation of phase, amplitude, and polarization of ThE at subwavelength\nscales15–22…''\n''…However, CP thermal emitters are crucial for applications such as circular dichroism (CD)\nspectroscopy15,37, polarization-encoded communication6, and high-resolution thermal imaging38, where\ndistinguishing left-handed CP (LCP) and right-handed CP (RCP) states boost device performance and\ninformation fidelity. While various approaches including chiral metamaterials17,39–42, plasmonic\nstructures37,38,43 and nano-antennas44, have been explored to generate CP-ThE, most existing designs are\nstatic, emitting only one fixed helicity at a preset wavelength without dynamic tuning capability.\nConsequently, practical systems often require multiple devices customized for specific wavelengths and\nhelicities, increasing system complexity, footprint, and fabrication demands. These challenges hinder\nthe development of compact, low-power, and reconfigurable photonic systems45…''\nReferences\n16. Liu, T., Guo, C., Li, W. & Fan, S. Thermal photonics with broken symmetries. eLight 2, 25 (2022).\n22. Wang, X. & Jacob, Z. Symmetry breaking in thermal photonics. Light Sci. Appl. 11, 342 (2022).\n37. Wang, X. et al. Observation of nonvanishing optical helicity in thermal radiation from symmetry-\nbroken metasurfaces. Sci. Adv. 9, eade4203 (2023).\n44. Khandekar, C. & Jacob, Z. Circularly Polarized Thermal Radiation From Nonequilibrium Coupled\nAntennas. Phys. Rev. Appl. 12, 014053 (2019).\n45. Picardi, M. F., Nimje, K. N. & Papadakis, G. T. Dynamic modulation of thermal emission—A\nTutorial. J. Appl. Phys. 133, 111101 (2023).\n2. The choice of material appears to play a crucial role in enabling the thermal tuning mechanism. Since\nthe analysis only focuses on Ge due to its large thermo-optic coefficient, a brief discussion of alternative\nmaterial platforms, including their potential advantages and limitations, would be valuable.\nOur response: We thank the reviewer for this insightful suggestion. We agree that discussing material\nselection places our work in a broader context. In the revised manuscript, we have expanded the\ndiscussion to include alternative platforms relevant to the mid-infrared regime. Specifically, we now\ncompare Ge with Silicon (standard CMOS material) and other high-index materials (e.g., chalcogenides\nor lead salts), weighing their thermo-optic coefficients against fabrication feasibility, thermal stability,\nand optical transparency.\nWe have added the following content in the revised manuscript:\n''…It is worth noting that the choice of Germanium represents a strategic trade-off between optical\nperformance, thermal sensitivity, and material stability. Although silicon is a mature alternative material\nwith a stable thermo-optic coefficient60, it has a lower refractive index and a narrower transparent range\n(i.e., the range where optical absorption is negligible) in the MIR region. On the other hand, alternative\nmaterials such as chalcogenide glasses or lead salts (e.g., PbTe) exhibit remarkably high refractive\nindices and large thermo-optic responses. However, their application is often constrained by lower\nthermal stability (e.g., low glass transition temperatures or phase-change risks) and complex fabrication\nrequirements incompatible with standard processes61. In contrast, Ge offers an optimal balance: it\ncombines a high refractive index for strong mode confinement and a large thermo-optic coefficient for\nefficient tuning within a broader transparent range, all while maintaining excellent thermal stability and\nfabrication maturity.''\nReference\n60. Della Valle, G. et al. Nonlinear Anisotropic Dielectric Metasurfaces for Ultrafast Nanophotonics.\nACS Photonics 4, 2129–2136 (2017).\n61. Ding, F., Yang, Y. & Bozhevolnyi, S. I. Dynamic Metasurfaces Using Phase‐Change Chalcogenides.\nAdv. Opt. Mater. 7, 1801709 (2019).\n3. The authors state that the results can be extrapolated to higher temperatures. Conversely, it would be\ninteresting to comment on the behavior at lower temperatures, particularly approaching the zero-\ntemperature limit, where the thermo-optic effect may become negligible. A brief discussion on this\nissue could provide additional physical insight.\nOur response: We appreciate the reviewer's suggestion to consider the low-temperature regime. We\nagree that discussing the temperature limit approaches absolute zero provides additional and valuable\ninsight. In the revised manuscript, we have added a brief discussion clarifying that both the material\nresponse (thermo-optic effect) and the radiation source (blackbody emission) diminish significantly at\ncryogenic temperatures.\nUpdated manuscript:\n''…Additionally, all results presented above focus on the high-temperature regime. As the\ntemperature decreases, particularly approaching the zero-temperature limit, the proposed tuning\nmechanism faces two fundamental constraints. First, the thermo-optic coefficient of Ge diminishes\nsignificantly as phonon populations freeze out and lattice anharmonicity weakens, rendering the\nrefractive index insensitive to temperature changes. Second, and more critically, governed by Planck’s\nlaw, the peak of the blackbody radiation shifts toward the far-infrared and millimeter-wave regions.\nConsequently, the thermal emission intensity within the target MIR window vanishes exponentially,\nmaking the device effectively inactive at cryogenic temperatures…''\n4. Finally, a few minor typographical errors should be corrected:\n- Line 129: \"[...] cyclic arrows in Fig. 1a [...]\", should read as \"[...] cyclic arrows in Fig. 1c [...]\";\n- Line 149: \"[...] refractive index based on based on spectral [...]\", should read as \"[...] refractive index\nbased on spectral [...]\";\n- Line 283: \"[...] regions in Figs. 5b and c highlight [...]\", should read as \"[...] regions in Figs. 5e and h\nhighlight [...]\".\nRespectfully, I hope these comments may help the authors further improve the manuscript.\nOur response: We thank the reviewer for the meticulous reading of our manuscript and for identifying\nthese typographical errors. All the specific errors listed above have been corrected in the revised version,\nand we have also taken this opportunity to thoroughly proofread the entire text to ensure linguistic\naccuracy.\nWe once again sincerely thank reviewer #1 for his/her time and the highly positive assessment of our\nwork. These valuable comments have helped us improve the clarity and overall quality of the\nmanuscript.\nReply to Reviewer #2\nReviewer #2\nThis work introduce a single thermal metasurface capable of generating coherent mid-infrared circularly\npolarized thermal emission with temperature-controlled wavelength tunability and reversible helicity\nswitching. The proposed device is based on a Ge anisotropic metasurface supporting two high-Q quasi-\nguided modes of opposite chirality, whose resonance wavelengths are tuned via the strong thermo-optic\neffect of Ge. This device enables LCP–RCP switching at a fixed wavelength without the use of active\nmaterials or external actuation. Experimental results demonstrate high temporal coherence, strong\ncircular dichroism and tunable helicity switching, in good agreement with numerical simulations.\nOverall, the manuscript presents an original and promising approach toward compact, passive and\ndynamically reconfigurable chiral thermal emitters.\nThe work is of clear fundamental interest and shows potential for applications across several\ntechnological domains. Therefore, it could be suitable for publication in Nature Communications.\nHowever, I recommend that the following points be addressed before final consideration\nOur response: We sincerely thank reviewer #2 for the insightful summary and positive assessment of\nour work. We heavily appreciate the recognition of the fundamental interest and potential applications\nof our proposed thermal metasurface. In response to the reviewer's recommendations, we have carefully\naddressed all the points raised below and revised the manuscript accordingly.\n1. The bibliography concerning earlier work on spectrally coherent thermal sources (for example those\nbased on edge modes, defect modes, photonic bandgaps and related concepts) should be expanded. In\nparticular, the following seminal works should be discussed and properly cited:\nE. Yablonovitch, Phys. Rev. Lett. 58, 2059 (1987).\nS. John, Phys. Rev. Lett. 58, 2486 (1987).\nM. M. Sigalas et al., Microwave Opt. Technol. Lett. 15, 153 (1997).\nS. Y. Lin, J. G. Fleming, and I. El-Kady, Appl. Phys. Lett. 83, 593 (2003).\nH. Sang, Z. Y. Li, and B. Y. Gu, Phys. Lett. A 331, 414 (2004).\nP. Ben-Abdallah and B. Ni, J. Appl. Phys. 97, 104910 (2005).\nOur response: We sincerely thank the reviewer for referring us to these seminal works. We agree that\nincorporating these pioneering works on photonic bandgaps, defect modes, and edge states is essential\nfor establishing the proper historical and physical context of our study. In the revised manuscript, we\nhave expanded the discussion in the Introduction to include and properly cite these references. These\nadditions clarify the connection between our approach and prior foundational studies, thereby\nstrengthening the background and motivation of our manuscript.\nWe have added the following content in the revised manuscript:\n''…The pursuit of spectrally coherent thermal sources traces back to the pioneering concepts of\nphotonic band engineering. Seminal works by Yablonovitch31 and John32 first proposed that periodic\ndielectric structures (photonic crystals) could inhibit spontaneous emission within a frequency band gap,\nthereby suppressing broadband thermal radiation. Building on this foundation, researchers\ndemonstrated that introducing structural defects or exploiting band-edge modes could effectively\nchannel the thermal energy into narrow spectral bands. For instance, Lin et al.33 experimentally realized\nhighly coherent thermal emission from the band edge of 3D tungsten photonic crystals. Similarly,\ntheoretical and experimental studies on defect modes in photonic lattices, such as those by Sigalas et\nal.34 and Sang et al.35, confirmed that breaking the periodicity can induce sharp transmission peaks\nwithin the forbidden gap. Additionally, Ben-Abdallah et al.36 demonstrated that single-defect Bragg\nstacks could enable high-power and narrow-band emission attributed to the coupling between defect\nstates and surface waves. These findings collectively highlight the potential of band-gap-engineered\nstructures in achieving dynamic and spectrally selective ThE, providing a strong foundation for high-\ncoherence thermal sources in advanced applications.''\nReferences\n31. Yablonovitch, E. Inhibited Spontaneous Emission in Solid-State Physics and Electronics. Phys.\nRev. Lett. 58, 2059–2062 (1987).\n32. John, S. Strong localization of photons in certain disordered dielectric superlattices. Phys. Rev. Lett.\n58, 2486–2489 (1987).\n33. Lin, S.-Y., Fleming, J. G. & El-Kady, I. Experimental observation of photonic-crystal emission\nnear a photonic band edge. Appl. Phys. Lett. 83, 593–595 (2003).\n34. Sigalas, M. M. et al. Dipole antennas on photonic band-gap crystals: Experiment and simulation.\nMicrow. Opt. Technol. Lett. 15, 153–158 (1997).\n35. Sang, H.-Y., Li, Z.-Y. & Gu, B.-Y. Stack-sequence dependent defect modes in one-dimensional\nphotonic crystals. Phys. Lett. A 331, 414–422 (2004).\n36. Ben-Abdallah, P. & Ni, B. Single-defect Bragg stacks for high-power narrow-band thermal\nemission. J. Appl. Phys. 97, 104910 (2005).\n2. The authors should provide a quantitative comparison of the degree of temporal coherence achieved\nin this work with that reported for previously demonstrated spectrally coherent thermal emitters.\nOur response: We thank the reviewer for this important suggestion. In response, we have added a\ncomparative table in the Supporting Information summarizing previously reported spectrally coherent\nthermal emitters, including their temporal coherence (Q-factor), spatial coherence, emission circular\ndichroism, and helicity-switching capability. This comparison provides a clear quantitative benchmark\nand highlights the distinguishing features—particularly the dynamic functionality—of our approach.\nUpdated main text:\n''…It is important to emphasize that this work primarily aims to demonstrate the unique capability of\ndynamic switching of CP states in coherent ThE (see table S1 in the Supplementary Information),\nfeaturing high ECD and broadband spectral tunability within a single device…''\nWe have added the following content in the Supplementary Information:\n''…4. Comparison to previously reported platforms\nTable S1 provides a detailed comparison of previously reported circularly polarized thermal\nemitters, benchmarking our device against existing platforms in the literature. It is worth noting that\nwhile some previous studies have demonstrated large Q-factors and high ECD values, few have realized\nbroad spectral tunability within a single device. More importantly, no single thermal emitter has yet\nachieved active helicity switching at a target frequency. Our work not only demonstrates superior\nperformance across these standard metrics but also provides a novel pathway for achieving reversible\nhelicity control and broadband tunability within a single device.\nSystem Normal output Q factor ECD Spectrally Tunable Helicity switching Ref.\nSIC Antenna No ~ 100 -- No No [2]\nSiC Grating No ~ 190 ~ 0.5 No No [3]\nZ-Shaped Platinum Metasurface Yes < 7 ~ 0.38 No No [4]\nF-Shape Gold Metasurface Yes ~ 10 ~ 0.6 No No [5]\nGeometric-Phase Si Metasurface Yes 128 ~ 0.73 Yes No [6]\nZ-Shaped Ge Metasurface Yes 211 0.835 Yes No [7]\nGe Corrugated Waveguide Metasurface Yes 304 0.91 Yes No [8]\nLC Shaped Gold Metasurface Yes ~ 15 0.4 No No [9]\nThis\nGe Brick Metasurface Yes > 150 > 0.8 Yes Yes\nwork\nTable S1 | Comparison of normal output capability, Q-factor, ECD, spectral tunability, and helicity\nswitching of CP thermal emission devices…''\nReferences\n2. Shitrit, N. et al. Spin-Optical Metamaterial Route to Spin-Controlled Photonics. Science 340, 724–\n726 (2013).\n3. Dahan, N., Gorodetski, Y., Frischwasser, K., Kleiner, V. & Hasman, E. Geometric Doppler Effect:\nSpin-Split Dispersion of Thermal Radiation. Phys. Rev. Lett. 105, 136402 (2010).\n4. Nguyen, A. et al. Large circular dichroism in the emission from an incandescent metasurface.\nOptica 10, 232 (2023).\n5. Wang, X. et al. Observation of nonvanishing optical helicity in thermal radiation from symmetry-\nbroken metasurfaces. Sci. Adv. 9, eade4203 (2023).\n6. Nolen, J. R., Overvig, A. C., Cotrufo, M. & Alù, A. Local control of polarization and geometric\nphase in thermal metasurfaces. Nat. Nanotechnol. 19, 1627–1634 (2024).\n7. Sun, K., Yang, B., Cai, Y., Kivshar, Y. & Han, Z. Circularly polarized thermal emission driven by\nchiral flatbands in monoclinic metasurfaces. Sci. Adv. 11, eadw0986 (2025).\n8. Sun, K. et al. Full polarization and high coherence control of thermal emissions via saddle-band\ndispersion engineering. Nat. Commun. 16, 8393 (2025).\n9. Chen, T. et al. Thermal Digital Imaging by Chirally Coded Metasurfaces. Laser Photonics Rev. 19,\n2500129 (2025).\n3. The assumption of infrared transparency of the Al₂O₃ spacer (n ≈ 1.55) should be explicitly justified\nin the relevant mid-infrared wavelength range.\nOur response: We appreciate the reviewer's attention to the material properties. We agree that\njustifying the transparency of the Al₂O₃ spacer is essential for the accuracy of our model. In the revised\nmanuscript, we have explicitly cited experimental optical constants (Ref. [59]) to confirm that Al O\n2 3\nexhibits negligible absorption and a stable refractive index within the relevant MIR band.\nUpdated main text:\n''…These two layers are separated by a low-refractive-index Al O (n ≈ 1.55) spacer, which exhibits\n2 3\nnegligible absorption in the targeted spectral range59…''\nReference\n59. Franta, D., Nečas, D., Ohlídal, I. & Giglia, A. Dispersion model for optical thin films applicable in\nwide spectral range. Proc SPIE 9628, 96281U (2015).\n4. The geometry and material composition of the thermal source should be described more clearly in\nthe main text. In particular, Fig. 1, which illustrates the helicity-switching principle, should be revised\nto provide a clearer and more explicit representation of the structure. This point is important and requires\nclarification.\nOur response: We thank the reviewer for emphasizing this critical point. We agree that a precise\ndescription of the geometry and material composition is vital for clarity. In response, we have\nredesigned Fig. 1 to provide a more explicit representation of the structure and the helicity-switching\nprinciple. Correspondingly, we have revised the main text to clearly specify the function and\ncomposition of each layer. Furthermore, we have ensured that detailed geometric parameters are\nexplicitly provided in Fig. 2.\nUpdated main text:\n''…The device architecture features an optically thick gold layer at the bottom to prevent light\ntransmission58 and ensure unidirectional emission. The topmost layer consists of a symmetry-breaking,\npartially etched Ge double-brick metasurface supporting nonlocal high-Q resonances. These two layers\nare separated by a low-refractive-index Al O (n ≈ 1.55) spacer, which exhibits negligible absorption\n2 3\nin the targeted spectral range59. The thickness of the spacer is optimized to facilitate impedance\nmatching of the entire structure to vacuum and minimize the influence of metal intrinsic losses on the\nupper nonlocal modes, thereby enhancing the temporal coherence of ThE27. When an electric current is\napplied to the bottom metal layer to heat the entire device, thermal fluctuations couple into the nonlocal\nhigh-Q chiral modes of the upper Ge structure, leading to a significant enhancement of ThE at the target\nwavelength. Importantly, the emitted CP-ThE can repeatedly switch between LCP and RCP at the target\nwavelength in response to temperature changes, as indicated by the cyclic arrows in Fig. 1c. The inset\non the right shows a top-view of the geometric structure. To further elucidate the thermal tuning\nmechanism, Fig. 1d presents the temperature-dependent refractive index of the amorphous Ge film due\nto the thermo-optic effect…''\n''…For the final device design, we selected the geometric parameters corresponding to the white\npentagram on the right in Fig. 2d (h = 220 nm, dy = 180 nm)…''\ndependent variations of the real part (n) and imaginary part (k) of the refractive index, as well as the\nemissivity spectrum, for metasurfaces incorporating a traditional phase-change material (a) and a thermo-\noptic material (b). c, Schematic illustration of the working principle of CP-ThE generation and temperature-\ndriven helicity reversal at wavelength λ . d, Thermo-optic effect of Ge: temperature-dependent variation of\n0\nits refractive index based on spectral measurement results. e, Helicity switching mechanism: a small\ntemperature increase ΔT enables handedness reversal at the target wavelength λ . f, Helicity-switching at a\n0\ntemperature-tunable wavelength λ .\n0\n5. The authors should discuss the dynamic response of the device, including estimates or direct\nmeasurements of the thermal inertia and helicity switching time, which are critical parameters for\nassessing practical applicability. In addition, the performance of the proposed thermal switching\napproach should be quantitatively compared with that of electrically reconfigurable metasurfaces,\nparticularly in terms of response speed and modulation capability.\nOur response: We appreciate the reviewer's insightful suggestion regarding the dynamic response and\npractical applicability of the device. In the revised manuscript, we have added a dedicated discussion\non the device's dynamic response, explicitly clarifying the difference between the limitations of our\nexperimental setup and the rapid switching achievable in practical applications. Furthermore, we have\nincorporated a quantitative comparison with electrically reconfigurable metasurfaces, highlighting the\ntrade-off between modulation speed and device scalability, as well as the suitability of our approach for\nspecific application scenarios.\nWe have added the following content in the revised manuscript:\n''…It is important to emphasize that switching speed in our current experimental setup was limited\nby the thermal inertia of the external heating stage (Bruker, A540). However, in practical applications,\nrapid temperature modulation rates of 104~105 K/s can be achieved by integrating micro-heaters for\nJoule heating65, which would significantly facilitate fast spectral adjustment and helicity switching\nspeed. Regarding the comparison with electrically reconfigurable metasurfaces66, a trade-off exists\nbetween speed and structural complexity. While electrical modulation schemes typically offer shorter\nresponse times, they require complex electrode networks and biasing circuits. In contrast, although the\nresponse speed of thermally tunable devices is constrained by thermal inertia, a key advantage lies in\ntheir simplicity and scalability. Since our approach does not require pixel-level wiring, it holds great\npotential for cost-effective, large-scale deployment. Crucially, for primary target applications such as\nMIR chiral sensing and molecular spectroscopy, the modulation speed offered by thermal tuning is\nsufficient. In these scenarios, spectral coverage and high emission quality are often more critical than\nnanosecond-level switching speeds…''\nReferences\n65. Lv, X., Wang, W., Liu, J. & Deng, J. Applications of rapid joule heating in rechargeable batteries:\nMaterial synthesis, structural engineering, and performance enhancement. Prog. Solid State Chem. 82,\n100570 (2026).\n66. Soma, G., Ariu, K., Karakida, S., Tsubai, Y. & Tanemura, T. Subvolt high-speed free-space\nmodulator with electro-optic metasurface. Nat. Nanotechnol. 20, 1625–1632 (2025)\n6. The manuscript should clarify the experimental uncertainties, including temperature stability, spectral\nresolution, polarization analysis accuracy and emissivity extraction errors.\nOur response: We appreciate the reviewer's valuable suggestion on the experimental data analysis. In\nthe revised manuscript, we have added a comprehensive discussion on the experimental uncertainties\nin both the Discussion and Conclusion, and the Methods sections. Specifically, we have clarified the\nimpact of finite collection angles on spectral broadening, the influence of temperature stability on\nspectral drift, and the systematic errors associated with polarization analysis and emissivity extraction.\nDetailed parameters regarding spectral resolution and signal averaging have also been explicitly stated.\nIn the revised manuscript, we have clarified these uncertainties at the relevant sections.\nUpdated manuscript:\n''…Several factors contribute to the experimental uncertainties. First, although two apertures were used\nto suppress background noise and restrict the collection of wide-angle ThE, a finite angular acceptance\nrange was inevitably included. Due to the angular dispersion of the nonlocal modes, this results in the\nsimultaneous collection of spectral components with slightly shifted central wavelengths, leading to\ninhomogeneous broadening of the emission spectrum and a reduction in the measured temporal\ncoherence (see Fig. S8). Second, given the strong temperature dependence of the emission wavelength,\ntemperature stability is a critical factor. Since the heating process was conducted in an open\nenvironment, minor convective temperature fluctuations could lead to slight spectral drift, further\ncontributing to the emission linewidth broadening. Nevertheless, a high Q-factor of up to 150 was\nachieved, demonstrating the robustness of our thermal control. Finally, the discrepancy between the\nmeasured emission CD and the theoretical values can be attributed to fabrication tolerances, as well as\nsystematic errors introduced by the finite extinction ratio of polarization analyzer and slight\ninaccuracies in the emissivity extraction process.''\n''…Both the blackbody reference and the fabricated sample were mounted onto a temperature-\ncontrolled heating stage. When the blackbody and the sample were heated to above 100°C, the emitted\npower was directed into the FTIR system and detected by an MCT detector. This comparative\nmeasurement setup ensures high accuracy in the emission measurements. All spectra were acquired\nusing the same FTIR spectrometer with a spectral resolution setting of 2 cm−1, which is sufficient to\nresolve the high-Q resonant features. To balance the signal-to-noise ratio and acquisition time, 15 scans\nwere averaged for each measurement. For clarity, the data are presented in wavelength units (μm) after\nconversion from wavenumber (cm−1).''\nOnce again, we sincerely appreciate the constructive feedback from reviewer #2, which has\nsignificantly improved the quality of the manuscript.\nReply to Reviewer #3\nReviewer #3\nI have carefully read the manuscript by Sun et al. titled “Helicity-selective and spectrally tunable chiral\nthermal emissions”. The authors report results on a smartly designed metasurface based on a multilayer\nof gold, alumina and germanium that allows switching the helicity of the thermal emission at a given\nwavelength by increasing or decreasing the device temperature. They start by describing the working\nprinciple of their device, based on the combination of the thermo-optical effect in Ge and the excitation\nof quasi guided modes with different helicity and different resonant wavelengths. They perform\nexhaustive simulations to characterize their device and how the different geometrical parameters of the\nunit cell imping on the thermal emission properties, specially its degree of circular polarization, and\nhow the change in T shifts the different emission bands. Finally, they fabricate their optimal device,\nand experimentally proved that their metasurface effectively allows the switching of the helicity over a\nbroad region of ~100 nm.\nThe reported results are highly interesting for the nanophotonics community, specially because\nthey provide solid experimental evidence, back up by extensive simulations, of a reconfigurable device.\nTheir results are sound, technically correct, and most of their conclusions are back-up by\nexperimental/simulation evidence (see comments below for aspects to improve). The manuscript is well\nwritten and structured, its length is adequate for its content, and all the figures, which are mostly clear\n(see comment below for improvement), are necessary. All in all, I could recommend publication of this\nmanuscript provided the author address the following minor issues:\nOur response: We sincerely thank reviewer #3 for the thorough evaluation of our manuscript and for\nthe positive and detailed assessment of our work. We appreciate the reviewer's recognition of our\nphysical concept, the comprehensive numerical analysis, and the experimental demonstration of\nhelicity-switchable thermal emission. We are also grateful for the constructive comments regarding\nclarity and presentation. We have addressed all the minor issues raised and revised the manuscript\naccordingly to further improve its clarity and completeness.\n1. In the abstract, the acronym CP is not defined, and it is not used again. So I would recommend writing\ndirectly circularly polarized, or whatever the authors intended to convey.\nOur response: We thank the reviewer for pointing this out. In the revised manuscript, the acronym\n''CP'' in the Abstract has been replaced by its full term, ''circularly polarized, '' to improve clarity.\nUpdated main text:\n''…we experimentally demonstrate helicity-switchable circularly polarized thermal emissions with high\ntemporal coherence (Q>150) and strong emission circular dichroism (> 0.8) over a ~100 nm wavelength\nrange through a temperature change of 250 K…''\n2. In the abstract (lines 28-30) and in the text (lines 180-188), it is suggested, or could be understood,\nthat they have achieved dynamically switchable full polarization control. This is not true, as the change\nfrom CP to LP is done with the unit cell design (no reconfigurability), while the helicity switching is\ndone with temperature changes. The authors should change the text so that this is clear and avoid\nconfusions.\nOur response: We thank the reviewer for this important clarification. We agree that the original\nwording could lead to confusion regarding the scope of polarization control. In the revised manuscript,\nwe have carefully revised the Abstract and the corresponding text to clearly distinguish between\npolarization-state engineering enabled by the unit-cell design and dynamic helicity switching\nachieved via temperature modulation. These revisions ensure that the manuscript does not imply\nfully reconfigurable polarization control and accurately reflects the physical mechanisms involved.\nUpdated main text:\n''…we experimentally demonstrate helicity-switchable circularly polarized thermal emissions with high\ntemporal coherence (Q>150) and strong emission circular dichroism (> 0.8) over a ~100 nm wavelength\nrange through a temperature change of 250 K. We further reveal that simple geometric design enables\nfull polarization tailoring…''\n''…This highlights the capability to access the full-Stokes parameter space through simple geometric\ndesign. In contrast, helicity switching in a single device is solely induced by temperature changes,\nleveraging the inherent temperature dependence of ThE. This approach eliminates the need for external\nelectrical or mechanical actuation, offering flexible control over the polarization state of emitted\nradiation…''\n''…The emitted ThE exhibits ultrahigh temporal coherence (Q > 150) and robust high ECD values\nacross the tunable range. Given the positions of RCP and LCP on the Poincaré sphere, these findings\nsuggest that combining simple geometric design with dynamic temperature actuation could even enable\ncoherent ThE across the entire sphere. This dynamic control of both helicity and emission wavelength\noffers a compact…''\n3. In the abstract (last line) and in some other parts of the text, they mention isomer identification, but,\ngiven that they are reporting a CP emitting device, they should better use enantiomer identification.\nOur response: We thank the reviewer for pointing out this terminology distinction. We agree that\nenantiomer identification is the more precise and chemically accurate term in the context of circularly\npolarized thermal emission. In the revised manuscript, we have replaced isomer identification with\nenantiomer identification throughout the abstract and the main text to ensure conceptual accuracy and\navoid ambiguity.\n4. Figure 1f can be confusing. When I saw it first I could not understand it quite well, and I had to read\nthe whole paper and, later, go back and understand it. It seems that at the same T and wavelength, one\ncan obtain both LCP and RCP. The figure is very nice, but they should separate in wavelengths the\ncontribution of LCP and RCP, as they do in Figure 3c. This is, each ECD spectra should be in a different\nline.\nOur response: We thank the reviewer for this careful and insightful comment. We agree that the\noriginal presentation of Fig. 1f was potentially confusing and might imply the simultaneous generation\nof both LCP and RCP at the same temperature and wavelength. To avoid this ambiguity, we have\nrevised Fig. 1f by clearly separating the spectral contributions of LCP and RCP into distinct wavelength\nranges, adopting the same visualization strategy as in Fig. 3c. In this revised figure, each ECD spectrum\nis shown on a separate line, which more clearly illustrates the helicity-switching mechanism and\nimproves the overall readability of the figure.\nUpdated Fig. 1:\nFig. 1 | Principle of helicity switching and wavelength tunability of CP-ThE. a and b, Temperature-\ndependent variations of the real part (n) and imaginary part (k) of the refractive index, as well as the\nemissivity spectrum, for metasurfaces incorporating a traditional phase-change material (a) and a thermo-\noptic material (b). c, Schematic illustration of the working principle of CP-ThE generation and temperature-\ndriven helicity reversal at wavelength λ . d, Thermo-optic effect of Ge: temperature-dependent variation of\n0\nits refractive index based on spectral measurement results. e, Helicity switching mechanism: a small\ntemperature increase ΔT enables handedness reversal at the target wavelength λ . f, Helicity-switching at a\n0\ntemperature-tunable wavelength λ .\n0\n5. Regarding the figures in the supplementary material, I would suggest the authors to order them in the\nsame way they are mentioned. I mean, Figure s1 is not mentioned until the end of the manuscript. This\nmakes their identification confusing (see next comment).\nOur response: We thank the reviewer for this helpful suggestion regarding the organization of the\nSupplementary Information. We have renumbered and reordered the figures in the Supplementary\nInformation to strictly align with their order of appearance in the main text. This revision ensures a\nlogical flow and facilitates easier identification of the supplementary content.\n6. In Line 166, when Figure S5 is mentioned. Is it really figure S5? In this figure there is not tuning of\ndx or delta whatsoever. That tuning is shown in Figure S2, but there is no Q factors there. Please, clarify.\nIn addition, in line 209, is it figure 3 or S3?\nOur response: We thank the reviewer for the meticulous attention to detail.\nRegarding Line 166: we apologize for the incorrect citation in the original manuscript. We have revised\nthe corresponding text to accurately cite Fig. 2c and Fig. S1 (in the reordered Supplementary\nInformation). Specifically, Fig. 2c illustrates the independent tuning of the radiative Q-factors via dx\nand δ, while Fig. S1 demonstrates the maintenance of high-Q factors across the wavevector range.\nRegarding Line 209: we confirm that the reference to Fig. S3 is correct. We have double-checked all\nfigure citations in the revised manuscript to ensure they strictly align with the reordered Supplementary\nInformation.\nUpdated main text:\n''…Importantly, the Q factors of QGM and QGM at the Γ point (see Fig. 2c) can be independently\nrad 1 2\nand continuously tuned by adjusting dx and δ, respectively. Additionally, under fixed perturbations,\nstable high-Q factors are maintained across a wide wavevector range in the band (see Fig. S1)…''\n7. Regarding the multipole decomposition of the quasi guided modes. Why is the LCP mode primary\nof magnetic dipole nature while the RCP is a mixture of toroidal and magnetic dipole? Please, elaborate\non this point.\nOur response: We thank the reviewer for this perceptive question regarding the physical origin of the\nmultipole decomposition. The different multipolar compositions of the LCP and RCP quasi-guided\nmodes originate from the distinct symmetry properties of the induced current distributions within the\nanisotropic metasurface. In the Supplementary Information, we have added a detailed explanation\nclarifying why the LCP mode is predominantly of magnetic dipole character, whereas the RCP mode\nexhibits a hybrid response involving both magnetic dipole and toroidal dipole contributions.\nUpdated Supplementary Information:\n''…The distinct multipolar characteristics of the two QGMs arise from their different field symmetries\nand the corresponding current configurations. As shown in the enlarged xy-plane cross-sectional\ndiagrams of calculated polarization current density distributions in Fig. S3a and d, the black arrows\nrepresent the distribution of polarization currents. For the LCP mode (QGM ), the displacement currents\n1\nwithin the structure form a dominant circulating loop. This distribution leads to a primarily magnetic\ndipole response. In contrast, the RCP mode (QGM ) exhibits a more complex topology characterized\n2\nby anti-parallel current loops (or head-to-tail arrangements of local magnetic moments). This specific\nconfiguration, where the magnetic moments are confined within the unit cell to form a vortex-like flux,\nis the physical hallmark of a toroidal dipole excitation. Consequently, QGM manifests as a hybrid\n2\nmode with significant contributions from both magnetic and toroidal dipoles, reflecting the broken in-\nplane symmetry of the anisotropic metasurface.\nFig. S3| Multipole decomposition analysis of the resonances associated with the LCP and RCP\nemission. a and b, The multipolar decomposition results of the QGM mode under LCP and RCP output,\n1\nwith insets showing the corresponding electric field enhancements. c and d, The multipolar decomposition\nresults of the QGM mode under LCP and RCP output. The enlarged illustrations in a and d show the electric\n2\nfield amplitude in the xy-plane, and the black arrows represent the distribution of polarization currents…''\nOnce again, we sincerely appreciate the constructive feedback from reviewer #3, which has\nsignificantly improved the quality of the manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have provided very detailed and satisfactory responses to all referee comments and have adequately addressed the concerns raised in my report. I therefore consider that the manuscript has been appropriately revised and, in line with my previous assessment, recommend its acceptance for publication in Nature Communications.", "reviewer_2": "The authors have adequately addressed all concerns raised by the reviewers and have revised the manuscript accordingly. The manuscript is now ready for publication.", "reviewer_3": "The authors have successfully addressed all my concerns. Congratulations."}, "author_response": ""}]

Decision letter