Multifunctional lithium niobate platform for photodetection and photoacoustic and thermoelastic gas sensing

Domain: neuroscience
Published: 2026-02-03
Editorial outcome: minor_revision

Abstract
Leveraging the intrinsic multi-physics nature of ferroelectric lithium niobate, we present a multi-functional platform (LN-MFP) that seamlessly integrates photoacoustic spectroscopy, light-induced thermoelastic spectroscopy and photodetection into a single on-chip device. The proposed LN-MFP operates over a broad spectral range spanning from the visible to the mid infrared. We experimentally demonstrate trace gas detection of nitrogen dioxide, water vapor, acetylene, carbon dioxide, methane and ammonia, achieving parts-per-billion detection limits. We implement a custom packaging solution where the LN-MFP chip and a 4.6 µm quantum cascade laser chip are mounted on a printed circuit board together with transimpedance amplification, demonstrating system-level integration. Using this co-packaged module, we demonstrate carbon monoxide detection via second-harmonic measurements, outlining a clear route towards fully integrated on-chip implementations. This compact, hybrid, multi-functional architecture markedly reduces system complexity and footprint compared with conventional benchtop systems and is intrinsically compatible with the rapidly developing lithium niobate integrated photonics ecosystem. The LN-MFP provides a core sensing building block for future all-lithium-niobate spectroscopic chips for environmental monitoring, point-of-care diagnostics and on-site chemical analysis.

Introduction
Lithium niobate (LN) has emerged as a key material for on-chip devices, thanks to its outstanding electro-optic, piezoelectric, and non-linear optical properties1. These unique characteristics make it a versatile platform for diverse integrated photonic applications2. Breakthroughs in LN photonics have propelled the development in high-performance components, including electro-optic modulators3–5, frequency shifters6, frequency combs7–9, pulse generators10, tunable lasers11, spectroscopic sensing12–14 and photonic integrated circuits15. Such advancements are critical enablers for optical communications, signal processing, and high-sensitivity sensing technologies16,17. Moreover, LN’s distinctive piezoelectric and thermoelastic properties unlock new possibilities for high-performance acoustic and acousto-optic devices, expanding its range of applications18–20. The advancement of LN-based on-chip devices is pivotal to the future of integrated photonics and electromagnetic spectrum technologies21. LN’s exceptional versatility positions it as a cornerstone for the next generation of compact, high-performance, and energy-efficient photonic systems.

Advancements in spectroscopic methodologies are expanding the frontiers photonics, offering unprecedented insights into molecular structures and dynamics. These breakthroughs are transforming fields such as chemistry, physics, and environmental science22–30. Among the spectroscopic methods, photoacoustic spectroscopy (PAS) stands out for its non-destructive nature and versatility across diverse type of samples over an extended range of wavelengths31–35. Light-induced thermoelastic spectroscopy (LITES) is an optical gas sensing technique that detects target gases by converting the sensor’s thermoelastic deformation, induced by modulated light absorption, into electrical signals. This enables contact-based detection with high sensitivity and selectivity, particularly when using materials with excellent piezoelectric properties36–38.

Despite their individual strengths, integrating spectroscopic detectors remains a major challenge for advancing micro-, nano- and on-chip spectroscopic devices. Conventional systems often rely on bulky optical components and complex configurations, limiting their suitability for miniaturized and portable sensing applications39. Key obstacles include large instrumentation, high power consumption, and the precise alignment of optical components, which hinder field deployment and real-time, on-site analysis. Furthermore, lack of seamless integration across multiple spectroscopic techniques restrict the development of versatile, compact platforms capable of delivering comprehensive analytical insights.

To overcome these challenges, this work introduces lithium niobate as a multi-functional integrated platform (LN-MFP) for spectroscopic sensing. By harnessing LN’s strong piezoelectric and thermoelastic properties, we develop an integrated sensor capable of performing diverse spectroscopic measurements within a single device. The LN-MFP seamlessly combines photoacoustic, thermoelastic, and photodetection functionalities, addressing the limitations of conventional spectroscopic systems. This integration not only minimizes physical footprint and system complexity but also enhances sensitivity and selectivity through the synergistic interplay of multiple detection mechanisms. Incorporating resonance mechanisms can enhance sensor performance, improving both sensitivity and selectivity across diverse applications40–42. In this work, we leverage lithium niobate’s piezoelectric properties and thermoelastic effects to detect and amplify signals via mechanical resonance, further optimized through a fork-shaped structure. This work has the potential to revolutionize on-chip spectroscopic devices by providing a compact, high-performance, and versatile solution that that overcomes the limitations of conventional bulky systems. The LN-MFP not only allows to advance the fundamental understanding of LN-based integrated platforms but also lays the groundwork for future innovations in the spectroscopic technologies. By addressing key integration challenges and enhancing spectroscopic detection capabilities, this research marks a pivotal step toward the next generation of multifunctional, on-chip spectroscopic sensors, driving progress in integrated photonics and advanced sensing technologies.

Results
Design and fabrication of the lithium niobate multi-functional platform

The design philosophy of the LN-MFP fundamentally differs from that of conventional quartz- or Si-based PAS/LITES devices. Lithium niobate is a ferroelectric crystal with 3m point-group symmetry and spontaneous polarization, leading to piezoelectric coefficients and electromechanical coupling factors more than one order of magnitude larger than those of quartz (for example, d22 ≈ 25 pC/N for LN versus d11 ≈ 2.3 pC/N for quartz). At the same time, LN exhibits strong electro-optic, nonlinear-optical, and pyroelectric effects that are absent in quartz and most MEMS materials. These intrinsic properties mean that LN-based sensors cannot be obtained by simply scaling or copying established quartz tuning fork designs; instead, the fork geometry, electrode layout, and fabrication process in this work are specifically engineered to harness LN’s multi-physics coupling. As a result, a single LN-MFP chip can simultaneously operate as a high-efficiency acoustic transducer for PAS, a thermoelastic detector, and a platform for LITES, thereby enabling multimodal operation that is not available in conventional quartz or MEMS devices.

The design of the LN-MFP is shown in Fig. 1a. Simulations performed with COMSOL Multiphysics software provided a detailed analysis of the electrical response of lithium niobate under varying stress and thermal conditions. The simulation results in Fig. 1b, depict the charge distribution across the LN-MFP during vibration. Positive charge accumulations appear in the red regions, while negative charges are shown in the blue regions. Notably, negative charges are concentrated near the gap between the tines, whereas positive charges accumulate near the device’s outer boundaries, forming a distinct bipolar distribution. This pattern suggests that mechanical deformation or thermal effects enhance the piezoelectric response, particularly in the marginal regions. Moreover, the symmetric charge distribution across both tines underscores their synchronized motion during vibration.

The LN-MFP is fabricated through an eight-step microfabrication process (Fig. 1c). It begins with slicing a y-cut 128° lithium niobate crystal block into thin wafers, each measuring 500 µm in thickness and 4 inches in diameter. The wafer edges are then refined using circular grinding to remove surfaces and ensured dimensional uniformity. Next, dual rotating pads polish the wafers to achieve a smooth, uniform finish. Finally, a spray-type cleaning system removes any contaminants and residues, preparing the wafer for subsequent processing. After cleaning, both sides of the wafers are coated with a 0.2 µm gold film. The wafers are then precisely diced into individual components. The structural design of the LN-MFP is fabricated using mechanical polishing and laser processing. A more specific and comprehensive description of the process flow was provided in Supplementary Note 3.

The tines of the LN-MFP are ~11.5 mm in length and 1.7 mm in width, with a gap of 1 mm between them. The base has a width of W ~ 7.6 mm, and features a circular fillet with a radius of r ~ 0.8 mm at the junction between the base and the tines. The LN-MFP has a frequency of 10,485 Hz and a Q factor of 1,621. An optical microscopy image of the fabricated LN-MFP is shown in Fig. 1d, while Fig. 1e and Fig. 1f present SEM images of the LN-MFP tine edge and the base fillet, respectively.

The design of the LN-MFP was guided by two primary factors: resonance frequency and the efficient coupling with laser sources. To minimize 1/f noise during spectroscopic detection, controlling the resonance frequency is crucial. In photoacoustic and photo-thermoelastic detection, the modulation frequency must be moderate, as certain molecular non-radiative relaxation processes can persist for up to 100 microseconds. Therefore, optimizing the resonance frequency is essential to maximizing device sensitivity while minimizing noise. Additionally, light sources like terahertz (THz) quantum cascade lasers (QCLs)43 or Fabry-Perot diode lasers44 often exhibit suboptimal beam quality. To accommodate these sources, the gap between the tines is intentionally widened. Microelectrodes were fabricated on both sides of the device using magnetron sputtering and masking techniques.

Photoacoustic detection with the LN-MFP

To assess the photoacoustic detection capabilities of the LN-MFP, we employed light sources of different types covering a broad spectral range, from visible to long-wave infra-red wavelengths, specifically at 450 nm, 1.3 µm, 1.5 µm, 2 µm, 3.3 µm, and 9.77 µm, shown in the Fig. 2a. This wide spectral operating range enables the detection of various gas species by targeting their characteristic absorption lines. All experiments were carried out at room temperature and pressure.

Nitrogen dioxide (NO₂) detection is crucial for environmental monitoring and public health protection. To measure NO₂ concentration, we used a visible (VIS) blue LED with a center wavelength of 450 nm as the light source. The target gas, supplied at a certified concentration of 5 ppm in N₂, was analysed, and the results are shown in Fig. 2b. Since the LED operated at a fixed wavelength, intensity modulation was applied to excite NO₂ molecules and generate acoustic waves. The LED emitted an output power of ~1.2 W, operating at a modulation frequency of 10,485 Hz with a 50% duty cycle. The LN-MFP achieved a signal-to-noise ratio (SNR) of ~132, corresponding to a minimum detection limit (MDL) of ~38 ppb.

The concentration of water vapor (H₂O) in air was measured using a 1.3 µm near-infrared (NIR) distributed feedback (DFB) semiconductor laser, with the results presented in Fig. 2c. The laser wavelength was scanned from 7185.2 cm⁻¹ to 7186.1 cm⁻¹, covering the water absorption line at 7185.6 cm⁻¹. At a water vapor concentration of 18,000 ppm, the peak voltage of the second harmonic signal (2f) was recorded at 0.0618 V. The 1σ standard deviation at a non-absorption wavelength was determined to be 3.65 × 10⁻⁶ V, resulting in a H₂O MDL of ~1 ppm.

The detection of acetylene (C₂H₂) is essential for ensuring safety in industrial processes and monitoring human activities. Acetylene at a concentration of 50 ppm is detected in N₂ using a 1.5 µm near-infrared semiconductor laser (Fig. 2d). The laser wavelength was scanned from 6506.99 cm⁻¹ to 6507.63 cm⁻¹, covering the C₂H₂ absorption line at 6507.4 cm⁻¹. The peak voltage of the 2f signal was recorded at 0.0477 V, while the modulation depth was to 0.44 cm-1. As a result, the 1σ standard deviation at a non-absorption wavelength was 5.99 × 10⁻⁶ V, yielding a MDL of ~63 ppb for C₂H₂.

Carbon dioxide (CO₂), a greenhouse gas closely linked to climate change, was detected using a 2 µm NIR DFB laser diode. A certified CO₂ gas mixture with a concentration of 1000 ppm in N₂ was analyzed, as shown in Fig. 2e. The laser wavelength was scanned from 4988.53 cm⁻¹ to 4988.77 cm⁻¹, covering the CO₂ absorption line at 4988.65 cm⁻¹. The 2f peak signal was recorded at 1.34 × 10⁻³ V, with a 1σ standard deviation of 1.14 × 10⁻⁵ V measured away from the absorption line. These results indicate that the LN-MFP achieved a MDL of ~9 ppm for CO₂.

Methane (CH₄), a major greenhouse gas and widely used energy source, was analyzed using a 3.3 µm mid-wave infrared (MWIR) interband cascade laser (ICL) in combination with the LN-MFP. The results are shown in Fig. 2f. The ICL wavelength was scanned from 2967.86 cm⁻¹ to 2969.33 cm⁻¹, covering multiple methane absorption lines. Methane exhibits four absorption lines at 2968.86 cm⁻¹, 2968.73 cm⁻¹, 2968.47 cm⁻¹, and 2968.4 cm⁻¹. A certified methane gas sample with a concentration of 100 ppm in N₂ was used for the measurement. At the strongest absorption peak, the signal voltage reached 0.019 V, with a 1σ standard deviation of 9.4 × 10⁻⁶ V measured away from the methane absorption lines. This corresponds to a MDL of ~49 ppb.

Ammonia (NH₃), a critical compound in both industrial and agricultural applications, was analyzed using a 9.77 µm long-wave infrared (LWIR) QCL. The results are shown in Fig. 2g. The laser wavelength was tuned from 1022.4 cm⁻¹ to 1023 cm⁻¹, covering the ammonia absorption line at 1022.76 cm⁻¹. A certified ammonia gas sample with a concentration of 1 ppm in N₂ was used for the measurement. The system achieved a SNR of ~145, corresponding to an MDL of ~7 ppb for NH₃.

Responses to varying target-gas concentrations were measured and the corresponding calibration curves are plotted in Fig. 2. As shown in Fig. 2, all channels exhibit excellent linearity, with linear-fit coefficients of R² > 0.99. To assess the long-term stability of the LN-MFP PAS sensor, we performed noise characterization and Allan-deviation analysis (Fig. 2). The 1σ noise levels were measured under a continuous flow of 200 sccm pure N₂. Figure 2b–g show the corresponding noise traces and Allan-deviation curves for the LN-MFP operated with a 450 nm LED, 1392 nm laser diode, 1536 nm laser diode, 2004 nm laser diode, 3370 nm ICL, and 9770 nm QCL. From the Allan deviation analysis, the 1σ MDLs, and optimal averaging times were extracted: the 450 nm (NO₂) channel achieves an MDL of ~2 ppb at an averaging time of 1000 s; the 1392 nm (H₂O), ~25 ppb at 6300 s; the 1536 nm (C₂H₂), ~3.5 ppb at 1000 s; the 2004 nm (CO₂), ~350 ppb at 1900 s; the 3370 nm (CH₄), ~2.5 ppb at 1200 s; and the 9770 nm (NH₃), ~350 ppt at 1000 s.

Table 1 provides summary of the sensitivities achieved together with the calculated normalized noise equivalent absorption coefficient (NNEA) values and the optimal modulation amplitude. The photoacoustic detection performance of the LN-MFP and the QTF-based PAS systems is summarized in Supplementary Note 10. These results demonstrate the LN-MFP’s capability to detect multiple gas species across a broad spectral range with high sensitivity, underscoring its versatility and potential for practical applications not limited to environmental monitoring and industrial process control.

Photodetection with the LN-MFP

Beyond its capabilities in photoacoustic detection, the LN-MFP demonstrates remarkable potential for photodetection across a broad spectral range, from visible light at 450 nm to long-wave infrared radiation at nearly 10 µm. To assess its photodetection performance, square-wave modulated lasers at various wavelengths were shined onto the LN-MFP most sensitive area. The modulated light intensity was gradually reduced to evaluate the device’s response, enabling the determination of its minimum detectable power and overall detectivity. The response curves of the LN-MFP were measured for light wavelength ranging from 450 nm to 9.77 µm (Fig. 3).

For the 450 nm LED as the modulation depth decreased from 0.904 cm⁻¹ to 3.57 × 10⁻⁴ cm⁻¹, the vibration amplitude of the LN-MFP correspondingly dropped from 1.08 V to 7.47 × 10⁻⁴ V. Using Equation (1) in the Supplementary Note 7, the sensitivity of the LN-MFP to 450 nm light was determined to be 260 nW, with a corresponding detectivity of 67.7 V/W. The diode laser operating at 1.3 µm was detuned from the H₂O absorption peak to avoid any power absorption occurring along the optical pathlength between laser and photodetector. For the 1.3 µm wavelength, as the modulation depth decreased from 1.15 cm⁻¹ to 2.98 × 10⁻⁴ cm⁻¹, the vibration amplitude correspondingly dropped from 2.47 × 10⁻¹ V to 1.03 × 10⁻⁴ V. The sensitivity was determined to be 445 nW, with a detectivity of 35.5 V/W. Similarly, for the 1.53 µm wavelength, decreasing the modulation depth from 0.59 cm⁻¹ to 1.12 × 10⁻³ cm⁻¹ led to a reduction in vibration amplitude from 1.07 × 10⁻¹ V to 3.1 × 10⁻⁴ V. The sensitivity of the LN-MFP to the 1.53 µm laser was then determined to be 695 nW, with a corresponding detectivity of 22.5 V/W. For the 2 µm NIR semiconductor laser, when the modulation depth was lowered from 0.33 cm⁻¹ to 4.95 × 10⁻³ cm⁻¹, the vibration amplitude simultaneously decreased from 2.76 × 10⁻² V to 2.02 × 10⁻⁴ V. Under these conditions, the sensitivity was measured at 693 nW, and the detectivity reached 15.3 V/W. At 3.37 µm the laser modulation depth was reduced from 0.34 cm⁻¹ to 7.22 × 10⁻³ cm⁻¹ and the vibration amplitude decreased from 8.03 × 10⁻³ V to 2.35 × 10⁻⁴ V. The sensitivity was calculated to be 1 µW, with a corresponding detectivity of 9.8 V/W. Finally, at 9.77 µm as the modulation depth decreased from 2.47 × 10⁻² cm⁻¹ to 8.25 × 10⁻⁵ cm⁻¹, the vibration amplitude dropped from 0.42 V to 1.73 × 10⁻³ V. The sensitivity to the 9.77 µm laser was determined to be 25 nW, with a detectivity of 3.73 × 10² V/W.

The performance of the LN-MFP is comprehensively assessed across the VIS, NIR, MWIR and LWIR spectral regions, spanning wavelengths from 450 nm to 9.77 µm (Fig. 3b). The results show that the photoresponsivity initially decreases as the wavelength transitions from the VIS to the LWIR region, followed by an increase in the LWIR region. This trend indicates that the LN-MFP achieves its highest sensitivity in the LWIR range. This behavior is attributed to the intrinsic optical absorption properties of LN in the investigated range, as illustrated in Fig. 3b, which also includes the absorption spectrum of LN, which was obtained by measuring the absorption of a 500 µm thick LN wafer using Fourier transform infrared spectroscopy. The enhanced absorption in the MIR and LWIR regions further underscore the LN-MFP’s potential for high-sensitivity applications, such as the fundamental absorption band of molecules. Figure 3c compares the photoresponsivity of the LN-MFP to that of four commercial mid-infrared detectors in response to a 9.7 µm laser. Notably, the LN-MFP exhibits superior photoresponsivity at 9.7 µm, outperforming these commercial mid-infrared photodetectors.

Table 2 summarizes the LN-MFP’s photodetection performance across various wavelengths. All measurements were conducted at room temperature without active cooling. Notably, the MDL and response rate could be significantly enhanced by incorporating cooling strategies, as lower temperatures generally reduce thermal noise and improve detector performance. This suggests that further optimization through thermal management could unlock even greater sensitivity for advanced spectroscopic applications.

Thermoelastic detection with the LN-MFP

Beyond its photoacoustic detection capabilities, the LN-MFP also exhibits strong potential for LITES. In this mechanism, target molecules absorb modulated laser radiation, producing localized heating and subsequent thermal expansion of a piezoelectric sensing element, in our case, the LN-MFP. This resulting mechanical deformation, enhanced by the LN-MFP’s inherent mechanical resonance, excites a piezoelectric response, generating a measurable electrical signal. Notably, this resonant amplification effect boosts the sensitivity of the system.

To evaluate the LITES performance of the LN-MFP system, six different light sources spanning from VIS to LWIR were employed to analyze different gases molecules and the results are shown in Fig. 4a. The laser beams propagated in free space, with the LN-MFP positioned at an optical path length of 2.5 cm. All experiments were conducted under laboratory conditions at ambient temperature and pressure, with N₂ as the carrier gas.

A 450 nm visible LED was used to detect NO₂. The 1f signal was obtained by alternately introducing pure N₂ and 5% NO₂, as shown in Fig. 4b. The modulation depth was set to ~100%, with a duty cycle of 50%. The LED output power was set as 330 mW. By analyzing the signals for pure N₂ and 5% NO₂, the LN-MFP system achieved a SNR of 3692, corresponding to a MDL of ~13.5 ppm.

For the evaluation of the thermoelastic detection capability of the LN-MFP in the near- and mid-IR regions, the noise level was determined as the 1σ standard deviation at non-absorbing wavelengths.

At 1.3 µm a near-infrared distributed feedback (DFB) laser was employed to target the H₂O absorption line at 7185.6 cm⁻¹, with the concentration fixed at 2% via a humidity controller. Under an optimized modulation depth of 0.4 cm⁻¹, the LN-MFP system generated a 2f LITES signal peak of 1.62 mV, as shown in Fig. 4c, yielding an MDL of ~59.3 ppm for H₂O. Similarly, a 1.5 µm near-infrared telecommunication laser was employed to detect C₂H₂ by targeting its absorption line at 6534.6 cm⁻¹. With the laser power set to 10.5 mW and a modulation depth of 0.68 cm⁻¹, the LN-MFP produced a 2f signal peak of 4.4 mV, while the noise resulted 9.16 µV, leading to an MDL of 10.4 ppm for C₂H₂ as depicted in Fig. 4d.

A 2 µm laser diode was employed to CO₂ at a 20% concentration. The laser was precisely tuned to target the CO₂ absorption line at 4988.65 cm⁻¹, and with a modulation amplitude of 0.47 cm⁻¹, the LN-MFP system generated a 2f signal peak of 3.1 mV, shown in the Fig. 4e. The baseline noise was determined to be 1.1 × 10⁻⁵ V, corresponding to a MDL of ~680 ppm for CO₂. At 3.3 µm, a mid-wave infrared (MWIR) interband cascade laser (ICL) at was used to measure CH₄ at a concentration of 2%. The combined contribution of four CH₄ absorption lines at 2968.86 cm⁻¹, 2968.73 cm⁻¹, 2968.47 cm⁻¹ and 2968.40 cm⁻¹ produces a 2f signal, with the modulation depth optimized to 0.27 cm⁻¹, yielding a peak amplitude of 0.101 V (Fig. 4f). The 1σ noise level was determined to be 112 µV, corresponding to a MDL of ~22 ppm for CH₄.

Finally to evaluate the thermoelastic detection capability of the LN-MFP at long-wave infrared region, a 9.7 µm quantum cascade laser (QCL) was employed for ammonia (NH₃) detection. The QCL was tuned from 1022.4 cm⁻¹ to 1023 cm⁻¹, covering the NH₃ absorption line at 1022.76 cm⁻¹. With an optimized modulation depth of ~0.15 cm⁻¹ for a 5% NH₃ sample, the LN-MFP achieved a SNR of 2014, yielding a MDL of ~25 ppm. The results were shown in Fig. 4g.

Similar to the PAS, calibration measurements for the LITES channels are summarized in Fig. 4. All channels exhibit excellent linearity with linear-fit R² > 0.99. To assess the long-term stability of the LN-MFP LITES sensor, we performed measurements identical to those used for the PAS characterization and computed Allan-deviation curves for each detection channel. From the Allan deviation analysis, the 1σ MDLs, and the corresponding optimal averaging times were extracted. The 450 nm (NO₂) channel achieves an MDL of ~1 ppm at an optimal averaging time of 450 s; the 1,392 nm (H₂O) channel, ~2.5 ppm at 1600 s; the 1536 nm (C₂H₂) channel, ~450 ppb at 1500 s; the 2004 nm (CO₂) channel, ~35 ppm at 1200 s; the 3370 nm (CH₄) channel, ~900 ppb at 1900 s; and the 9770 nm (NH₃) channel ~1.2 ppm at 1200 s.

Table 3 provides an overview of the LN-MFP’s thermoelastic detection performance for various gas species across the range 450 nm-9.77 µm. The table detail the light sources type, the target molecules, the laser power, the modulation amplitude, the sensitivity, and the normalized noise-equivalent absorption (NNEA) achieved. Note that we implemented a very short optical path of only 2.5 cm and the sensitivity and NNEA could be simply improved by employing multi-pass cells or resonant cavities. A comparison with state-of-the-art LITES is provided in the Supplementary Note 10.

The integration of thermoelastic detection into the LN-MFP demonstrates its versatility as a multifunctional spectroscopic platform. The spectral selectivity of both modes arises from the intrinsic ro-vibrational fingerprints of target molecules. The dual functionality does not alter this selectivity but extends its applicability across varying gases and conditions. By incorporating both PAS and LITES detection modes, the device enables adaptive gas sensing across a wide range of environments and measurement scenarios. In PAS mode, direct gas–sensor interaction provides high-sensitivity detection within a compact volume, ideal for confined or small-scale applications. Conversely, LITES operates in a non-contact configuration, suitable for sealed, corrosive, or otherwise harsh environments. This dual-mode architecture combines the sensitivity and compactness of contact-based detection with the robustness and flexibility of non-contact operation, enabling comprehensive spectroscopic analysis and positioning the LN-MFP as a versatile platform for next-generation sensing technologies.

PCB-level Co-packaged LN-MFP Sensor Module

Based on the abovementioned multifunctional capabilities, we next implemented a PCB-level co-packaged LN-MFP sensor module and evaluated its performance. As shown in the photograph in Fig. 5a, the wire-bonded QCL chip and LN-MFP are co-mounted on a common silicon carrier and installed on a custom 3.5 cm × 4.5 cm PCB that integrates the drive and readout electronics, including a transimpedance amplifier to pre-condition the LN-MFP charge output. An optical-microscope close-up of the coupling between the QCL facet and the LN-MFP tine gap is shown in Fig. 5b. The PCB features a stepped electrode pad to interface with the bottom-side electrode of the LN-MFP, while the top electrode is wire-bonded for signal extraction.

As proof of concept experiment, a QCL chip with the central wavelength of 4.6 µm was used. The output wavelength of this QCL chip was tuned by injection current to cover the CO absorption line at 2179.77 cm-1. The LN-MFP is coupled to the QCL chip with a distance of ~100 µm. No collimating optics were used in the system. The optical coupling efficiency was estimated to be ~29% (Fig. 5b and c). We have initially optimized the modulation amplitude to 0.36 cm-1. We employed a trumpet-shaped gas interface that directly covers the chip. The gas is fed to the sensor through this interface, flowing onto the chip at a rate of 100 sccm, consistent with standard experimental conditions, to help minimize noise. The 2f signal enables detection of 8000 ppm CO in N₂ using a PCB-level co-packaged LN-MFP–QCL module (Fig. 5d). This result demonstrates that the LN-MFP can serve as a compact and efficient sensing element in a hybrid QCL-LN configuration and highlighting a realistic pathway toward future fully integrated on-chip spectroscopic systems.

Furthermore, by co-packaging the LN-MFP chip with a mid-infrared QCL chip and the associated drive and readout electronics on a compact PCB, we demonstrate that the LN-MFP can be seamlessly interfaced with semiconductor laser sources using standard microelectronics assembly techniques. Experimental results reveal high sensitivity and selectivity across a broad spectral range, achieving detection limits suitable for real-world applications in environmental monitoring and industrial process control. Even though the current implementation is at the PCB level rather than monolithic, this hybrid QCL–LN architecture already reduces the footprint and alignment complexity of the spectrometer and establishes the LN-MFP as a practical building block for future fully integrated lithium-niobate spectroscopic chips.

Methods
Resonance evaluation

The resonance frequency of the lithium niobate multi-functional platform (LN-MFP) was characterized through electrical excitation to optimize its performance for spectroscopic applications. The experimental setup is depicted in Supplementary Fig. 1. A function generator, controlled by LabView software, produced sinusoidal voltage signals with a constant amplitude of 40 mV and frequencies ranging from 10,470 Hz to 10,500 Hz. These signals were applied to one terminal of the LN-MFP, inducing mechanical vibrations through the piezoelectric effect. The output current from the LN-MFP, which corresponds to its mechanical response, was converted into a voltage signal using a transimpedance amplifier with a 10 MΩ feedback resistor. This amplified signal was then processed by a lock-in amplifier, which was synchronized with the excitation frequency generated by the function generator. By selectively amplifying signals at the excitation frequency while suppressing noise at other frequencies, the lock-in amplifier enhanced the signal-to-noise ratio, ensuring precise detection and analysis of the LN-MFP’s vibrational response.

The demodulated signals were recorded and analyzed by a computer, enabling precise characterization of the LN-MFP’s resonance behavior. To accurately determine the center resonance frequency, a Lorentzian fitting was applied to the resonance curve. This precise frequency identification is essential for optimizing the device’s sensitivity and selectivity, as the resonance frequency plays a critical role in amplifying mechanical vibrations induced by photoacoustic and photothermal-elastic effects.

Experimental setup

The experimental setup for photoacoustic and light-induced thermoelastic detection using the LN-MFP is illustrated in Supplementary Fig. 2. A function generator provided modulation signals to the laser driver, utilizing either amplitude modulation (AM) or wavelength modulation (WM) depending on the characteristics of the light source. For non-tunable light sources, such as light-emitting diodes (LEDs), AM was employed by directly applying square or sinusoidal modulation signals to modulate the light intensity. In contrast, for tunable semiconductor lasers, including laser diodes (LDs), ICLs, or QCLs, WM was implemented by superimposing a sinusoidal signal onto a triangular waveform. The slow triangular wave enabled wavelength scanning by varying the injection current, while the superimposed sinusoidal dither signal enabled high-frequency modulation for lock-in detection.

The modulated light was collimated and directed either through the LN-MFP tines or onto its surface, depending on the detection mode. In PAS detection, the collimated light beam passed through the gap between the LN-MFP tines, where it interacts with gas molecules in the chamber. This interaction led to modulated molecular absorption, generating acoustic waves that were subsequently detected. In LITES detection, the light was instead focused directly onto the LN-MFP surface, where it induced thermal expansion and mechanical deformation through the thermoelastic effect.

The LN-MFP was enclosed within a gas chamber designed to enable controlled interaction with target gas molecules. The gas flow system comprised standard gas cylinders supplying calibrated analyte concentrations, mass flow controllers for precise flow regulation, pressure gauges for system stabilization, temperature controllers to ensure consistent operating temperatures, and a vacuum pump for efficient gas circulation. This setup provided precise control over gas composition and environmental conditions, ensuring optimal conditions for spectroscopic measurements.

Electrical signals generated by the LN-MFP, resulting from its mechanical response to the modulated light, were first amplified using a transimpedance preamplifier. The amplified signals were then demodulated by a lock-in amplifier synchronized with the modulation frequency, effectively enhancing detection sensitivity by filtering out noise and isolating the signal corresponding to target gas absorption. The lock-in amplifier was configured with a 1s time constant and a 12 dB/octave low-pass filter slope, corresponding to an effective detection bandwidth of 0.25 Hz. Processed signals were recorded using data acquisition software, and gas concentrations were determined based on the signal amplitude and pre-established calibration curves.

For alignment and verification, the transmitted or reflected light beam was captured by a camera after interacting with the LN-MFP. This enabled real-time monitoring of optical alignment, ensuring optimal interaction between the light and the sensing platform.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript presents an integrated lithium niobate platform (LN-MFP) that combines photoacoustic spectroscopy, light-induced thermoelastic spectroscopy, and broadband photodetection into a single fork-shaped piezoelectric device. The authors demonstrate trace gas detection over a broad spectral range, and the integration of multiple sensing functionalities within a single lithium niobate structure is promising, especially in terms of compact photonic sensor development. The experimental results clearly show the device functions. However, I believe the manuscript, in its current form, does not meet the publication standards of Nature Communications due to substantial limitations in rigor, design transparency, and technical completeness.\n1. The manuscript provides only schematics (Fig. 1a and Fig. 5a) without actual images of the fabricated LN-MFP device and the integrated QCL-LN system. Given that this work claims a significant advancement in integration and experimental realization, the absence limits the manuscript’s transparency and reproducibility. I strongly recommend the authors provide those below.\n(1) Optical microscope and SEM images of the fabricated LN-MFP\n(2) Photographs of the packaged sensor, including the wire-bonded QCL and LN-MFP on the PCB\n(3) Actual images of the experimental setup used for sensing demonstrations\n2. The manuscript introduces a fork-shaped LN structure but does not provide any systematic design optimization or performance trade-off analysis. While dimensions are reported, there is no discussion of how the geometry was selected to optimize resonance, signal strength, or multifunctional operation. In addition, I strongly recommend the authors clarify whether the geometry was optimized specifically for PAS, LITES, photodetection, or a compromise among them.\n3. The MDLs are estimated using 1-sigma noise, but the authors do not specify detailed measurement conditions, such as (1) lock-in amplifier time constant, (2) detection bandwidth, and (3) integration time or sampling rate. I believe this omission limits reproducibility and meaningful comparison with other platforms. In addition, it is strongly recommended to include Allan deviation analysis to evaluate long-term stability and optimal averaging time, as is standard practice in gas sensing.\n4. The description of light coupling between the QCL chip and the LN-MFP is vague. The ~100 μm free-space coupling distance is mentioned without supporting figures, optics specification, beam alignment tolerance, or coupling efficiency estimation. I believe that this is critical given the compact packaging and the divergence of QCL beams in the mid-IR. I recommend the authors (1) provide a photo and diagram of the actual optical alignment, (2) specify whether any collimating optics were used and, (3) discuss the stability of this coupling under practical conditions.\n5. Please include a calibration curve or concentration-response plots for the target gas species in the measurements with the LN-MFP device.\n6. Please provide the temporal response time of each sensing mode.\n7. The manuscript lacks sufficient detail regarding the fabrication process of the LN-MFP device. To improve reproducibility and technical clarity, please provide a more specific and comprehensive description of the process flow, including details on substrate preparation, lithographic patterning, etching methods (e.g., ICP parameters, mask materials), metallization, and any relevant bonding or release techniques.\n8. A summary/benchmark table comparing the LN-MFP sensor’s performance metrics (e.g., MDL, responsivity, bandwidth, response time) with existing PAS, LITES, and pyroelectric sensors would enhance the impact of the work.", "reviewer_2": "The manuscript describes a lithium niobate multifunctional integrated platform (LN-MFP) for spectroscopic gas sensing. Specifically, the authors present a systematic study of a lithium niobate tuning fork for photoacoustic, thermoelastic/photodetection-based sensing. It is of interest to see the direct bonding of the QCL chip to the LN-MFP, showing potential for integrated spectroscopic applications. This work will attract researchers from different areas such as spectroscopy, sensing, integrated photonics, etc. The following issues should be addressed before this work can be considered for publication.\n1. One key application shown in this study is photoacoustic spectroscopy. The authors show a good record of adopting tuning forks (especially quartz tuning fork, QTF) in gas sensing with demonstrated high sensitivity across a broad spectral range. However, in the manuscript, the authors seem to only report the PAS gas sensing results without providing sufficient discussion. A detailed comparison with state-of-the-art tuning fork-based PAS gas sensing systems is needed to highlight the key advantages and advancements offered by the designed LN tuning fork.\n2. In addition, the following information should be added to strengthen the technical content:\n(1) Experimental setup for each light source is suggested to be documented in the supplement.\n(2) Better to include N₂ background signals in Fig. 2 for direct comparison.\n3. Regarding the demonstration of thermoelastic detection (LITES), the reviewer notices tuning forks have achieved high sensitivity in the visible, near-IR, and mid-IR. The manuscript should compare with state-of-the-art LITES results and clarify how the LN tuning fork advances beyond these results.\n4. Following the previous question, some specific concerns:\n(1) The claim that \"resonant amplification significantly boosts sensitivity\" is confusing, as the LITES performance is notably worse than PAS. It is noticed that the author used a 2.5 cm optical length, but the results are still unsatisfied to support your claims. Further discussion is needed to clarify this discrepancy.\n(2) Clarify how dual PAS/LITES functionality enables comprehensive analysis. It is not straightforward for readers to understand how combining them together can leverage the sensing power of LN-MFP.\n(3) Clarify the relationship between spectral selectivity and dual functionality, as selectivity is primarily determined by ro-vibrational fingerprints.\n5. It seems there is an overlap between the photodetection functionality with the thermoelastic sensing mechanism. Is it more reasonable to move this section before LITES as it relates to the fundamental mechanism? In addition, the modulated photodetection using tuning forks has been previously demonstrated, so it is more interesting to make a comparison with state-of-art photodetection using tuning forks, highlighting key pros and cons. Also, it is better to make a fair comparison of noise-equivalent power (NEP) with commercial photodetectors, which is more important in photodetection.\n6. LN is an important photonic platform used in many areas as described in the first paragraph of Introduction. However, the authors missed mentioning the spectroscopic sensing applications of using LN. May be not too many in this topic, but at least there are a couple of pioneering work recently reported on fabricating LN rib waveguide and using it for on-chip photothermal gas sensing. As spectroscopic sensing the key topic of this paper, the authors should make a more complete literature to strengthen this part.\nAdditional concerns:\n(1) Why is the tuning fork placed horizontally rather than vertically?\n(2) It is more interesting to include a photo/microscope image of the on-chip LN-MFP sensor in Figure 5.\n(3) The results show the signal of 8000 ppm CO. Please discuss limitations and potential solutions.\n7. Regarding the design and fabrication of the LN-MFP, a discussion and comparison with existing LN, quartz, silicon, and polymer tuning forks should be added. The key parameters (resonant frequency, quality factor) should be moved to the main text, with detailed simulation and characterization data properly referenced in supplementary materials. The key parameters like resonant frequency, quality factor, and dimensions will be useful for readers to understand the advancements of the proposed device.", "reviewer_3": "The paper presents a new device made from lithium niobate (LiNbO₃) that can do three things: detect sound made by light (photoacoustic), sense heat changes (thermoelastic), and respond to light across a wide range from visible to long-wave infrared. The experiments show it could be useful for small integrated sensors. The authors used the special electric and optical properties of LiNbO₃ to build a small sensor that does multiple things, which is both creative and technically important. I think this paper could be published if some issues are fixed. Here are my concerns:\n-The paper says they added an acoustic resonator around the LN-MFP to make the signal stronger. According to the Supporting Information, the signal became 79 times bigger. But how big is the resonator compared to the tuning fork? How are they placed relative to each other? Did they try to match the size of the resonator to the frequency of the tuning fork? Also, did they test or simulate how well the resonator and tuning fork work together?\n-In the thermoelastic detection part, the laser was pointed at the side of the tuning fork prong instead of the front. Why was that done? Does lighting from the side help create more thermal stress or better vibration? Were any comparisons done between side and front illumination to see which works better?\n-The paper shows simulations saying that the shape and position of the electrodes affect how the device vibrates. But there’s no experiment to prove this. Have the authors made real devices with different electrode shapes and tested them, like checking their frequency response or how they vibrate?\n-The authors used a Y-cut 128° LiNbO₃ crystal but didn’t explain why. Since LiNbO₃ has different properties depending on how it's cut (like Z-cut, X-cut, etc.), maybe this cut gives better piezoelectric effect or mechanical quality. Could the authors say what makes this cut better for their device? Did they compare it with other cuts in simulations or tests?\n-The paper mentions using a lock-in amplifier to get the 2f signal, but doesn't say how it was set up. Important settings like time constant, filter slope, reference phase, and bandwidth can affect the signal quality. Could the authors give more details about these settings?\n-In many sensing methods like QEPAS and LITES, quartz is usually used because it’s stable and has good resonance. Why did the authors choose LiNbO₃ instead? Is it because it can do more than one thing, has better sensitivity, or can be used with light on a chip? A comparison with quartz would help explain this choice.\n-The paper combines photoacoustic, thermoelastic, and photodetection into one device, but it’s not clear why all three are needed together. What advantages does this combination give? It would help to have a comparison with using them separately.\n-For the photodetection tests, the authors changed the light power to see how the device responds. But since LiNbO₃ absorbs different wavelengths differently, how do they know the change in signal is only because of power and not wavelength? Were the measurements done at fixed wavelengths? Or were power and wavelength controlled separately?\n-In the conclusion, the authors talk about making a small photonic chip from this platform. But how will they turn this lab-scale device into something small enough for a chip? There are challenges like tiny fabrication, connecting parts on a wafer, and packaging. Also, how will they add light sources directly onto the same chip?\n-The paper introduces a new multi-purpose LiNbO₃ platform, but it doesn’t compare it clearly with existing technologies like QEPAS or LITES. Could the authors give a table or explanation comparing key performance numbers like detection limit, speed, and noise equivalent power?"}, "author_response": "Response to reviewers’ comments\nDear Reviewers:\nPlease find attached our point-by-point responses to all comments from the three reviewers. We\nhave revised the manuscript accordingly.\nPlease Note: reviewers’ comments are in black; our comments are in red italic; the original\npaper text is in red and revised text is in blue.\nReviewer #1:\nThis manuscript presents an integrated lithium niobate platform (LN-MFP) that combines\nphotoacoustic spectroscopy, light-induced thermoelastic spectroscopy, and broadband\nphotodetection into a single fork-shaped piezoelectric device. The authors demonstrate trace\ngas detection over a broad spectral range, and the integration of multiple sensing\nfunctionalities within a single lithium niobate structure is promising, especially in terms of\ncompact photonic sensor development. The experimental results clearly show the device\nfunctions. However, I believe the manuscript, in its current form, does not meet the\npublication standards of Nature Communications due to substantial limitations in rigor, design\ntransparency, and technical completeness.\n1. The manuscript provides only schematics (Fig. 1a and Fig. 5a) without actual images of the\nfabricated LN-MFP device and the integrated QCL-LN system. Given that this work claims a\nsignificant advancement in integration and experimental realization, the absence limits the\nmanuscript’s transparency and reproducibility. I strongly recommend the authors provide\nthose below.\n(1) Optical microscope and SEM images of the fabricated LN-MFP\nWe sincerely thank the Reviewer for this constructive suggestion. Optical-microscope and SEM\nimages of the fabricated LN-MFP have now been added to Fig. 1 (panels d–f) in main article\npage 6.\n1 / 63\n“Fig. 1 | Lithium Niobate Multi-Functional Platform. a Design of the LN-MFP. b Simulated\nsurface charge distribution of LN-MFP during vibration. The red regions correspond to positive\ncharge accumulations, blue regions to negative ones. c Manufacturing process of LN-MFP; d\nOptical microscope image of the fabricated LN-MFP; e SEM image of the edge of the LN-MFP\ntine. f SEM image of the fillet of LN-MFP base.”\nRelated descriptions are added in main article page 5 line 24:\n“Figure 1d shows an optical microscope image of the fabricated LN-MFP, while Fig. 1e and\nFig. 1f present SEM images of the LN-MFP tine edge and the base fillet, respectively.”\n(2) Photographs of the packaged sensor, including the wire-bonded QCL and LN-MFP on the\nPCB\nThanks for the comment. The photograph of the packaged sensor was added to the main\narticle page 18, Fig.5a.\nThe wire-bonded QCL chip and the LN-MFP are co-mounted on a common silicon carrier\nand installed on a 3.5 cm × 4.5 cm PCB that integrates the readout electronics. The QCL\nemission facet was aligned to the LN-MFP prong gap under an optical microscope. The free-\nspace separation between the QCL facet and the prong edge is ~100 µm.\n2 / 63\nFig. 5 | On chip LN-MFP sensor. a Photograph of the packaged on-chip sensor.\nRelated descriptions are added in main article page 17 Line 23. We replaced:\n“As shown in Fig. 5(a), we realized a custom-designed 3.5 cm×4.5 cm printed circuit board\n(PCB) with integrated transimpedance amplifier function to pre-process the charge output of\nLN-MFP and a QCL chip.”\nwith\n“As displayed in the photograph shown in Fig. 5a, the wire-bonded QCL chip and LN-MFP are\nco-mounted on a common silicon carrier and installed on a custom 3.5 cm × 4.5 cm PCB that\nintegrates the drive and readout electronics, including a transimpedance amplifier to pre-\ncondition the LN-MFP charge output.”\n(3) Actual image of the experimental setup used for sensing demonstrations\nA photo of the experimental setup was added in Supplementary Section 10, page 17 Fig. 7s.\n3 / 63\nFig. 7s | Photo of the employed experimental setup. PC: personal computer, DM: detection\nmodule.\nRelated descriptions are added to the Supplementary page 16 line 8:\n“Figure 7s shows a photo of the experimental setup used for the LN-MFP measurements. A\nfunction generator (Tektronix AFG3102) supplied the modulation waveform to the light\nsources. The sources, as listed in the Table 5s, were driven and temperature-stabilized by a\ncurrent/TEC controller (Thorlabs ITC4005QCL). The emitted beam was routed through the\ndetection module. A camera (Dataray WinCamD-IR-BB) was employed for beam profiling and\n4 / 63\nalignment. The electrical output from the detection module was demodulated by a lock-in\namplifier (SRS SR830). A personal computer was used for instrument control and data\nacquisition.”\nTable 5s. The wavelength and model of seven light sources.\nLaser Powe IC\nWavelengt Modulatio Temperatur\nsourc Model r (mA\nh (nm) n method e (℃)\ne (mW) )\nVIS\nNICHIA 450 450 1200 AM 600 /\nLED\nNIR NTT Electronics\n1390 19 FM 84 28\nLD Corporation\nNIR NTT Electronics\n1536 3700* FM 90 25\nLD Corporation\nNIR NTT Electronics\n2004 10.6 FM 120 18\nLD Corporation\nMWI\nNANOPLUS 3370 6.5 FM 86 17\nR ICL\nQCL Chinese Academy\n4590 14 FM 270 45\nChip of Sciences\nLWIR HEATHYPHOTO\n9770 82.3 FM 420 20\nQCL N\nIC: injection current; AM: amplitude modulation; FM: frequency modulation; *Amplified\nby EDFA\n2. The manuscript introduces a fork-shaped LN structure but does not provide any systematic\ndesign optimization or performance trade-off analysis. While dimensions are reported, there\nis no discussion of how the geometry was selected to optimize resonance, signal strength, or\nmultifunctional operation. In addition, I strongly recommend the authors clarify whether the\ngeometry was optimized specifically for PAS, LITES, photodetection, or a compromise among\n5 / 63\nthem.\nWe thank the reviewer for this important comment. Below we detail the design objectives and\nthe technical rationale behind the chosen fork geometry. We describe the systematic study\nperformed to identify the final dimensions as a practical compromise among photoacoustic\n(PAS), light-induced thermoelastic (LITES) and photodetection performance. The following\ncontent has already been added to the Supplementary page 3 section 2.\n“The resonance frequency and quality factor (Q) of the LN-MFP are governed by both\nmaterial properties and geometric configuration. In this work, the geometry was carefully\noptimized to balance the competing requirements of its multifunctional operation, including\nPAS, LITES, and photodetection. To guide this optimization, we present a developed\nsystematic design procedure detailing how the geometric parameters of the LN-MFP were\nselected and refined.\n(1) Optimization for Resonance\nThe geometry and resonance of the LN-MFP were optimized by varying two key geometric\nparameters: the tine length 𝑙 and width w, while keeping all other parameters fixed (see Fig.\n1s a for definitions). A parametric finite-element sweep of l and w revealed that the fundamental\nresonance f decreases monotonically with increasing l and with decreasing w (Fig. 1s b). In the\n0\nsimulated design space f ranged from ~ 34.5 kHz (l =6 mm, w = 2 mm) to ~2.8 kHz (l = 18\n0\nmm, w =1 mm).\nFor the LN-MFP, excessively high resonant frequency 𝑓 yields shorter vibration period,\n0\nlimiting the time available for the device to accumulate acoustic energy. Since the light-source\nmodulation must match the LN-MFP resonance frequency, high f₀ values require faster\nmodulation rates, which reduce the efficiency of photoacoustic generation for molecules with\nlong vibrational-to-translational (V–T) relaxation times. For example, CO₂ exhibits V–T\nrelaxation times on the order of 100 μs (~ 10 kHz), and modulation beyond this frequency\nsignificantly diminishes photoacoustic generation efficiency. By contrast, very low f increases\n0\nsusceptibility to low-frequency environmental disturbances such as mechanical vibration, air\ncurrents and 1/f (pink) noise.\nBalancing these effects, an optimal frequency f ~ 10.5 kHz was selected, corresponding\n0\n6 / 63\nto tine dimensions of l=11.5 mm, w=1.7 mm.\nFig. 1s | a Schematic of the LN-MFP showing the defined geometric parameters: tine length l,\nbase width W, tine width w, inter-tine gap g, thickness t, and fillet radius r. b Finite-element\nsimulation of the fundamental resonant frequency f as a function of tine length 𝑙 and width w.\na frequency range from ~34.5 kHz to ~2.8 kHz across the design space.\n(2) Optimization for PAS\nThe LN-MFP geometry was optimized for photoacoustic spectroscopy (PAS) through coupled\nacoustic–structural finite-element simulations. In the acoustic excitation model, a cylindrical\npressure source was positioned within the tine gap to reproduce the spatial distribution of the\nphotoacoustic pressure wave and to induce symmetric tine oscillation (see Fig. 2s a). For each\ngeometric configuration the total piezoelectric charge generated by the device was calculated.\nThe resulting contour map of normalized piezoelectric charge exhibits a clear and\nmonotonic dependence on tine dimensions (Fig. 2s b). The PAS signal strength increases with\ngreater tine length l and with smaller tine width w. Physically, longer and more slender tines\nexhibit larger tip displacements and stronger strain localization near the tine base under acoustic\nexcitation, hereby generating a higher piezoelectric charge output and enhancing signal\namplitude.\n7 / 63\nFig. 2s | a Simulated deformation and strain distribution of the LN-MFP under PAS; b\nNormalized piezoelectric charge as a function of tine length l and tine width w.\n(3) Optimization for LITES and photodetection\nBecause both LITES and direct photodetection originate from light-induced heating, they were\nanalyzed within a unified thermoelastic framework. In this model, a localized heat source was\napplied at the LN-MFP base to simulate the photothermal excitation induced by incident light\n(see Fig. 3s a). For each geometric configuration, the total piezoelectric charge collected by the\nelectrode was computed. The resulting contour map of normalized piezoelectric charge in (Fig.\n3s b) reveals a systematic increase with the tine width w and only a weak dependence on tine\nlength l. This trend arises from several factors. First, wider tines provide a larger heated volume\nand absorption area for a given illumination spot, leading to greater overall thermal expansion\nand a larger bending moment. Second, increasing w enlarges the electromechanically active\nvolume and electrode-covered area, enhancing conversion of localized strain into piezoelectric\ncharge. Third, because the thermoelastic excitation is concentrated near the base, variations in\ntine length l exert minimal influence on the thermal-to-mechanical coupling in this region.\n8 / 63\nFig. 3s | a Simulated deformation and strain distribution of the LN-MFP under LITES and\nphotodetection excitation; b Contour map of normalized piezoelectric charge as a function of\ntine length l and tine width w.\n(4) Other geometry parameters\n-Tine gap g\nTo accommodate diverse optical sources, from visible LEDs to mid-IR QCLs, and to\nbalance optical and acoustic requirements, the tine gap was fixed at g=1.0 mm. The gap size is\ngoverned by two competing effects. For LITES and photodetection, light is incident on the\nouter tine surfaces, whereas in PAS operation, the excitation beam must traverse the gap\nwithout contacting the electrodes. If the gap is too narrow, the laser spot overlaps the metalized\ntines, introducing significant photothermal background noise. Conversely, if the gap is too wide,\nthe acoustic pressure field weakens, reducing acoustic-to-mechanical coupling efficiency.\nTherefore, a 1.0 mm gap represents a practical compromise between these competing optical\nand acoustic constraints.\n- Device thickness t\nCommercial lithium-niobate wafers are typically available in thicknesses of ~500 μm.\nWithin the explored design-space, variation in device thickness produced only minor effects on\nthe fundamental resonance compared to the dominant influence of tine length and width. Its\nimpact on electromechanical and thermoelastic responses was similarly limited. Considering\n9 / 63\nfactors such as wafer availability, fabrication precision, mechanical robustness and process\nrepeatability, a thickness of t=500 μm was selected for the fabricated LN-MFP devices.\n(5) Conclusion\nIn summary, a comprehensive finite-element parameter sweep combined with\nmultiphysics simulation was conducted to optimize the LN-MFP geometry across three key\nperformance dimensions: resonant behavior, photoacoustic (PAS) response, and\nthermoelastic/photodetection response. The inherently opposing design requirements of PAS\nand LITES/photodetection necessitated a balanced configuration. The finalized structure, with\nl=11.5 mm, w =1.7 mm, g=1.0mm, and t=500 μm, exhibits a fundamental resonant frequency\nnear 10.5 kHz. This optimized design achieves high PAS sensitivity alongside strong\nLITES/photodetection responsivity, ensuring multifunctional operation with excellent\nmechanical stability, fabrication compatibility and system integration compatibility.”\n3. The MDLs are estimated using 1-sigma noise, but the authors do not specify detailed\nmeasurement conditions, such as (1) lock-in amplifier time constant, (2) detection bandwidth,\nand (3) integration time or sampling rate. I believe this omission limits reproducibility and\nmeaningful comparison with other platforms. In addition, it is strongly recommended to\ninclude Allan deviation analysis to evaluate long-term stability and optimal averaging time, as\nis standard practice in gas sensing.\nMany thanks for the comment. According to the reviewer’s suggestion, we added the\nrequested measurement conditions and Allan deviation analysis to revised manuscript.\nIn main article page 25 line 4, we added:\n“The lock-in amplifier was configured with a 1 s time constant and a 12 dB/octave low-pass\nfilter slope, corresponding to an effective detection bandwidth of 0.25 Hz.”\nRegarding the Allan deviation analysis, for PAS, we added the following sentences in main\narticle page 8 line 18:\n“To assess the long-term stability of the LN-MFP PAS sensor, we performed noise\ncharacterization and Allan-deviation analysis (see Fig. 2). The 1σ noise levels were measured\nunder a continuous flow of 200 sccm pure N₂. Figures 2b–g show the corresponding noise traces\n10 / 63\nand Allan-deviation curves for the LN-MFP operated with a 450 nm LED, 1392 nm laser diode,\n1536 nm laser diode, 2004 nm laser diode, 3370 nm ICL, and 9770 nm QCL. From the Allan\ndeviation analysis, the 1σ MDLs, and optimal averaging times were extracted: the 450 nm (NO₂)\nchannel achieves an MDL of ~2 ppb at an averaging time of 1,000 s; the 1,392 nm (H₂O),\n~25 ppb at 6,300 s; the 1,536 nm (C₂H₂), ~3.5 ppb at 100 s; the 2,004 nm (CO₂), ~350 ppb at 1,900\ns; the 3,370 nm (CH₄), ~2.5 ppb at 1,200 s; and the 9,770 nm (NH₃), ~350 ppt at 1,100 s.\nFig. 2 | LN-MFP for photoacoustic detection. a Spectral coverage with excitation sources\nfrom visible to long-wave infrared and the targeted analytes. b-g present representative\nmeasurements for each target gas: the main plot in each panel shows a typical 2f signal acquired\nwith the indicated light source and sample concentration; the left inset shows the concentration-\ncalibration (linear fit); the top-right inset shows a 1σ noise level obtained with pure N₂; and the\nbottom-right inset shows the Allan-deviation curves used to extract the1σ minimum detection\nlimit (MDL) and optimal averaging times. Extracted MDLs (1σ) and averaging times are: b\n11 / 63\nMDL ~2 ppb at 1,000 s; c MDL ~25 ppb at 6,300 s; d MDL ~3.5 ppb at 1,000 s; e MDL ~350\nppb at ~1,900 s; f MDL ~2.5 ppb at 1,200 s; g MDL ~350 ppt at 1,100 s. VIS LED: visible light\nemitting diode; NIR LD: near-infrared laser diode; MWIR ICL: mid-wave interband cascade\nlaser; LWIR QCL: long-wave quantum-cascade laser.\nAnd also, for LITES, in main article page 16 line 10:\n“To assess the long-term stability of the LN-MFP LITES sensor, we performed measurements\nidentical to those used for the PAS characterization and computed Allan-deviation curves for\neach detection channel. From the Allan deviation analysis, the 1σ MDLs, and the corresponding\noptimal averaging times were extracted. The 450 nm (NO₂) channel achieves an MDL of ~1\nppm at an optimal averaging time of 450 s; the 1,392 nm (H₂O) channel, ~2.5 ppm at 1,600s;\nthe 1,536 nm (C₂H₂) channel, ~450 ppb at 1500 s; the 2,004 nm (CO₂) channel, ~35 ppm at\n1,200 s; the 3,370 nm (CH₄) channel, ~900 ppb at 1,900 s; and the 9,770 nm (NH₃) channel\n~1.2 ppm at 1,200 s.”\n12 / 63\nFig. 4 | LN-MFP for thermoelastic detection. a Spectral coverage of excitation sources from\nvisible to long-wave infrared, together with the corresponding targeted analytes. b-g\nRepresentative measurements for each target gas species: the main plots shows typical 2f signal\nacquired with the indicated light source and gas concentration. The left inset displays the\nconcentration-calibration (linear fit); the top-right insets show the 1σ noise level measured for\npure N₂; and the bottom-right insets present the Allan-deviation curves used to extract the 1σ\nMDL and optimal averaging times. Extracted MDLs (1σ) and optimal averaging times are: b\nMDL ~ 1 ppm at 450 s; c MDL ~ 2.5 ppm at 1,600 s; d MDL ~ 450 ppb at 1500 s; e MDL ~ 35\nppm at 1,200 s; f MDL ~900 ppb at 1,900 s; g MDL ~1.2 ppm at 1,200 s.\n13 / 63\n4. The description of light coupling between the QCL chip and the LN-MFP is vague. The ~100\nμm free-space coupling distance is mentioned without supporting figures, optics specification,\nbeam alignment tolerance, or coupling efficiency estimation. I believe that this is critical given\nthe compact packaging and the divergence of QCL beams in the mid-IR. I recommend the\nauthors (1) provide a photo and diagram of the actual optical alignment, (2) specify whether\nany collimating optics were used and, (3) discuss the stability of this coupling under practical\nconditions.\nWe thank the reviewer for this valuable comment.\n(1) provide a photo and diagram of the actual optical alignment\nWe have replaced the original Fig. 5 with an expanded Fig. 5 in main article page 18 to show\nthe photo and diagram.\n14 / 63\nFig. 5 | On chip LN-MFP sensor. a Photograph of the packaged on-chip sensor. b Optical-\nmicroscope image of the coupling region showing the QCL emission facet aligned with the LN-\nMFP prong gap. c Schematic diagram of the free-space optical coupling. d 2f photoacoustic\nspectrum of 8,000 ppm CO in N₂ measured using the integrated LN-MFP–QCL assembly.\nThe expanded Fig.5 includes: a a high-resolution photograph of the packaged on-chip\nassembly showing the wire-bonded QCL chip and the LN-MFP on the silicon carrier; b an\noptical-microscope close-up of the coupling region showing the QCL facet and the tine gap;\nand c a schematic that reproduces the measured geometry and beam path.\n(2) specify whether any collimating optics were used\nIn main article, page 18 line 5, we add the following sentence:\n“As shown in Fig. 5b and 5c, no collimating optics were used in the system.”\nNo external collimating or focusing optics were employed in the on-chip coupling\nexperiments described in this work. The QCL was operated as a bare die, emitting freely across\nthe gap toward the LN-MFP. Prior to alignment, the QCL emission pattern was characterized\nusing a mid-IR camera to verify beam divergence and ensure proper coupling geometry.\n15 / 63\n(3) discuss the stability of this coupling under practical conditions\nAs shown in Fig. 5b–c, the QCL chip was aligned directly to the LN-MFP tine gap without the\nuse of collimating optics. The QCL far-field, measured with a mid-IR camera, exhibited a\ndivergence of ~70°. The tine gap was measured under an optical microscope as ≈1.0 mm,\nand the separation between the QCL and LN-MFP was ~100 µm. The QCL chip was soldered on\na silicon carrier, and after fine alignment under the microscope, the LN-MFP was fixed in\nposition using conductive silver paste. This simple free-space coupling configuration enables\nproof-of-concept PAS measurements (see Fig. 5d). Because no precision collimation optics were\nemployed, the coupling tolerances are relatively relaxed. Only a portion of the emitted light\nneeds to overlap the tine gap to generate a detectable signal, which simplifies on-chip\nalignment and ensuring mechanically stability. However, the absence of beam shaping\nelements also allows partial stray illumination of the tine faces, introducing photothermal\nbackground, reducing optical coupling efficiency and potentially increasing noise that limits\nlong-term stability.\nTo further improve coupling efficiency and robustness, future designs will explore: (i) the\nintegration of a micro-collimator or compact mid-IR lens between the QCL facet and the tine\ngap to reduce divergence and enhance beam overlap with the gap; (ii) incorporation of a\nmolded microlens or etched micro-optic on the silicon carrier; (iii) monolithic on-chip coupling\nstrategies such as integrated waveguide or butt-coupling to eliminate free-space tolerances;\nand (iv) hermetic, thermally-matched packaging with mechanical stabilization to minimize drift\nduring thermal cycling.\nTo make it clear to the readers, the following sentences have been added to the\nSupplementary page 13 Section 8：\n“(1) Optical-alignment and stability.\nThe QCL chip was aligned directly to the LN-MFP tine gap without external collimating optics.\nThe QCL far-field characterized using a mid-IR camera, exhibited a divergence of ~70°. The\ntine gap, under an optical microscope was ~1.0 mm and the separation between QCL facet and\nLN-MFP was ~100 µm. The QCL chip was soldered to a silicon carrier, and after fine optical\nalignment under the microscope, the LN-MFP was secured in position with conductive silver\n16 / 63\npaste. This free-space coupling configuration enables robust proof-of-concept PAS\nmeasurements with relaxed lateral alignment tolerances.\n(2) Limitations and mitigation.\nIn the absence of beam-shaping optics, part of the emitted power falls outside the LN-MFP tine\ngap, leading to stray illumination of the tine surfaces. This produces photothermal background\nsignals, reduces optical coupling efficiency, and can ultimately limit long-term stability and the\nachievable minimum detection limit (MDL). To enhance coupling efficiency and robustness in\nfuture iterations, several strategies will be explored: (i) integrating a micro-collimator or\ncompact mid-IR lens between the QCL facet and the tine gap to reduce divergence and improve\noverlap; (ii) incorporating molded or etched microlenses on the silicon carrier to shape the beam\nat the chip level; (iii) implementing monolithic on-chip coupling approaches, such as integrated\nwaveguide or butt-coupling to eliminate free-space tolerances and further stabilize the optical\nalignment.”\n17 / 63\n5. Please include a calibration curve or concentration-response plots for the target gas species\nin the measurements with the LN-MFP device.\nWe thank the reviewer for this important suggestion. We have added the calibration curves to\nFig.2 and Fig.4 in the main article page 9 and page 14.\nFig. 2 | LN-MFP for photoacoustic detection. a Spectral coverage with excitation sources\nfrom visible to long-wave infrared and the targeted analytes. b-g present representative\nmeasurements for each target gas: the main plot in each panel shows a typical 2f signal acquired\nwith the indicated light source and sample concentration; the left inset shows the concentration-\ncalibration (linear fit); the top-right inset shows a 1σ noise level obtained with pure N₂; and the\nbottom-right inset shows the Allan-deviation curves used to extract the1σ minimum detection\nlimit (MDL) and optimal averaging times. Extracted MDLs (1σ) and averaging times are: b\nMDL ~2 ppb at 1,000 s; c MDL ~25 ppb at 6,300 s; d MDL ~3.5 ppb at 1,000 s; e MDL ~350\nppb at ~1,900 s; f MDL ~2.5 ppb at 1,200 s; g MDL ~350 ppt at 1,100 s. VIS LED: visible light\nemitting diode; NIR LD: near-infrared laser diode; MWIR ICL: mid-wave interband cascade\n18 / 63\nlaser; LWIR QCL: long-wave quantum-cascade laser.\nRelated description in main article page 8 line 16:\n“Responses to varying target-gas concentrations were measured and the corresponding\ncalibration curves are plotted in Fig. 2. As shown in Fig. 2, all channels exhibit excellent\nlinearity, with linear-fit coefficients of R2>0.99.”\nFig. 4 | LN-MFP for thermoelastic detection. a Spectral coverage of excitation sources from\nvisible to long-wave infrared, together with the corresponding targeted analytes. b-g\nRepresentative measurements for each target gas species: the main plots shows typical 2f signal\nacquired with the indicated light source and gas concentration. The left inset displays the\nconcentration-calibration (linear fit); the top-right insets show the 1σ noise level measured for\npure N₂; and the bottom-right insets present the Allan-deviation curves used to extract the 1σ\nMDL and optimal averaging times. Extracted MDLs (1σ) and optimal averaging times are: b\nMDL ~ 1 ppm at 450 s; c MDL ~ 2.5 ppm at 1,600 s; d MDL ~ 450 ppb at 1500 s; e MDL ~ 35\nppm at 1,200 s; f MDL ~900 ppb at 1,900 s; g MDL ~1.2 ppm at 1,200 s.\n19 / 63\nRelated description in main article page 16 line 8:\n“Similar to the PAS, calibration measurements for the LITES channels are summarized in Fig.\n4. All channels exhibit excellent linearity with linear-fit R2>0.99.”\n6. Please provide the temporal response time of each sensing mode.\nThanks for the comment. In this work, the lock-in amplifier and data acquisition module were\nconfigured to output one data point per second, corresponding to a theoretical minimum\nresponse time of 1 s.\nIt is noteworthy that the theoretical temporal response times of PAS, LITES, and\nphotodetection in this work are significantly shorter than 1 s. For optical detection techniques\nbased on resonance principles, the absorbed laser power is accumulated within the resonator\nover Q oscillation periods, where Q denotes the quality factor of the resonator [1]. The time\nrequired for the signal amplitude to rise from zero to a steady-state level, the response time,\ncan be estimated by the theoretical model τ = Q / f₀ [2]. For the LN-MFP, with a resonance\nfrequency f₀ = 10,485 Hz and a Q factor of 1,621, the corresponding theoretical response time\nis approximately 150 ms. However, in this work, to guarantee stable operation of the LN-MFP\nsystem and to average the signal fluctuations, the output rate was intentionally limited to one\ndata point per second. Therefore, the effective temporal response time of the entire LN-MFP\nsensing system is approximately 1 s.\nIn the revised manuscript, we added the following sentence in the main article page 25\nline 4 to provide the response time:\n“The lock-in amplifier was configured with a 1 s time constant and a 12 dB/octave low-\npass filter slope, corresponding to an effective detection bandwidth of 0.25 Hz.”\nReference\n[1] Kosterev A A, Bakhirkin Y A, Curl R F, et al. Quartz-enhanced photoacoustic spectroscopy[J].\nOptics letters, 2002, 27(21): 1902-1904.\n[2] Zhang H, Jin W, Hu M, et al. Investigation and optimization of a line-locked quartz enhanced\nspectrophone for rapid carbon dioxide measurement[J]. Sensors, 2021, 21(15): 5225.\n20 / 63\n7. The manuscript lacks sufficient detail regarding the fabrication process of the LN-MFP\ndevice. To improve reproducibility and technical clarity, please provide a more specific and\ncomprehensive description of the process flow, including details on substrate preparation,\nlithographic patterning, etching methods (e.g., ICP parameters, mask materials),\nmetallization, and any relevant bonding or release techniques.\nThanks for the comment. According to the reviewer’s suggestion, we provided a more specific\nand comprehensive description of the process flow in the revised manuscript and the\nSupplementary.\nIn the revised main article page 5 line 19, we add:\n“A more specific and comprehensive description of the process flow was provided in the\nSupplementary Section 3.”\nIn the Supplementary page 7 line 15 Section 3, we add the following text:\n“The LN-MFP device was fabricated on a double-side-polished Y+128°-cut LiNbO₃ wafer with\na thickness of 500 μm. The process began with wafer slicing, edge grinding, and double-side\npolishing to ensure surface flatness and uniform thickness followed by ultrasonic cleaning to\nremove microscopic contaminants.\nA Cr/Au (20/200 nm) bilayer was then deposited to form the electrode layer using an\nelectron-beam evaporation system (model DE400, Texas Instruments, USA) under a base\nvacuum of 3 × 10⁻⁶ Torr. The Cr adhesion layer was evaporated at 30 W with a rate of 0.5 Å/s\nfor 400 s, followed by deposition of the Au layer at 40 W, 0.8 Å/s for 2,500 s ensuring uniform\nfilm adhesion and high surface conductivity.\nSubsequently, 355 nm UV photolithography was employed to define the electrode\ngeometry directly on the wafer surface. Both electrode patterning and device dicing were\nperformed using a UV laser (5 W, 20 kHz repetition rate, 30 μs pulse width), achieving\nmaskless fabrication with ±10 μm precision. During electrode definition, each exposure lasted\n10 s, sufficient to pattern the Cr/Au electrodes. For dicing, the same laser system operated for\nover 1200 s, fully cutting through the 500 μm-thick LN substrate.\nFollowing laser processing, an “optical-grade dicing” and polishing step was implemented\n21 / 63\nto refine structural precision. The LN-MFPs were mounted on a UV-sensitive adhesive film\nattached to a stainless-steel polishing frame. A high-precision optical dicing system equipped\nwith a 56 mm-diameter, 150 µm-thick resin-bonded diamond wheel was used to polish the\nsidewalls at a rotation speed of 10, 000 rpm and a feed rate of 0.2 mm/s, with deionized-water\ncirculation maintaining a stable temperature of 18 °C.\nFinally, the fabricated LN-MFP was integrated onto the chip via gold wire bonding\ntechnology. The bonding process used controlled heat, pressure, and ultrasonic energy to\nconnect the surface electrodes of the LN-MFP to the silicon carrier. The bonded gold wires\nwere 20 μm in diameter and the backside electrode was attached with conductive silver paste\nto ensure reliable grounding and enhanced electrical contact.”\n8. A summary/benchmark table comparing the LN-MFP sensor’s performance metrics (e.g.,\nMDL, responsivity, bandwidth, response time) with existing PAS, LITES, and pyroelectric\nsensors would enhance the impact of the work.\nMany thanks for the comment. Three detailed tables about PAS, LITES, and pyroelectric sensors\nwere added to the Supplementary page 13 section 9:\n“In photoacoustic spectroscopy, the key parameters that govern detection performance are the\nlaser wavelength, optical power, and integration time. These parameters together with the\nminimum detection level (MDL), relative MDL (accounting for the proportionality of the\nphotoacoustic signal to laser power), and normalized noise equivalent absorption (NNEA)\ncoefficient, are summarized in Table 2s. Notably, the LN-MFP sensor is chip-scale offering\ninherent advantages in compactness, robustness, and ease of system integration.\n22 / 63\nTable 2s | PAS detection of trace gases using LN-MFP and QTF.\nTarget Detector Wavelength IT Power MDL RMDL NNEA\nanalyte type (μm) (s) (mW) (ppb) (ppmW) (cm-1WHz-1/2)\nLN-MFP 0.45 1000 1200 2 2.4×10-3 /\nNO\nLN-MFP 1.39 6300 19 25 4.75×10-4 5.82×10-9\nH O\nLN-MFP 1.53 1000 3700 3.5 1.3×10-2 2.7×10-8\nC H\n2 2\nQTF [9] 1.53 370 5 21 1.05×10-4 /\nLN-MFP 2 1900 10.6 350 3.71×10-3 2.53×10-8\nCO\nLN-MFP 3.3 1200 6.5 2.5 1.63×10-5 8.36×10-9\nCH\nNH LN-MFP 9.7 1100 82.3 0.35 2.88×10-5 6.58×10-10\nfunctional platform; IT: Integration time; QTF: quartz tuning fork; MDL: minimum detection limit;\nRMDL: relative minimum detection limit\nLN-MFP’s competitive photoacoustic performance arises from several complementary\nphysical and practical factors. First, single-crystal lithium niobate exhibits substantially higher\neffective piezoelectric coupling (for appropriate crystal orientations and tensor components)\nthan α-quartz, enabling LiNbO₃ devices to generate stronger electrical signals—and thus higher\nsignal-to-noise ratios—for a given mechanical strain [5]. Second, the device geometry and\nresonant frequency were carefully optimized to balance acoustic coupling, thermoelastic drive\nand readout bandwidths ensuring robust multimodal transduction even for slowly relaxing\nspecies such as CO₂. Finally, lithium niobate’s mature and rapidly evolving photonics\necosystem, featuring broad optical transparency, strong electro-optic properties and wafer-level\nprocessing, facilitates the integration of on-chip optics, waveguides and micro-collimation\ncomponents that are challenging to implement in conventional quartz tuning-fork systems [6].\nThe chip-scale monolithic LN-MFP also minimizes parasitic capacitance and interconnect\nlosses, enabling compact low-noise electronics near the transducer, thereby lowering the noise\nfloor and enhancing NNEA.\n23 / 63\nTable 3s | Photodetector using LN-MFP and pyroelectric sensors.\nCharacterized NEP Response Responsivity\nDetector type\nwavelength range (W/√Hz) time (V/W)\nLN-MFP 450−9770 nm 5.65×10-8 1 s 373\nQTF [12] 1540 nm / 1 s 47.4\nLiNbO [13] / 1.623×10-7 1.9 s /\nSb Te −Bi Te [16] 600−700 nm 8.0 × 10−9 341 μs 38\n2 3 2 3\nLN-MFP: lithium niobate multi-functional platform; QTF: quartz tuning fork.\nA concise comparison of the LN-MFP with representative pyroelectric and tuning-fork\ndetectors is provided in Table 3s. In the current prototype the LN-MFP demonstrates high\nresponsivity (~373 V W⁻¹) and a noise-equivalent power on the order of 5.6×10⁻⁸ W Hz⁻¹/²;\nmeasured with a 1 s lock-in time constant (corresponding to ~0.25 Hz detection bandwidth).\nThe device operates across a broad wavelength range from 450 nm to 9.77 µm and integrates\nPAS, LITES and direct photodetection functionalities on a single chip. The reported\nperformance reflects a deliberate, system-level optimization, balancing geometry and readout\nto meet the combined requirements of all three sensing modalities. Although the LN-MFP is\nnot necessarily optimized for any single modality, it uniquely combines high per-power\nresponsivity, multimodal functionality and compatibility with wafer-scale photonics. Several\nstraightforward engineering routes remain to further improve the absolute MDL, NEP and\nbandwidth, including the use of low-noise transimpedance amplifiers, micro-collimation or\nwaveguide coupling to enhance optical power delivery, and tailored absorptive coatings at the\ntine base. These improvements will be explored in future work to further enhance performance\nwhile preserving the platform’s integration advantages.\n24 / 63\nTable 4s | LITES detection of trace gases using LN-MFP and QTF.\nTarget Wavelength IT AL MDL RMDL\nDetector type\nanalyte (μm) (s) (cm) (ppm) (ppmm)\nNO LN-MFP 0.45 450 2.5 1 2.5×10-2\nH O\nLN-MFP 1.53 1500 2.5 0.45 1.13×10-2\nC H\n2 2\nQTF [18] 1.53 70 20 0.36 7.2×10-2\nLN-MFP 2 1200 2.5 35 8.75×10-1\nCO\nLN-MFP 3.3 1900 2.5 0.9 2.25×10-2\nCH\nNH LN-MFP 9.7 1200 2.5 1.2 3×10-2\nplatform; MDL: minimum detection limit; RMDL: relative minimum detection limit; IT: Integration\ntime;\nFor LITES, the key parameters include the laser wavelength, absorption length (AL), and\nintegration time, as summarized in Table 4s. Since the MDL is directly proportional to the AL,\nthe comparison includes not only the MDL but also the relative MDL for a fair and\ncomprehensive evaluation. Table 4s shows that the LN-MFP attains superior RMDL for most\nchannels (H₂O, C₂H₂, CO₂, NO₂, NH₃), with CH₄ as the only minor exception. This advantage\nstems from three factors: larger effective piezoelectric coupling of Y+128° LiNbO₃, an\nelectrode layout that maximizes charge collection at regions of peak modal strain, and a\nresonance (~10.5 kHz) deliberately tuned to enhance thermoelastic/acoustic energy build-up\nfor slowly relaxing species.\nReference\n5. Yue, W. et al. Crystal orientation dependence of piezoelectric properties in LiNbO and\n3\n6. Boes, A. et al. Lithium niobate photonics: Unlocking the electromagnetic spectrum. Science\n379, eabj4396 (2023).\n7. Breitegger, P. et al. Towards low-cost QEPAS sensors for nitrogen dioxide detection.\nPhotoacoustics 18, 100169 (2020).\n25 / 63\n8. Rousseau R. et al. Monolithic double resonator for quartz enhanced photoacoustic\nspectroscopy. Appl. Sci. 11: 2094 (2021).\n9. Wang, R., Qiao, S., He, Y., & Ma, Y. Highly sensitive laser spectroscopy sensing based on\na novel four-prong quartz tuning fork. Opto-Electron. Adv. 8, 240275-1 (2025).\n10. Zhang, Y. et al. Continuous real-time monitoring of carbon dioxide emitted from human\nskin by quartz-enhanced photoacoustic spectroscopy. Photoacoustics 30, 100488 (2023).\n11. Wu H. et al. Atmospheric CH measurement near a landfill using an ICL-based QEPAS\n4\nsensor with VT relaxation self-calibration. Sens. Actuator B-Chem. 297 126753 (2019).\n12. Zhou, S. et al. Realization of a infrared detector free of bandwidth limit based on quartz\ncrystal tuning fork. Opt. Laser Technol. 113, 261-265 (2019).\n13. Aleks, M. et al. An overview of microelectronic infrared pyroelectric detector. Eng. Sci. 16,\n82-89 (2021).\n14. Suen, J. Y. et al. Multifunctional metamaterial pyroelectric infrared detectors. Optica 4,\n276-279 (2017).\n15. Kane, S. R. et al. Characterizing pyroelectric detectors for quantitative synchrotron\nradiation measurements. Sensor. Actuat. A-Phys. 387, 116406 (2025).\n16. Mauser, K. W. et al. Resonant thermoelectric nanophotonics. Nat. Nanotechnol. 12, 770-\n775 (2017).\n17. Xu, L. et al. Multigas sensing technique based on quartz crystal tuning fork-enhanced laser\nspectroscopy. Anal. Chem. 92 14153-14163 (2020).\n18. He, Y. et al. Hydrogen-enhanced light-induced thermoelastic spectroscopy sensing.\nPhotonics Res. 13 194-200 (2024).\n19. Bojęś, P. et al. Dual-band light-induced thermoelastic spectroscopy utilizing an antiresonant\nhollow-core fiber-based gas absorption cell. Appl. Phys. B 129, 177 (2023).\n20. Shang, Z. et al. Robust and compact light-induced thermoelastic sensor for atmospheric\nmethane detection based on a vacuum-sealed subminiature tuning fork. Photoacoustics 42,\n100691 (2025).”\nReviewer #2:\nThe manuscript describes a lithium niobate multifunctional integrated platform (LN-MFP) for\nspectroscopic gas sensing. Specifically, the authors present a systematic study of a lithium\nniobate tuning fork for photoacoustic, thermoelastic/photodetection-based sensing. It is of\ninterest to see the direct bonding of the QCL chip to the LN-MFP, showing potential for\nintegrated spectroscopic applications. This work will attract researchers from different areas\nsuch as spectroscopy, sensing, integrated photonics, etc. The following issues should be\naddressed before this work can be considered for publication.\n1. One key application shown in this study is photoacoustic spectroscopy. The authors show a\ngood record of adopting tuning forks (especially quartz tuning fork, QTF) in gas sensing with\ndemonstrated high sensitivity across a broad spectral range. However, in the manuscript, the\nauthors seem to only report the PAS gas sensing results without providing sufficient\ndiscussion. A detailed comparison with state-of-the-art tuning fork-based PAS gas sensing\nsystems is needed to highlight the key advantages and advancements offered by the designed\nLN tuning fork.\nThank you for the reviewer’s valuable suggestion. A detailed comparison table has been added\nto the Supplementary.\nIn the main article (page 9 line 14), we have included the sentence:\n“The photoacoustic detection performance of the LN-MFP and the QTF-based PAS\nsystems is summarized in Supplementary Table 2s.”\nIn the Supplementary (page 14 line 2), we have included the following sentences:\n“LN-MFP’s competitive photoacoustic performance arises from several complementary\nphysical and practical factors. First, single-crystal lithium niobate exhibits substantially higher\neffective piezoelectric coupling (for appropriate crystal orientations and tensor components)\nthan α-quartz, enabling LiNbO₃ devices to generate stronger electrical signals—and thus higher\nsignal-to-noise ratios—for a given mechanical strain [5]. Second, the device geometry and\nresonant frequency were carefully optimized to balance acoustic coupling, thermoelastic drive\nand readout bandwidths ensuring robust multimodal transduction even for slowly relaxing\n26 / 63\nspecies such as CO₂. Finally, lithium niobate’s mature and rapidly evolving photonics\necosystem, featuring broad optical transparency, strong electro-optic properties and wafer-level\nprocessing, facilitates the integration of on-chip optics, waveguides and micro-collimation\ncomponents that are challenging to implement in conventional quartz tuning-fork systems [6].\nThe chip-scale monolithic LN-MFP also minimizes parasitic capacitance and interconnect\nlosses, enabling compact low-noise electronics near the transducer, thereby lowering the noise\nfloor and enhancing NNEA.”\nTable 2s | PAS detection of trace gases using LN-MFP and QTF.\nTarget Detector Wavelength IT Power MDL RMDL NNEA\nanalyte type (μm) (s) (mW) (ppb) (ppmW) (cm-1WHz-1/2)\nLN-MFP 0.45 1000 1200 2 2.4×10-3 /\nNO\nLN-MFP 1.39 6300 19 25 4.75×10-4 5.82×10-9\nH O\nLN-MFP 1.53 1000 3700 3.5 1.3×10-2 2.7×10-8\nC H\n2 2\nQTF [9] 1.53 370 5 21 1.05×10-4 /\nLN-MFP 2 1900 10.6 350 3.71×10-3 2.53×10-8\nCO\nLN-MFP 3.3 1200 6.5 2.5 1.63×10-5 8.36×10-9\nCH\nNH LN-MFP 9.7 1100 82.3 0.35 2.88×10-5 6.58×10-10\nfunctional platform; IT: Integration time; QTF: quartz tuning fork; MDL: minimum detection limit;\nRMDL: relative minimum detection limit\nReference\n5. Yue, W. et al. Crystal orientation dependence of piezoelectric properties in LiNbO and\n3\n6. Boes, A. et al. Lithium niobate photonics: Unlocking the electromagnetic spectrum. Science\n379, eabj4396 (2023).\n7. Breitegger, P. et al. Towards low-cost QEPAS sensors for nitrogen dioxide detection.\nPhotoacoustics 18, 100169 (2020).\n27 / 63\n8. Rousseau R. et al. Monolithic double resonator for quartz enhanced photoacoustic\nspectroscopy. Appl. Sci. 11: 2094 (2021).\n9. Wang, R., Qiao, S., He, Y., & Ma, Y. Highly sensitive laser spectroscopy sensing based on\na novel four-prong quartz tuning fork. Opto-Electron. Adv. 8, 240275-1 (2025).\n10. Zhang, Y. et al. Continuous real-time monitoring of carbon dioxide emitted from human\nskin by quartz-enhanced photoacoustic spectroscopy. Photoacoustics 30, 100488 (2023).\n11. Wu H. et al. Atmospheric CH measurement near a landfill using an ICL-based QEPAS\n4\nsensor with VT relaxation self-calibration. Sens. Actuator B-Chem. 297 126753 (2019).”\n28 / 63\n2. In addition, the following information should be added to strengthen the technical content:\n(1) Experimental setup for each light source is suggested to be documented in the\nsupplement.\n(2) Better to include N₂ background signals in Fig. 2 for direct comparison.\nThanks for the comment.\n(1) In accordance with the reviewer’s recommendation, the experimental setup for each light\nsource is documented.\nIn the Supplementary page 18, we add a Table 5s:\nTable 5s. The wavelength and model of seven light sources.\nLaser Wavelength Power Modulation IC Temperature\nModel\nsource (nm) (mW) method (mA) (℃)\nVIS\nNICHIA 450 450 1200 AM 600 /\nLED\nNIR NTT Electronics\n1390 19 FM 84 28\nLD Corporation\nNIR NTT Electronics\n1536 3700* FM 90 25\nLD Corporation\nNIR NTT Electronics\n2004 10.6 FM 120 18\nLD Corporation\nMWIR\nNANOPLUS 3370 6.5 FM 86 17\nICL\nQCL Chinese Academy of\n4590 14 FM 270 45\nChip Sciences\nLWIR\nHEATHYPHOTON 9770 82.3 FM 420 20\nQCL\nIC: injection current; AM: amplitude modulation; FM: frequency modulation; *Amplified by EDFA\nIn addition, in the Supplementary Page 16 line 8, we add:\n“Figure 7s shows a photo of the experimental setup used for the LN-MFP measurements. A\n29 / 63\nfunction generator (Tektronix AFG3102) supplied the modulation waveform to the light sources.\nThe sources, as listed in the Table 5s, were driven and temperature-stabilized by a current/TEC\ncontroller (Thorlabs ITC4005QCL). The emitted beam was routed through the detection\nmodule. A camera (Dataray WinCamD-IR-BB) was employed for beam profiling and alignment.\nThe electrical output from the detection module was demodulated by a lock-in amplifier (SRS\nSR830). A personal computer was used for instrument control and data acquisition.”\nFig. 7s | Photo of the experimental setup. PC: personal computer, DM: detection module.\n30 / 63\n(2) Thanks for the valuable suggestion. We have already included N₂ background signals in\nFig. 2, as shown in the revised main article, page 9.\nFig. 2 | LN-MFP for photoacoustic detection. a Spectral coverage with excitation sources\nfrom visible to long-wave infrared and the targeted analytes. b-g present representative\nmeasurements for each target gas: the main plot in each panel shows a typical 2f signal acquired\nwith the indicated light source and sample concentration; the left inset shows the concentration-\ncalibration (linear fit); the top-right inset shows a 1σ noise level obtained with pure N₂; and the\nbottom-right inset shows the Allan-deviation curves used to extract the1σ minimum detection\nlimit (MDL) and optimal averaging times. Extracted MDLs (1σ) and averaging times are: b\nMDL ~2 ppb at 1,000 s; c MDL ~25 ppb at 6,300 s; d MDL ~3.5 ppb at 1,000 s; e MDL ~350\nppb at ~1,900 s; f MDL ~2.5 ppb at 1,200 s; g MDL ~350 ppt at 1,100 s. VIS LED: visible light\nemitting diode; NIR LD: near-infrared laser diode; MWIR ICL: mid-wave interband cascade\nlaser; LWIR QCL: long-wave quantum-cascade laser.\n31 / 63\n3. Regarding the demonstration of thermoelastic detection (LITES), the reviewer notices\ntuning forks have achieved high sensitivity in the visible, near-IR, and mid-IR. The manuscript\nshould compare with state-of-the-art LITES results and clarify how the LN tuning fork advances\nbeyond these results.\nThanks for the comment. The comparisons with state-of-the-art LITES results have been added\nas Table 4s to the Supplementary page 16, as follows:\nTable 4s | LITES detection of trace gases using LN-MFP and QTF.\nTarget Wavelength IT AL MDL RMDL\nDetector type\nanalyte (μm) (s) (cm) (ppm) (ppmm)\nNO LN-MFP 0.45 450 2.5 1 2.5×10-2\nH O\nLN-MFP 1.53 1500 2.5 0.45 1.13×10-2\nC H\n2 2\nQTF [18] 1.53 70 20 0.36 7.2×10-2\nLN-MFP 2 1200 2.5 35 8.75×10-1\nCO\nLN-MFP 3.3 1900 2.5 0.9 2.25×10-2\nCH\nNH LN-MFP 9.7 1200 2.5 1.2 3×10-2\nplatform; MDL: minimum detection limit; RMDL: relative minimum detection limit; IT: Integration\ntime;\nExamining the RMDL entries in Table 4s shows that the LN-MFP delivers better RMDL for most\ntested channels (H₂O, C₂H₂, CO₂, NO₂, NH₃) compared with the cited QTF results. The only\nchannel in the table where a cited QTF entry currently shows a slightly better RMDL is CH₄.\nThe superior RMDL of the LN-MFP arises from several complementary factors:\n(1) Higher piezoelectric transduction per unit strain\nFor the relevant device orientations, single-crystal LiNbO₃ provides larger effective\npiezoelectric coupling than α-quartz. Consequently, for the same local thermoelastic or\nacoustic strain the LN prongs generate more collected charge at the electrodes, improving the\nelectrical SNR and lowering RMDL.\n(2) Optimized electrode geometry and charge collection\n32 / 63\nThe LN-MFP electrode layout was properly designed to collect charge from regions of\nmaximum modal strain (near the tine base). This increases the fraction of mechanically\ngenerated charge that is measured versus many standard QTF electrode patterns, improving\nper-power readout efficiency.\n(3) Resonant tuning matches thermoelastic / molecular dynamics\nThe chosen resonance (~10.5 kHz) and geometry were selected to maximize\nthermoelastic/ acoustic energy accumulation for slowly relaxing species (e.g., CO₂). This\nresonance matching increases the thermoelastic drive per unit delivered optical power for such\nspecies and thus yields better RMDL in those channels.\nIn the Supplementary page 15 line 7, we added the following clarification:\n“Table 4s shows that the LN-MFP attains superior RMDL for most gas species (H₂O, C₂H₂,\nCO₂, NO₂, NH₃), with CH₄ as the only minor exception. This advantage stems from three factors:\nlarger effective piezoelectric coupling of Y+128° LiNbO₃, an electrode layout that maximizes\ncharge collection at regions of peak modal strain, and a resonance (~10.5 kHz) properly tuned\nto enhance thermoelastic/acoustic energy build-up for slowly relaxing species.”\nReference\n17. Xu, L. et al. Multigas sensing technique based on quartz crystal tuning fork-enhanced laser\nspectroscopy. Anal. Chem. 92 14153-14163 (2020).\n18. He, Y. et al. Hydrogen-enhanced light-induced thermoelastic spectroscopy sensing.\nPhotonics Res. 13 194-200 (2024).\n19. Bojęś, P. et al. Dual-band light-induced thermoelastic spectroscopy utilizing an antiresonant\nhollow-core fiber-based gas absorption cell. Appl. Phys. B 129, 177 (2023).\n20. Shang, Z. et al. Robust and compact light-induced thermoelastic sensor for atmospheric\nmethane detection based on a vacuum-sealed subminiature tuning fork. Photoacoustics 42,\n100691 (2025).\n4. Following the previous question, some specific concerns:\n(1) The claim that \"resonant amplification significantly boosts sensitivity\" is confusing, as the\nLITES performance is notably worse than PAS. It is noticed that the author used a 2.5 cm\n33 / 63\noptical length, but the results are still unsatisfied to support your claims. Further discussion is\nneeded to clarify this discrepancy.\n(2) Clarify how dual PAS/LITES functionality enables comprehensive analysis. It is not\nstraightforward for readers to understand how combining them together can leverage the\nsensing power of LN-MFP.\n(3) Clarify the relationship between spectral selectivity and dual functionality, as selectivity is\nprimarily determined by ro-vibrational fingerprints.\nThanks for the comment.\n(1) To avoid the confusion, we deleted the sentence:\n“Notably, this resonant amplification effect significantly boosts the sensitivity of the system.”\n(2) The integration of these two complementary detection modes enables comprehensive and\nversatile gas sensing under various environmental conditions.\nSpecifically, in the PAS mode, gas molecules are excited by a modulated laser beam that\npasses through the narrow tine gap of the LN-MFP. The resulting acoustic pressure is\nmechanically amplified by the resonant vibration of the fork structure. In photoacoustic\ndetection, the analyte gas surrounds the LN-MFP, enabling high-sensitivity detection within a\ncompact volume. However, when the target gas is corrosive or contains impurities, the LN-MFP\nmay suffer from surface degradation or contamination, leading to reduced performance.\nIn contrast, when operated in the LITES mode, the LN-MFP functions in a non-contact\ndetection configuration. In this case, the laser passes through the target gas, which can be\nenclosed in a sealed chamber if corrosive or contains for example particulates, and then\nirradiates the surface of the LN-MFP. The light induced thermoelastic deformation generates a\nmeasurable signal. Unlike photoacoustic processes relying on acoustic wave propagation\nthrough gas, this mechanism eliminates direct contact between gas and transducer.\nConsequently, LITES enables reliable detection even in extreme or corrosive environments\nwhere physical interaction between the gas and transducer could otherwise degrade\nperformance.\nBy integrating both PAS and LITES modes on the same LN-MFP platform, the device can\nadaptively switch between a compact PAS configuration and a non-contact LITES configuration,\ndepending on the target gas and environmental conditions. This dual-mode functionality\nenables the system to accommodate a wide range of applications, from trace gas analysis with\nonly minute gas volumes available to reliable monitoring under harsh industrial or atmospheric\nconditions. There by achieving comprehensive and flexible sensing capability within a unified,\nminiaturized device architecture.\nTo make it clear to the readers, in the revised main article in page 17 line 7, we replaced\nthe original paragraph:\n“The integration of thermoelastic detection into the LN-MFP demonstrates its versatility\nas a multifunctional spectroscopic platform. The capability to perform both photoacoustic and\nthermoelastic measurements within a single device highlights its potential for comprehensive\nspectroscopic analysis. Such dual functionality proves especially valuable for applications\nrequiring both high sensitivity and spectral selectivity across a broad spectral range, further\npositioning the LN-MFP as an advanced tool for next-generation spectroscopic sensing\ntechnologies.”\nwith the following:\n“The integration of thermoelastic detection into the LN-MFP demonstrates its versatility\nas a multifunctional spectroscopic platform. The spectral selectivity of both modes arises from\nthe intrinsic ro-vibrational fingerprints of target molecules. The dual functionality does not alter\nthis selectivity but extends its applicability across varying gases and conditions. By\nincorporating both PAS and LITES detection modes, the device enables adaptive gas sensing\nacross a wide range of environments and measurement scenarios. In PAS mode, direct gas–\nsensor interaction provides high-sensitivity detection within a compact volume, ideal for\nconfined or small-scale applications. Conversely, LITES operates in a non-contact\nconfiguration, suitable for sealed, corrosive or otherwise harsh environments. This dual-mode\narchitecture combines the sensitivity and compactness of contact-based detection with the\nrobustness and flexibility of non-contact operation, enabling comprehensive spectroscopic\nanalysis and positioning the LN-MFP as a versatile platform for next-generation sensing\ntechnologies.”\n35 / 63\n(3) The spectral selectivity of the LN-MFP system originates from the intrinsic ro-vibrational\nfingerprints of target molecules, which fundamentally govern the absorption characteristics in\nboth PAS and LITES. Since both techniques rely on absorption-based signal generation, their\nselectivity is inherently molecular rather than device-dependent. The dual PAS/LITES\nfunctionality does not modify this intrinsic selectivity. Instead, it broadens the operational\nscope of the system. In the PAS mode, direct gas–sensor interaction allows high-sensitivity\ndetection of strong absorption lines within confined volumes, while the LITES mode enables\nnon-contact measurements of weaker transitions or corrosive gases over larger or sealed\nspaces. Moreover, the broadband optical transparency and mechanical stability of the LN-MFP\nsupport excitation from the near- to mid-infrared, granting access to multiple absorption bands.\nConsequently, the dual-mode architecture preserves the molecular origin of spectral selectivity\nwhile extending its practical applicability across diverse gases, spectral regions, and\nenvironmental conditions.\nTo make it clear to readers, in main article page 17 line 7, we added the following\nsentences:\n“The integration of thermoelastic detection into the LN-MFP demonstrates its versatility\nas a multifunctional spectroscopic platform. The spectral selectivity of both modes arises from\nthe intrinsic ro-vibrational fingerprints of target molecules. The dual functionality does not alter\nthis selectivity but extends its applicability across varying gases and conditions.”\n5. It seems there is an overlap between the photodetection functionality with the\nthermoelastic sensing mechanism. Is it more reasonable to move this section before LITES as\nit relates to the fundamental mechanism? In addition, the modulated photodetection using\ntuning forks has been previously demonstrated, so it is more interesting to make a comparison\nwith state-of-art photodetection using tuning forks, highlighting key pros and cons. Also, it is\nbetter to make a fair comparison of noise-equivalent power (NEP) with commercial\nphotodetectors, which is more important in photodetection.\nThanks for the comment.\n(1) We have moved section “Photodetection with the LN-MFP” before the section\n36 / 63\n“Thermoelastic detection with the LN-MFP”.\n(2) We have added Table 3S to supplementary page 15, comparing the LN-MFP with\nrepresentative pyroelectric and tuning-fork detectors.\nTable 3s | Photodetector using LN-MFP and pyroelectric sensors.\nCharacterized NEP Response Responsivity\nDetector type\nwavelength range (W/√Hz) time (V/W)\nLN-MFP 450−9770 nm 5.65×10-8 1 s 373\nQTF [12] 1540 nm / 1 s 47.4\nLiNbO [13] / 1.623×10-7 1.9 s /\nSb Te −Bi Te [16] 600−700 nm 8.0 × 10−9 341 μs 38\n2 3 2 3\nLN-MFP: lithium niobate multi-functional platform; QTF: quartz tuning fork.\nReference\n12. Zhou, S. et al. Realization of a infrared detector free of bandwidth limit based on quartz\ncrystal tuning fork. Opt. Laser Technol. 113, 261-265 (2019).\n13. Aleks, M. et al. An overview of microelectronic infrared pyroelectric detector. Eng. Sci. 16,\n82-89 (2021).\n14. Suen, J. Y. et al. Multifunctional metamaterial pyroelectric infrared detectors. Optica 4,\n276-279 (2017).\n15. Kane, S. R. et al. Characterizing pyroelectric detectors for quantitative synchrotron\nradiation measurements. Sensor. Actuat. A-Phys. 387, 116406 (2025).\n16. Mauser, K. W. et al. Resonant thermoelectric nanophotonics. Nat. Nanotechnol. 12, 770-\n775 (2017).\nIn the present prototype the LN-MFP exhibits high responsivity (~373 V/W) and noise-\nequivalent power on the order of 5.6×10⁻⁸ W/√Hz; these values were measured with a 1 s lock-\nin time constant (~0.25 Hz detection bandwidth). The device supports a characterized\n37 / 63\nwavelength range from 450 nm to 9.77 µm and integrates PAS, LITES and direct photodetection\nmodalities on a single chip. It is important to note that the reported performance represents a\ndeliberate, system-level compromise: the geometry and readout were optimized to satisfy the\nrequirements of all three sensing modalities. While the LN-MFP is not necessarily optimized for\nany single sensing modality, it achieves a balanced performance across all functions. Its\ncombination of high per-power responsivity, multimodal capability, and compatibility with\nwafer-scale photonics distinguishes it from existing devices. Moreover, several straightforward\nengineering improvements remain to enhance the absolute MDL, NEP, and bandwidth—such\nas implementing low-noise transimpedance amplifiers, integrating micro-collimators or\nwaveguide coupling to improve optical power delivery, and applying tailored absorptive\ncoatings at the tine base. These developments will be pursued in future work to further\nadvance each modality while preserving the platform’s high level of integration.\n38 / 63\n(3) A fair comparison of the noise-equivalent power (NEP) between the LN-MFP and the\ncommercial photodetector has been included.\nIn the main article page 13, we replaced the original Table 3 with the following:\nTable 2 LN-MFP for photodetection from VIS to LWIR.\nLight Wavelength MDL NEP Photoresponsivity\nDetector type\nsources (µm) (nW) (W/√Hz) (V/W)\nVIS LED 0.45 260 5.88×10-7 67.7\n1.39 445 1×10-6 35.5\nNIR LD 1.53 695 1.57×10-6 22.5\nLN-MFP\n2 693 1.56×10-6 15.3\nMIR ICL 3.37 1000 2.26×10-6 9.8\nLWIR QCL 9.77 25 5.65×10-8 373\nHPPD-B-D-10.6-10 LWIR QCL 9.77 8000 1.8×10-5 258\n6. LN is an important photonic platform used in many areas as described in the first paragraph\nof Introduction. However, the authors missed mentioning the spectroscopic sensing\napplications of using LN. May be not too many in this topic, but at least there are a couple of\npioneering work recently reported on fabricating LN rib waveguide and using it for on-chip\nphotothermal gas sensing. As spectroscopic sensing the key topic of this paper, the authors\nshould make a more complete literature to strengthen this part.\nAdditional concerns:\n(1) Why is the tuning fork placed horizontally rather than vertically?\n(2) It is more interesting to include a photo/microscope image of the on-chip LN-MFP sensor\nin Figure 5.\n(3) The results show the signal of 8000 ppm CO. Please discuss limitations and potential\nsolutions.\nThanks for the comment. We have added the most recent publications on spectroscopic sensing\nin the main article page 3 line 4.\n“Recent breakthroughs in LN photonics have propelled the development in high-performance\n39 / 63\ncomponents, including electro-optic modulators [3-5], frequency shifters [6], frequency combs\n[7-9], pulse generators [10], tunable lasers [11], spectroscopic sensing [12-14] and photonic\nintegrated circuits [15].”\nReference\n[12] Shim, J. et al. Room-temperature waveguide-integrated photodetector using bolometric\neffect for mid-infrared spectroscopy applications. Light Sci. Appl. 14, 125 (2025).\n[13] Hwang, A. Y. et al. Mid-infrared spectroscopy with a broadly tunable thin-film lithium\nniobate optical parametric oscillator. Optica 10, 1535-1542 (2023).\n[14] Yan, Y. et al. On-chip photothermal gas sensor based on a lithium niobate rib waveguide.\nSensor Actuat. B-chem. 405, 135392 (2024).\nAdditional concerns:\n(1) As shown in Fig. 1, the LN-MFP has a total length of approximately 1.5 cm. During the on-\nchip fabrication process, the device was positioned horizontally on the silicon substrate,\nprimarily due to chip-size limitations and packaging considerations. The LN-MFP operates\nthrough the resonant vibration of its tines, a mechanism independent of its overall orientation.\nTo prevent mechanical damping and ensure free tine motion, a micromachined recess was\ncreated beneath the fork region using photolithography, allowing the tines to remain\nsuspended without contacting the substrate. Moreover, the horizontal configuration enables\nin-plane optical detection, which increases the effective light–gas interaction length and\nimproves detection sensitivity. Therefore, horizontal arrangement represents a practical and\nrobust design choice that balances mechanical performance, optical efficiency, and integration\nfeasibility.\n(2) The photograph of the on-chip LN-MFP sensor was added to main article Figure 5.\n40 / 63\nFig. 5 | On chip LN-MFP sensor. a Photograph of the packaged on-chip sensor. b Optical-\nmicroscope image of the coupling region showing the QCL emission facet aligned with the LN-\nMFP tine gap. c Schematic diagram of the free-space optical coupling. d 2f photoacoustic\nspectrum of 8,000 ppm CO in N₂ measured using the integrated LN-MFP–QCL assembly.\n(3) As shown in the revised Fig. 5c, the laser emitted from the QCL diverges by approximately\n70° across a ~100 μm gap without any collimating optics, causing part of the beam to\nilluminate the LN-MFP surface. This unintended illumination introduces additional\nphotothermal noise, thereby degrading the signal-to-noise ratio. In future work, we plan to\nincorporate micro collimating optics or integrated LN waveguides to confine the laser\npropagation path and prevent stray light from reaching the tines. Such improvements are\nexpected to significantly reduce optical noise, enhance coupling efficiency, and ultimately\nimprove the detection limit and dynamic range of the on-chip LN-MFP sensor.\nIn the revised main article page 19 line 14, we replaced the sentences:\n“Specifically, our goal is to merge a mid-infrared quantum cascade laser (QCL) chip with\nthe LN-MFP using advanced chip fabrication techniques, thereby consolidating both the light\n41 / 63\nsource and the detector onto a single platform.”\nWith the following:\n“Specifically, our goal is to integrate a mid-infrared quantum cascade laser (QCL) chip\nand optical waveguide with the LN-MFP through advanced microfabrication techniques,\nthereby unifying the light source, waveguide, and detector on a single monolithic platform.”\n7. Regarding the design and fabrication of the LN-MFP, a discussion and comparison with\nexisting LN, quartz, silicon, and polymer tuning forks should be added. The key parameters\n(resonant frequency, quality factor) should be moved to the main text, with detailed\nsimulation and characterization data properly referenced in supplementary materials. The key\nparameters like resonant frequency, quality factor, and dimensions will be useful for readers\nto understand the advancements of the proposed device.\nThanks for the constructive suggestion.\nIn accordance with the reviewer’s recommendation, we have substantially revised both the\nmain text and the Supplementary to provide a detailed discussion of the LN-MFP’s design and\nfabrication.\n(1) Discussion and comparison with existing tuning forks\nWe added the following discussion and comparison to the Supplementary in page 2:\n“1. Discussion and Comparison Regarding the Design and Fabrication of Tuning Forks\nThe design and fabrication of tuning forks are strongly governed by the intrinsic material\nproperties and the target application. The fabrication methods of representative tuning forks are\nsummarized in Table 1s. Quartz tuning forks, typically produced by photolithography and wet\nchemical etching from Z-cut quartz wafers, exhibit excellent frequency stability, high\nmechanical Q-factor, and low temperature drift, making them ideal for precision sensing and\ntiming applications. Silicon tuning forks, fabricated using photolithography and deep reactive\nion etching (DRIE) within standard MEMS processes, offer high fabrication reproducibility\nand seamless compatibility with on-chip integration, however their lack of intrinsic\npiezoelectricity limits direct signal transduction. In contrast, LiNbO₃ tuning forks, fabricated\nby UV photolithography and optical-grade dicing, leverage lithium niobate’s strong\n42 / 63\npiezoelectric and electro-optic properties to enable efficient transduction and potential coupling\nwith optical or acoustic fields. Nonetheless, their brittle crystalline nature and processing\ncomplexity impose strict requirements on machining precision. Polymer-based (PVDF) tuning\nfork, formed through film casting, mechanical shaping, and electrode deposition, provide a\nflexible, lightweight, and low-cost alternative. Despite their lower Q-factor and reduced\nfrequency stability compared with crystalline counterparts, PVDF designs are advantageous for\nflexible, wearable, or chemically harsh sensing environments. Overall, the trade-off between\nprecision, cost, mechanical robustness, and integration feasibility dictates the optimal material\nand fabrication strategy for a given tuning-fork application.”\nTable 1s Design and fabrication method of existing tuning forks.\nMaterials Piezoelectric Fabrication method Ref\nThis\nLiNBO3 yes UV photolithography and optical grade dicing\nwork\nquartz yes photolithography and etching [1]\nsilicon no photolithography and etching [2]\npolymer yes film casting and mechanical processing [3]\naluminum no mechanical processing [4]\nReferences\n1. Lin, H. et al. Application of standard and custom quartz tuning forks for quartz-enhanced\nphotoacoustic spectroscopy gas sensing. Appl. Spectrosc. Rev. 58, 562-584 (2023).\n2. Lavrik, N. V. & Datskos., P. G. Optically read Coriolis vibratory gyroscope based on a silicon\ntuning fork. Microsyst. Nanoeng. 5, 47 (2019).\n3. Liu, K. et al. A novel photoacoustic spectroscopy gas sensor using a low cost polyvinylidene\nfluoride film. Sens. Actuator B-Chem. 277, 571-575 (2018).\n4. Pan, Y. et al. Miniaturized and highly-sensitive fiber-optic photoacoustic gas sensor based\non an integrated tuning fork by mechanical processing with dual-prong differential\nmeasurement. Photoacoustics 34, 100573 (2023).\n43 / 63\n(2) key parameters with detailed simulation\nThe key parameters including resonant frequency and quality factor has been added to the\nmain article in page 5 line 23：\n“The LN-MFP has a frequency of 10,485 Hz and a Q factor of 1,621.”\nThe detailed simulation and characterization data has been added to the Supplementary\nsection 2 page 3 line 1:\n“The resonance frequency and quality factor (Q) of the LN-MFP are governed by both material\nproperties and geometric configuration. In this work, the geometry was carefully optimized to\nbalance the competing requirements of its multifunctional operation, including PAS, LITES,\nand photodetection. To guide this optimization, we present a developed systematic design\nprocedure detailing how the geometric parameters of the LN-MFP were selected and refined.\n(1) Optimization for Resonance\nThe geometry and resonance of the LN-MFP were optimized by varying two key geometric\nparameters: the tine length 𝑙 and width w, while keeping all other parameters fixed (see Fig.\n1s a for definitions). A parametric finite-element sweep of l and w revealed that the fundamental\nresonance f decreases monotonically with increasing l and with decreasing w (Fig. 1s b). In the\n0\nsimulated design space f ranged from ~ 34.5 kHz (l =6 mm, w = 2 mm) to ~2.8 kHz (l = 18\n0\nmm, w =1 mm).\nFor the LN-MFP, excessively high resonant frequency 𝑓 yields shorter vibration period,\n0\nlimiting the time available for the device to accumulate acoustic energy. Since the light-source\nmodulation must match the LN-MFP resonance frequency, high f₀ values require faster\nmodulation rates, which reduce the efficiency of photoacoustic generation for molecules with\nlong vibrational-to-translational (V–T) relaxation times. For example, CO₂ exhibits V–T\nrelaxation times on the order of 100 μs (~ 10 kHz), and modulation beyond this frequency\nsignificantly diminishes photoacoustic generation efficiency. By contrast, very low f increases\n0\nsusceptibility to low-frequency environmental disturbances such as mechanical vibration, air\ncurrents and 1/f (pink) noise.\nBalancing these effects, an optimal frequency f ~ 10.5 kHz was selected, corresponding\n0\nto tine dimensions of l=11.5 mm, w=1.7 mm.\n44 / 63\nFig. 1s | a Schematic of the LN-MFP showing the defined geometric parameters: tine length l,\nbase width W, tine width w, inter-tine gap g, thickness t, and fillet radius r. b Finite-element\nsimulation of the fundamental resonant frequency f as a function of tine length 𝑙 and width w.\na frequency range from ~34.5 kHz to ~2.8 kHz across the design space.\n(2) Optimization for PAS\nThe LN-MFP geometry was optimized for photoacoustic spectroscopy (PAS) through coupled\nacoustic–structural finite-element simulations. In the acoustic excitation model, a cylindrical\npressure source was positioned within the tine gap to reproduce the spatial distribution of the\nphotoacoustic pressure wave and to induce symmetric tine oscillation (see Fig. 2s a). For each\ngeometric configuration the total piezoelectric charge generated by the device was calculated.\nThe resulting contour map of normalized piezoelectric charge exhibits a clear and\nmonotonic dependence on tine dimensions (Fig. 2s b). The PAS signal strength increases with\ngreater tine length l and with smaller tine width w. Physically, longer and more slender tines\nexhibit larger tip displacements and stronger strain localization near the tine base under acoustic\nexcitation, hereby generating a higher piezoelectric charge output and enhancing signal\namplitude.\n45 / 63\nFig. 2s | a Simulated deformation and strain distribution of the LN-MFP under PAS; b\nNormalized piezoelectric charge as a function of tine length l and tine width w.\n(3) Optimization for LITES and photodetection\nBecause both LITES and direct photodetection originate from light-induced heating, they were\nanalyzed within a unified thermoelastic framework. In this model, a localized heat source was\napplied at the LN-MFP base to simulate the photothermal excitation induced by incident light\n(see Fig. 3s a). For each geometric configuration, the total piezoelectric charge collected by the\nelectrode was computed. The resulting contour map of normalized piezoelectric charge in (Fig.\n3s b) reveals a systematic increase with the tine width w and only a weak dependence on tine\nlength l. This trend arises from several factors. First, wider tines provide a larger heated volume\nand absorption area for a given illumination spot, leading to greater overall thermal expansion\nand a larger bending moment. Second, increasing w enlarges the electromechanically active\nvolume and electrode-covered area, enhancing conversion of localized strain into piezoelectric\ncharge. Third, because the thermoelastic excitation is concentrated near the base, variations in\ntine length l exert minimal influence on the thermal-to-mechanical coupling in this region.\n46 / 63\nFig. 3s | a Simulated deformation and strain distribution of the LN-MFP under LITES and\nphotodetection excitation; b Contour map of normalized piezoelectric charge as a function of\ntine length l and tine width w.\n(4) Other geometry parameters\n-Tine gap g\nTo accommodate diverse optical sources, from visible LEDs to mid-IR QCLs, and to\nbalance optical and acoustic requirements, the tine gap was fixed at g=1.0 mm. The gap size is\ngoverned by two competing effects. For LITES and photodetection, light is incident on the\nouter tine surfaces, whereas in PAS operation, the excitation beam must traverse the gap\nwithout contacting the electrodes. If the gap is too narrow, the laser spot overlaps the metalized\ntines, introducing significant photothermal background noise. Conversely, if the gap is too wide,\nthe acoustic pressure field weakens, reducing acoustic-to-mechanical coupling efficiency.\nTherefore, a 1.0 mm gap represents a practical compromise between these competing optical\nand acoustic constraints.\n- Device thickness t\nCommercial lithium-niobate wafers are typically available in thicknesses of ~500 μm.\nWithin the explored design-space, variation in device thickness produced only minor effects on\nthe fundamental resonance compared to the dominant influence of tine length and width. Its\nimpact on electromechanical and thermoelastic responses was similarly limited. Considering\n47 / 63\nfactors such as wafer availability, fabrication precision, mechanical robustness and process\nrepeatability, a thickness of t=500 μm was selected for the fabricated LN-MFP devices.\n(5) Conclusion\nIn summary, a comprehensive finite-element parameter sweep combined with\nmultiphysics simulation was conducted to optimize the LN-MFP geometry across three key\nperformance dimensions: resonant behavior, photoacoustic (PAS) response, and\nthermoelastic/photodetection response. The inherently opposing design requirements of PAS\nand LITES/photodetection necessitated a balanced configuration. The finalized structure, with\nl=11.5 mm, w =1.7 mm, g=1.0mm, and t=500 μm, exhibits a fundamental resonant frequency\nnear 10.5 kHz. This optimized design achieves high PAS sensitivity alongside strong\nLITES/photodetection responsivity, ensuring multifunctional operation with excellent\nmechanical stability, fabrication compatibility and system integration compatibility.”\n48 / 63\nReviewer #3:\nThe paper presents a new device made from lithium niobate (LiNbO₃) that can do three things:\ndetect sound made by light (photoacoustic), sense heat changes (thermoelastic), and respond\nto light across a wide range from visible to long-wave infrared. The experiments show it could\nbe useful for small integrated sensors. The authors used the special electric and optical\nproperties of LiNbO₃ to build a small sensor that does multiple things, which is both creative\nand technically important. I think this paper could be published if some issues are fixed. Here\nare my concerns:\n-The paper says they added an acoustic resonator around the LN-MFP to make the signal\nstronger. According to the Supporting Information, the signal became 79 times bigger. But\nhow big is the resonator compared to the tuning fork? How are they placed relative to each\nother? Did they try to match the size of the resonator to the frequency of the tuning fork?\nAlso, did they test or simulate how well the resonator and tuning fork work together?\nWe thank the reviewer for this insightful comment regarding the acoustic resonator design and\nits coupling with the LN-MFP. We have now provided detailed information and analysis in both\nthe revised manuscript and the Supplementary Materials to clarify the geometry, placement,\nand resonance matching between the acoustic resonator and the LN-MFP.\nFirst, to help readers understand the dimensions of the acoustic resonator, we have added\nthe dimensions of the acoustic resonator description in the Supplementary page 10 line 18:\n“After optimization, the acoustic resonator was designed with a length of 13 mm, an inner\ndiameter of 2.6 mm, and an outer diameter of 3 mm.”\nSecond, regarding resonance matching, the acoustic resonator’s longitudinal dimension\nwas designed to match the acoustic wavelength corresponding to the LN-MFP’s fundamental\nfrequency (~10.5 kHz). This was intentionally done so that the standing acoustic wave inside\nthe resonator couples efficiently to the mechanical resonance of the fork. As shown in the\nrevised Fig. 5s(c), the introduction of the resonator shifted the system’s resonance from 10,485\nHz to 10,490 Hz, indicating acoustic–mechanical coupling between the resonator and the LN-\nMFP. The quality factor (Q) slightly decreased to ~1300 due to increased acoustic loading,\n49 / 63\nconfirming that the two components are indeed coupled. We have added the description in the\nSupplementary page 10 line 20:\n“According to Fig. 5s c, the resonator shifted the system’s resonance from 10,485 Hz to 10,488\nHz, indicating acoustic–mechanical coupling between the resonator and the LN-MFP. The\nquality factor decreased to ~1300 due to resonant coupling.”\nIn Supplementary page 10, we replaced original Fig. 2s with the following Fig. 5s:\nFig. 5s | The LN-MFP coupled with AmRs. a Simulated and measured PAS signals as a\nfunction of the laser beam position along the LN-MFP tines. b PAS signal amplitude as the\nfunction of an acoustic resonator’s length. c Resonance curves measured for a bare LN-MFP\nand a LN-MFP with an AmRs; d PAS 2f signal based on H O measurements when using a bare\n-In the thermoelastic detection part, the laser was pointed at the side of the tuning fork prong\ninstead of the front. Why was that done? Does lighting from the side help create more thermal\n50 / 63\nstress or better vibration? Were any comparisons done between side and front illumination\nto see which works better?\nWe thank the reviewer for this insightful comment regarding the laser incident position. This\nenhancement arises because, when the laser irradiates the bottom side of the prong, the\noptical path inside the LN substrate is longer, leading to more effective light absorption and\nstronger localized heating near the base region where mechanical strain is maximized. This\ngenerates a larger thermoelastic deformation and, consequently, a stronger piezoelectric\nresponse. In contrast, front illumination produces a shallower absorption depth and a less\nefficient thermal-stress distribution.\nTherefore, the bottom-side illumination geometry was selected for all LITES and\nphotodetection measurements to achieve stronger thermal excitation, improved signal-to-\nnoise ratio, and better mechanical-electrical conversion efficiency. To make it clear to the\nreaders, in the Supplementary in page 11 line 14：\n“As shown in Fig. 6s a, we experimentally compared the photodetection responses when the\nlaser irradiated either the bottom side (black curve) or the front surface (red curve) of the LN-\nMFP tine. The results clearly indicate that side illumination produces a stronger signal, with a\npeak amplitude more than twice that of front illumination.”\nIn Supplementary page 12, we replaced original Fig. 3s with the following Fig. 6s:\nFig. 6s | Optimization of the laser incident position. a LITES signal of the LN-MFP as a\nfunction of the laser beam position along the left tuning fork tine. b Normalized photodetection\nsignal measurements when laser was incident to the side and to the front of the LN-MFP tine\n51 / 63\nsurface.\n-The paper shows simulations saying that the shape and position of the electrodes affect how\nthe device vibrates. But there’s no experiment to prove this. Have the authors made real\ndevices with different electrode shapes and tested them, like checking their frequency\nresponse or how they vibrate?\nTo validate the simulation results regarding the effect of electrode geometry on the vibration\nperformance of the LN-MFP, and in accordance with the reviewer's comments, we fabricated\nand tested three electrode configurations, as shown in Fig. 2r. The LN-MFP contains six\nelectrodes on both sides. In Configuration I, the two side electrodes on one surface are\nconnected to the center electrode on the other side, corresponding to the optimal simulated\ndesign. In Configuration II, only the two side electrodes are connected, while the center\nelectrode remains open-circuited. In Configuration III, all three electrodes on one surface are\nconnected. Frequency sweep experiments show that Configuration I achieves the highest\nresonant amplitude, while Configuration II shows a modest decrease due to the unconnected\ncenter electrode. Configuration III exhibits an amplitude drop of approximately 70% due to the\ncancellation caused by the opposite charges on the center and side electrodes. These results\nconfirm the high agreement between experimental measurements and simulations,\ndemonstrating that electrode configuration significantly influences the electromechanical\ncoupling efficiency of the LN-MFP.\nFig. 1r a Three types of electrode distributions. Red and blue indicate positive and negative\npoles. Black indicates open circuit. b Normalized resonance amplitude of different electrode\n52 / 63\ndistribution.\n-The authors used a Y-cut 128° LiNbO₃ crystal but didn’t explain why. Since LiNbO₃ has\ndifferent properties depending on how it's cut (like Z-cut, X-cut, etc.), maybe this cut gives\nbetter piezoelectric effect or mechanical quality. Could the authors say what makes this cut\nbetter for their device? Did they compare it with other cuts in simulations or tests?\nWe thank the reviewers for their valuable questions. The choice of Y-shaped, 128° LiNbO₃ for\nthe fabrication of LN-MFP was based on literature evidence and our finite element simulation\nresults. LiNbO₃ is known for its strong anisotropy—different crystal facets exhibit distinct\npiezoelectric coefficients, dielectric constants, and elastic properties, all of which directly affect\nthe electromechanical coupling efficiency and vibration characteristics of the tuning fork-like\nstructure. Y-shaped, 128° LiNbO₃ has been widely used in magnetic field sensing, atomic force\nmicroscopy (AFM), and oil density and viscosity measurements.\nFig. 2r Charge distribution generated by the piezoelectric effect of different properties analyzed\nby FEM.\nTo verify the optimal orientation for LN-MFP, we performed a COMSOL Multiphysics\nsimulation and compared the piezoelectric charge distribution of Z-cut, X-cut, Y-cut, and Y-cut\n128° LiNbO₃ wafers, as shown in Figure 3r. The results clearly show that the Y-cut 128°\nconfiguration produces the highest surface charge density. Therefore, choosing Y-cut 128° to\nprepare LN-MFP is the best solution.\n-The paper mentions using a lock-in amplifier to get the 2f signal, but doesn't say how it was\n53 / 63\nset up. Important settings like time constant, filter slope, reference phase, and bandwidth can\naffect the signal quality. Could the authors give more details about these settings?\nWe thank the reviewer for this valuable comment and the opportunity to clarify the lock-in\ndetection configuration used in our measurements. All second-harmonic (2f) demodulation\nsignals were acquired using a Stanford Research SR830 lock-in amplifier, which was\nsynchronized with the sinusoidal reference signal generated by a Tektronix AFG3102C function\ngenerator. This generator provided both the modulation signal to the laser driver and the\nreference input to the lock-in amplifier, ensuring precise phase synchronization between optical\nmodulation and signal demodulation.\nFor all measurements, the reference frequency was set to match the resonant frequency\nof the LN-MFP (≈10.48 kHz). The time constant was set to 300 ms with a 12 dB/oct filter slope,\nproviding a good balance between noise suppression and temporal response. The reference\nphase was optimized manually for each measurement to maximize the in-phase (X) component\nof the 2f signal, typically around 0° ± 2°. The detection bandwidth was determined by the\nchosen time constant and modulation frequency, corresponding to approximately 0.5–1 Hz in\neffective bandwidth.\nThis configuration ensured a high signal-to-noise ratio and stable demodulation of weak\nphotoacoustic and thermoelastic signals, while maintaining consistent measurement\nconditions across all PAS, LITES, and photodetection experiments. We explicitly report the lock-\nin amplifier settings time constant and filter slope, as well as the integration time.\nIn the main article page 25 Line 4, we added:\n“The lock-in amplifier was configured with a 1 s time constant and a 12 dB/octave low-pass\nfilter slope, corresponding to an effective detection bandwidth of 0.25 Hz.”\n-In many sensing methods like QEPAS and LITES, quartz is usually used because it’s stable and\nhas good resonance. Why did the authors choose LiNbO₃ instead? Is it because it can do more\nthan one thing, has better sensitivity, or can be used with light on a chip? A comparison with\nquartz would help explain this choice.\nWe appreciate the reviewer’s insightful question. To provide a direct comparison between\n54 / 63\nquartz and LiNbO₃, two tuning-fork sensors with identical geometries and dimensions were\nfabricated using the same process described in the main text. Their resonance characteristics\nwere then experimentally measured by point-scanning detection, and the results are presented\nin Fig. 4r.\nAs shown, the quartz-based sensor exhibits a resonant frequency approximately 700 Hz\nlower than that of the LiNbO₃-based sensor, mainly due to the intrinsic differences in density\nand Young’s modulus between the two materials. Furthermore, because LiNbO₃ possesses a\nmuch higher piezoelectric coefficient (e.g., d₃₃ ~ 27 pm/V compared to quartz’s d₁₁ ~ 2.3 pm/V),\nthe resonance amplitude of the LiNbO₃ device is significantly higher, confirming stronger\nelectromechanical coupling and improved signal transduction efficiency.\nBeyond its superior piezoelectricity, LiNbO₃ also offers excellent electro-optic and\nnonlinear optical properties, enabling integration of photoacoustic, thermoelastic, and\nphotodetection functionalities within a single platform. This multifunctional capability makes\nLiNbO₃ a far more versatile material than quartz, particularly for future on-chip photonic\nimplementations where compact, multimodal sensing and optical integration are desired.\nFig. 3r Resonance curves measured of quartz and LiNbO₃ sensors of the same size.\n-The paper combines photoacoustic, thermoelastic, and photodetection into one device, but\nit’s not clear why all three are needed together. What advantages does this combination give?\nIt would help to have a comparison with using them separately.\nWe thank the reviewer for this valuable comment and the opportunity to clarify the advantages\n55 / 63\nof integrating photoacoustic (PAS), light-induced thermoelastic (LITES), and photodetection\nfunctionalities within a single LN-MFP platform. The multifunctional design is not a simple\ncombination of sensing mechanisms, but rather a strategic integration that enables adaptive,\ncomprehensive, and highly stable sensing under a wide range of environmental conditions.\nSpecifically, in the PAS mode, the modulated laser beam passes through the narrow tine\ngap of the LN-MFP and excites gas molecules within the confined region. The resulting acoustic\npressure waves are mechanically amplified by the resonant fork structure, providing high-\nsensitivity gas detection in compact volumes—especially when coupled with an acoustic micro-\nresonator. This makes PAS particularly suitable for trace-gas analysis in controlled or enclosed\nenvironments where acoustic coupling is strong.\nIn contrast, the LITES mode operates through non-contact thermal transduction. Here, the\nlaser irradiates the bottom sidewall region of the LN-MFP rather than directly passing through\nthe gas. The absorbed optical energy induces periodic thermoelastic deformation in the LN\nsubstrate, generating detectable signals even under extreme or corrosive conditions where\ndirect gas–transducer contact could impair performance or reliability. Thus, LITES provides a\nrobust alternative detection mechanism in harsh or contamination-prone environments.\nBy integrating both PAS and LITES modes on the same LN-MFP platform, the device can\nadaptively switch between a compact PAS configuration and a non-contact LITES configuration,\ndepending on the target gas and environmental conditions. This dual-mode functionality\nenables the system to accommodate a wide range of applications, from trace gas analysis with\nonly minute gas volumes available to reliable monitoring under harsh industrial or atmospheric\nconditions. There by achieving comprehensive and flexible sensing capability within a unified,\nminiaturized device architecture.\nTo make it clear to the readers, in the revised main article in page 17 line 11, we replaced\nthe original paragraph:\n“The integration of thermoelastic detection into the LN-MFP demonstrates its versatility\nas a multifunctional spectroscopic platform. The capability to perform both photoacoustic and\nthermoelastic measurements within a single device highlights its potential for comprehensive\nspectroscopic analysis. Such dual functionality proves especially valuable for applications\n56 / 63\nrequiring both high sensitivity and spectral selectivity across a broad spectral range, further\npositioning the LN-MFP as an advanced tool for next-generation spectroscopic sensing\ntechnologies.”\nwith the following:\n“The integration of thermoelastic detection into the LN-MFP demonstrates its versatility\nas a multifunctional spectroscopic platform. The spectral selectivity of both modes arises from\nthe intrinsic ro-vibrational fingerprints of target molecules. The dual functionality does not alter\nthis selectivity but extends its applicability across varying gases and conditions. By\nincorporating both PAS and LITES detection modes, the device enables adaptive gas sensing\nacross a wide range of environments and measurement scenarios. In PAS mode, direct gas–\nsensor interaction provides high-sensitivity detection within a compact volume, ideal for\nconfined or small-scale applications. Conversely, LITES operates in a non-contact\nconfiguration, suitable for sealed, corrosive or otherwise harsh environments. This dual-mode\narchitecture combines the sensitivity and compactness of contact-based detection with the\nrobustness and flexibility of non-contact operation, enabling comprehensive spectroscopic\nanalysis and positioning the LN-MFP as a versatile platform for next-generation sensing\ntechnologies.”\nThe built-in photodetection functionality further strengthens the system by enabling real-\ntime monitoring of laser power, wavelength stability, and signal referencing—functions\ntypically requiring separate detectors. This capability ensures self-calibration and enhances the\nstability and accuracy of both PAS and LITES measurements.\nBy integrating PAS, LITES, and photodetection within the same LN-MFP device, the system\ncan flexibly switch between high-sensitivity PAS and robust LITES detection, while maintaining\nreal-time optical feedback and system diagnostics. This tri-modal configuration expands the\noperational versatility of the device—from high-precision laboratory gas spectroscopy to\nindustrial, atmospheric, or corrosive environment monitoring—all within a unified and\nminiaturized platform.\n-For the photodetection tests, the authors changed the light power to see how the device\n57 / 63\nresponds. But since LiNbO₃ absorbs different wavelengths differently, how do they know the\nchange in signal is only because of power and not wavelength? Were the measurements done\nat fixed wavelengths? Or were power and wavelength controlled separately?\nWe thank the reviewer for this insightful question. All photodetection experiments in this study\nwere conducted under fixed-wavelength conditions to ensure that the observed signal\nvariations originated solely from changes in optical power rather than wavelength-dependent\nabsorption.\nSpecifically, for each of the six laser sources (covering the visible to mid-infrared range),\nthe output power was modulated electrically by applying a square-wave signal with a 50%\nduty cycle and 100% modulation depth to the injection current of the laser diode. This\nmodulation scheme enabled periodic variation of the emitted optical power at a constant\nwavelength, allowing accurate characterization of the LN-MFP’s photodetection response to\npower changes.\nAlthough LiNbO₃ exhibits wavelength-dependent optical absorption, in the current\nexperiments each test was performed using a single, fixed laser wavelength, corresponding to\nthe absorption features of specific target gases. The differences among wavelengths were\ninvestigated by employing different laser sources (450 nm–9.77 µm) rather than by tuning a\nsingle laser across a wide spectral range. Therefore, under each measurement condition, the\ndetected signal variations were attributed exclusively to changes in optical power, not to\nwavelength drift.\n-In the conclusion, the authors talk about making a small photonic chip from this platform.\nBut how will they turn this lab-scale device into something small enough for a chip? There are\nchallenges like tiny fabrication, connecting parts on a wafer, and packaging. Also, how will\nthey add light sources directly onto the same chip?\nAccording to the reviewer’s suggestion, we have added a detailed description in the\nSupplementary page 8 line16 section 3:\n“The chip fabrication process began with the design and preparation of a transimpedance\npreamplifier PCB board, which serves as the signal conditioning and amplification interface for\n58 / 63\nthe LN-MFP device. A gold-coated silicon substrate was then patterned using photolithography\nto define the electrode layout. To enable free vibration of the LN-MFP tines, a micro-machined\nrecess was etched beneath the LN-MFP region on the silicon substrate, ensuring that the fork\nstructure remained suspended and mechanically isolated from the base.\nSubsequently, the processed silicon substrate was bonded to the preamplifier PCB,\nestablishing the mechanical foundation for electrical integration. The LN-MFP was then\nattached to the silicon substrate using conductive silver paste, forming a robust electrical\ncontact between the LN-MFP’s bottom electrode and the silicon-based electrodes. In parallel,\nthe QCL chip was mounted onto the same silicon substrate using a specialized adhesive or\nsolder material, enabling optical alignment with the LN-MFP region.\nFinally, gold wire bonding was employed to complete the electrical interconnections. The\nsurface electrodes of the LN-MFP and QCL chip were bonded to the silicon electrodes, which\nwere in turn connected to the corresponding pads on the PCB. This integration process ensured\nprecise electrical continuity, compact structural assembly, and efficient signal transmission,\nproviding the foundation for a stable and miniaturized chip-scale LN-MFP sensing system.”\n-The paper introduces a new multi-purpose LiNbO₃ platform, but it doesn’t compare it clearly\nwith existing technologies like QEPAS or LITES. Could the authors give a table or explanation\ncomparing key performance numbers like detection limit, speed, and noise equivalent power?\nMany thanks for the comment. Three detailed tables about PAS, LITES, and pyroelectric sensors\nwere added to the Supplementary page 13 Section 9:\n“9. Comparison of the performance of the LN-MFP with existing PAS, LITES, and\npyroelectric sensors\n“In photoacoustic spectroscopy, the key parameters that govern detection performance are the\nlaser wavelength, optical power, and integration time. These parameters together with the\nminimum detection level (MDL), relative MDL (accounting for the proportionality of the\nphotoacoustic signal to laser power), and normalized noise equivalent absorption (NNEA)\ncoefficient, are summarized in Table 2s. Notably, the LN-MFP sensor is chip-scale offering\ninherent advantages in compactness, robustness, and ease of system integration.\n59 / 63\nTable 2s | PAS detection of trace gases using LN-MFP and QTF.\nTarget Detector Wavelength IT Power MDL RMDL NNEA\nanalyte type (μm) (s) (mW) (ppb) (ppmW) (cm-1WHz-1/2)\nLN-MFP 0.45 1000 1200 2 2.4×10-3 /\nNO\nLN-MFP 1.39 6300 19 25 4.75×10-4 5.82×10-9\nH O\nLN-MFP 1.53 1000 3700 3.5 1.3×10-2 2.7×10-8\nC H\n2 2\nQTF [9] 1.53 370 5 21 1.05×10-4 /\nLN-MFP 2 1900 10.6 350 3.71×10-3 2.53×10-8\nCO\nLN-MFP 3.3 1200 6.5 2.5 1.63×10-5 8.36×10-9\nCH\nNH LN-MFP 9.7 1100 82.3 0.35 2.88×10-5 6.58×10-10\nfunctional platform; IT: Integration time; QTF: quartz tuning fork; MDL: minimum detection limit;\nRMDL: relative minimum detection limit\nLN-MFP’s competitive photoacoustic performance arises from several complementary\nphysical and practical factors. First, single-crystal lithium niobate exhibits substantially higher\neffective piezoelectric coupling (for appropriate crystal orientations and tensor components)\nthan α-quartz, enabling LiNbO₃ devices to generate stronger electrical signals—and thus higher\nsignal-to-noise ratios—for a given mechanical strain [5]. Second, the device geometry and\nresonant frequency were carefully optimized to balance acoustic coupling, thermoelastic drive\nand readout bandwidths ensuring robust multimodal transduction even for slowly relaxing\nspecies such as CO₂. Finally, lithium niobate’s mature and rapidly evolving photonics\necosystem, featuring broad optical transparency, strong electro-optic properties and wafer-level\nprocessing, facilitates the integration of on-chip optics, waveguides and micro-collimation\ncomponents that are challenging to implement in conventional quartz tuning-fork systems [6].\nThe chip-scale monolithic LN-MFP also minimizes parasitic capacitance and interconnect\nlosses, enabling compact low-noise electronics near the transducer, thereby lowering the noise\nfloor and enhancing NNEA.\n60 / 63\nTable 3s | Photodetector using LN-MFP and pyroelectric sensors.\nCharacterized NEP Response Responsivity\nDetector type\nwavelength range (W/√Hz) time (V/W)\nLN-MFP 450−9770 nm 5.65×10-8 1 s 373\nQTF [12] 1540 nm / 1 s 47.4\nLiNbO [13] / 1.623×10-7 1.9 s /\nSb Te −Bi Te [16] 600−700 nm 8.0 × 10−9 341 μs 38\n2 3 2 3\nLN-MFP: lithium niobate multi-functional platform; QTF: quartz tuning fork.\nA concise comparison of the LN-MFP with representative pyroelectric and tuning-fork\ndetectors is provided in Table 3s. In the current prototype the LN-MFP demonstrates high\nresponsivity (~373 V W⁻¹) and a noise-equivalent power on the order of 5.6×10⁻⁸ W Hz⁻¹/²;\nmeasured with a 1 s lock-in time constant (corresponding to ~0.25 Hz detection bandwidth).\nThe device operates across a broad wavelength range from 450 nm to 9.77 µm and integrates\nPAS, LITES and direct photodetection functionalities on a single chip. The reported\nperformance reflects a deliberate, system-level optimization, balancing geometry and readout\nto meet the combined requirements of all three sensing modalities. Although the LN-MFP is\nnot necessarily optimized for any single modality, it uniquely combines high per-power\nresponsivity, multimodal functionality and compatibility with wafer-scale photonics. Several\nstraightforward engineering routes remain to further improve the absolute MDL, NEP and\nbandwidth, including the use of low-noise transimpedance amplifiers, micro-collimation or\nwaveguide coupling to enhance optical power delivery, and tailored absorptive coatings at the\ntine base. These improvements will be explored in future work to further enhance performance\nwhile preserving the platform’s integration advantages.\n61 / 63\nTable 4s | LITES detection of trace gases using LN-MFP and QTF.\nTarget Wavelength IT AL MDL RMDL\nDetector type\nanalyte (μm) (s) (cm) (ppm) (ppmm)\nNO LN-MFP 0.45 450 2.5 1 2.5×10-2\nH O\nLN-MFP 1.53 1500 2.5 0.45 1.13×10-2\nC H\n2 2\nQTF [18] 1.53 70 20 0.36 7.2×10-2\nLN-MFP 2 1200 2.5 35 8.75×10-1\nCO\nLN-MFP 3.3 1900 2.5 0.9 2.25×10-2\nCH\nNH LN-MFP 9.7 1200 2.5 1.2 3×10-2\nplatform; MDL: minimum detection limit; RMDL: relative minimum detection limit; IT: Integration\ntime;\nFor LITES, the key parameters include the laser wavelength, absorption length (AL), and\nintegration time, as summarized in Table 4s. Since the MDL is directly proportional to the AL,\nthe comparison includes not only the MDL but also the relative MDL for a fair and\ncomprehensive evaluation. Table 4s shows that the LN-MFP attains superior RMDL for most\nchannels (H₂O, C₂H₂, CO₂, NO₂, NH₃), with CH₄ as the only minor exception. This advantage\nstems from three factors: larger effective piezoelectric coupling of Y+128° LiNbO₃, an\nelectrode layout that maximizes charge collection at regions of peak modal strain, and a\nresonance (~10.5 kHz) deliberately tuned to enhance thermoelastic/acoustic energy build-up\nfor slowly relaxing species.\nReference\n5. Yue, W. et al. Crystal orientation dependence of piezoelectric properties in LiNbO and\n3\n6. Boes, A. et al. Lithium niobate photonics: Unlocking the electromagnetic spectrum. Science\n379, eabj4396 (2023).\n7. Breitegger, P. et al. Towards low-cost QEPAS sensors for nitrogen dioxide detection.\nPhotoacoustics 18, 100169 (2020).\n62 / 63\n8. Rousseau R. et al. Monolithic double resonator for quartz enhanced photoacoustic\nspectroscopy. Appl. Sci. 11: 2094 (2021).\n9. Wang, R., Qiao, S., He, Y., & Ma, Y. Highly sensitive laser spectroscopy sensing based on\na novel four-prong quartz tuning fork. Opto-Electron. Adv. 8, 240275-1 (2025).\n10. Zhang, Y. et al. Continuous real-time monitoring of carbon dioxide emitted from human\nskin by quartz-enhanced photoacoustic spectroscopy. Photoacoustics 30, 100488 (2023).\n11. Wu H. et al. Atmospheric CH measurement near a landfill using an ICL-based QEPAS\n4\nsensor with VT relaxation self-calibration. Sens. Actuator B-Chem. 297 126753 (2019).\n12. Zhou, S. et al. Realization of a infrared detector free of bandwidth limit based on quartz\ncrystal tuning fork. Opt. Laser Technol. 113, 261-265 (2019).\n13. Aleks, M. et al. An overview of microelectronic infrared pyroelectric detector. Eng. Sci. 16,\n82-89 (2021).\n14. Suen, J. Y. et al. Multifunctional metamaterial pyroelectric infrared detectors. Optica 4,\n276-279 (2017).\n15. Kane, S. R. et al. Characterizing pyroelectric detectors for quantitative synchrotron\nradiation measurements. Sensor. Actuat. A-Phys. 387, 116406 (2025).\n16. Mauser, K. W. et al. Resonant thermoelectric nanophotonics. Nat. Nanotechnol. 12, 770-\n775 (2017).\n17. Xu, L. et al. Multigas sensing technique based on quartz crystal tuning fork-enhanced laser\nspectroscopy. Anal. Chem. 92 14153-14163 (2020).\n18. He, Y. et al. Hydrogen-enhanced light-induced thermoelastic spectroscopy sensing.\nPhotonics Res. 13 194-200 (2024).\n19. Bojęś, P. et al. Dual-band light-induced thermoelastic spectroscopy utilizing an antiresonant\nhollow-core fiber-based gas absorption cell. Appl. Phys. B 129, 177 (2023).\n20. Shang, Z. et al. Robust and compact light-induced thermoelastic sensor for atmospheric\nmethane detection based on a vacuum-sealed subminiature tuning fork. Photoacoustics 42,\n100691 (2025).”\n63 / 63"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed my previous comments well overall, and the quality of the revised manuscript has improved.\nHowever, I still have several remaining concerns.\n1. While the multifunctional operation of the LN-MFP platform is interesting, the level of novelty remains limited. The individual sensing mechanisms (PAS, LITES, and thermoelastic photodetection) have been demonstrated previously using QTF-based and MEMS-based systems, and the performance improvements presented here appear incremental rather than introducing a fundamentally new advancement. The manuscript should clearly describe the specific scientific or technological contribution that is unique to this LN-based approach, and how it advances the current state of the art. I believe that strengthening this aspect is very important for meeting the expectations of Nature Communications.\nAdditionally, the term “on-chip integration”, especially in the abstract, may lead readers the impression of monolithic or wafer-scale integration. In this work, however, the demonstrated QCL-LN configuration is based on PCB-level co-packaging rather than true chip-scale integration. I recommend clarifying this distinction in both the abstract and the main text to avoid overstating the integration level. In addition, it would be helpful for the authors to more explicitly describe what the key novelty and strengths of this work are, although the integration remains at the PCB level. Providing a clear justification of the scientific value and technological impact of this work, despite not achieving on-chip integration, would strengthen the manuscript’s positioning.\n2. Although the authors provide additional details regarding the free-space alignment between the QCL and the LN-MFP, the optical coupling efficiency is still not reported. Since key performance metrics such as NNEA and MDL depend on the optical power actually delivered to the sensing region, the absence of a coupling-efficiency estimate introduces uncertainty in the quantitative interpretation of the results. Please clarify how the NNEA values were calculated rigorously.", "reviewer_2": "The authors have made significant revision in the manuscript and well answered most of my questions. Here I have only a few minor comments for authors' consideration and clarification.\n(1) I appreciate the authors added N₂ background signals in Fig. 2 for direct comparison, but there is significant non-zero signal in (b) and (c). Please explain and mention whether it is an issue for applications.\n(2) The authors made detailed response to the previous question Q3 regarding comparison to LITES. I notice this part has been added to supplemental, but nothing is mentioned in the main text. Please inform readers this part is discussed in the supplemental information.\n(3) In the previous question Q5, the Table 3s compares pyroelectric detectors. Is it possible to include photovoltaic detector in the table as well, considering its wide use in spectroscopy.", "reviewer_3": "The authors have carefully revised the manuscript accroding to my comments. I am conent with the revisions and suggesting acceptance now."}, "author_response": "Response to reviewers’ comments\nDear reviewers:\nPlease find enclosed our answers to the three referees’ comments. We have addressed all questions raised by both reviewers, and revised our manuscript, accordingly.\nPlease Note: reviewers’ comments are in black; our comments are in red italic; the original paper text is in red and revised text is in blue.\nReviewer #1:\nThe authors have addressed my previous comments well overall, and the quality of the revised manuscript has improved. However, I still have several remaining concerns.\n1. While the multifunctional operation of the LN-MFP platform is interesting, the level of novelty remains limited. The individual sensing mechanisms (PAS, LITES, and thermoelastic photodetection) have been demonstrated previously using QTF-based and MEMS-based systems, and the performance improvements presented here appear incremental rather than introducing a fundamentally new advancement. The manuscript should clearly describe the specific scientific or technological contribution that is unique to this LN-based approach, and how it advances the current state of the art. I believe that strengthening this aspect is very important for meeting the expectations of Nature Communications.\nWe thank the reviewer for this important comment. We fully agree that it is essential to clearly articulate what is unique about the present LN-based approach. Our key point is that this work does not simply transplant known mechanisms onto a different substrate instead, it establishes a new material and system-level platform in which these mechanisms are unified and exploited in ways that are not attainable with quartz or conventional MEMS technologies.\nThe most fundamental distinction between lithium niobate (LN) and existing technologies lies in its intrinsic multifunctionality and strong compatibility with on-chip integration. LN exhibits significantly richer multi-physical-field coupling—encompassing piezoelectric, pyroelectric, electro-optic, and photoelastic effects—than quartz. In recent years, extensive studies on LN have enabled a broad range of optoelectronic components, including light sources [Nature 615, 411-417 (2023)], optical waveguides [Nat. Photon. 18, 218-223 (2024)], optical isolators [Nat. Photon. 17, 666-671 (2023)], and optical modulators [Nat. Commun. 16, 2281 (2025)]. Although these technologies target applications in optical communication or sensing, truly practical LN-based sensing applications remain scarce.\nIn this work, we take the lead in fabricating devices directly on LN wafers and successfully demonstrate multiple sensing-related functionalities spanning the visible to the mid-infrared spectral range. Moreover, we integrate a mid-infrared quantum cascade laser (QCL) with the LN multi-functional platform and validate its capability for gas sensing. Therefore, this contribution of this work should be viewed as a system-level innovation rather than an isolated device-level demonstration.\nIn the revised manuscript, we now highlight four aspects of novelty:\n(1) Material-level novelty: intrinsically multifunctional LN platform.\nThe most fundamental difference between lithium niobate (LN) and existing quartz- or Si-based technologies is LN’s intrinsic multifunctionality and strong multi-physical-field coupling. LN simultaneously exhibits strong piezoelectric, pyroelectric, electro-optic, and photoelastic effects, whereas quartz provides only comparatively weak piezoelectric coupling and lacks electro-optic and pyroelectric responses. Over the past years, these material properties have enabled a rich ecosystem of LN-based optoelectronic components, including light sources, optical waveguides, optical isolators, and optical modulators [Nature 615, 411–417 (2023); Nat. Photon. 18, 218–223 (2024); Nat. Photon. 17, 666–671 (2023); Nat. Commun. 16, 2281 (2025)]. However, despite this progress, practical and fully integrated LN-based sensing implementations remain scarce. Our work fills this gap by realizing a multifunctional sensing platform directly on LN wafers and by demonstrating that LN can serve as the core material for a unified PAS/LITES/photodetection platform spanning the visible to the mid-infrared.\n(2) Fundamental material properties and a design paradigm distinct from quartz.\nThe fabrication processes and intrinsic physical properties of LN are fundamentally different from those of quartz. LN is a ferroelectric crystal with spontaneous polarization and belongs to the 3m point group, whereas quartz is non-ferroelectric (point group 32) and lacks spontaneous polarization. As a result, LN exhibits piezoelectric coefficients and electromechanical coupling factors more than an order of magnitude higher than those of quartz (e.g., d ≈ 25 pC/N for LN 22 versus d₁₁ ≈ 2.3 pC/N for quartz), and additionally offers strong electro-optic, nonlinear-optical, and pyroelectric effects that quartz completely lacks [Nat. Commun. 14, 4856 (2023)]. Because of these fundamental differences in crystal symmetry and ferroelectric nature, LN-based sensors cannot be designed by simply applying “scaled-down” quartz-QTF paradigms. The fork-shaped resonator geometry, electrode layout, and fabrication flow used in this work are specifically engineered to exploit LN’s unique multi-physics response. These details were fully demonstrated in the previous revision. It is precisely these new design and manufacturing strategies that enable the superior multimodal performance of the LN-MFP platform presented here.\n(3) Mechanism-level advances for PAS, photodetection, and LITES on LN.\n(a) Photoacoustic spectroscopy (PAS).\nA defining characteristic of PAS is that the signal amplitude scales inversely with the interaction volume. By developing an on-chip–integrable PAS transducer based on LN, we can simultaneously leverage (i) the strong PAS scaling at small volumes and (ii) the inherent capability of a wafer-scale photonic platform. LN wafers can be micro-fabricated and co-integrated with optoelectronic or MEMS subsystems to realize compact PAS sensors, which is extremely challenging with quartz QTFs. Moreover, LN’s much larger piezoelectric coefficients and electromechanical coupling factors translate directly into higher acoustic-to-electric conversion efficiency: under the same acoustic pressure, LN generates a substantially larger electrical output than quartz. LN’s relatively high dielectric permittivity (ε₁₁ ≈ 85, ε₃₃ ≈ 30) further enables a strong polarization response under high-frequency and high-field operation. These material advantages provide a fundamentally different and improved operating regime for PAS compared with QTF-based sensors.\n(b) Thermoelastic photodetection.\nLN exhibits strong electro-optic and nonlinear-optical properties that quartz fundamentally lacks. As a non-centrosymmetric ferroelectric crystal, LiNbO₃ supports a strong linear electro-optic (Pockels) effect and pronounced photorefractive and pyroelectric responses, enabling direct optical-to-electrical energy conversion and efficient optical phase/intensity modulation. Quartz, by contrast, does not exhibit second-order nonlinearity and is limited to weak piezo-photoelastic effects, making it unsuitable for high-performance thermoelastic photodetection. Furthermore, LN devices can operate with ultrafast response and extremely wide bandwidths, reaching the GHz–THz regime [Nat. Commun. 16, 8864 (2025)]. When exploited as photothermal or thermoelastic photodetectors, LN devices benefit simultaneously from thermoelastic and Joule-heating effects, setting a much higher intrinsic ceiling on sensitivity and bandwidth than quartz- or Si-based detectors.\n(c) Light-induced thermoelastic spectroscopy (LITES).\nIn LITES, the same LN-MFP serves as the multi-physics transducer. When LN is used as both optical modulation and detection medium, several performance enhancements become available. LN’s broad electro-optic bandwidth allows rapid laser frequency modulation and improved frequency stability, enhancing both measurement speed and spectral resolution. LN-based devices also naturally support phase-sensitive detection schemes, enabling precise extraction of both amplitude and phase of the absorption signal. Because LN provides a single material platform for both high-speed modulators and sensitive detectors, future LN-based LITES systems can integrate modulator and detector within the same material, enabling high-frequency modulation and lock-in detection in a unified architecture. Such co-integration is not achievable with quartz-based or conventional MEMS sensors and can significantly reduce the detection limits while expanding dynamic range [Nat. Commun. 16, 8864 (2025)]. Additionally, LN’s high Curie temperature (T ≈ C 1210 °C), large pyroelectric coefficient (p ≈ 40 µC m⁻² K⁻¹), and excellent thermal robustness allow stable operation over wide temperature ranges and under high optical intensity, making LN particularly suitable for harsh-environment sensing (e.g., combustion diagnostics) beyond the reach of most QTF- or MEMS devices.\n(4) System-level innovation and compatibility with the LN photonics ecosystem.\nAs summarized in the new Table 1r of the revised manuscript, recent works have established LN as a leading platform for integrated light sources, frequency combs, high-speed modulators, and wavelength converters spanning the visible to the mid-infrared [Science 379, eabj4396 (2023) and related references]. These studies repeatedly highlight the potential of LN integrated photonics for spectroscopy, but a multifunctional sensing “front end” compatible with this ecosystem has been missing. Our work addresses this gap by introducing an LN-based multifunctional platform (LN-MFP) that unifies photoacoustic spectroscopy, light-induced thermoelastic spectroscopy, thermoelastic photodetection and, at the system level, a mid-infrared QCL source within a single hybrid module. The sensor is fabricated using commercially available LN wafers, assembled into a PCB-level co-packaged LN-MFP–QCL module, and experimentally validated for gas sensing. This demonstrates that the proposed LN platform is not only physically distinct from quartz QTFs and traditional MEMS sensors, but also technologically compatible with the existing LN integrated-photonics infrastructure, thereby enabling future “all-lithium-niobate” spectroscopic chips.\nTable 1r Overview of state-of-the-art lithium niobate integrated photonic devices.\nArticle Device Ref.\nUltrafast tunable photonic-integrated extended-DBR Pockels\nLight Source [1]\nlaser\nProgrammable multifunctional integrated microwave photonic\nModulator [2]\ncircuit on thin-film lithium niobate\nBroadband electro-optic frequency comb generation in a\nLight Source [3]\nlithium niobate microring resonator\nCascaded nonlinear down-conversion in poling-free lithium\nLight Source [4]\nniobate nanophotonic waveguides\nSub-1 Volt and high-bandwidth visible to near-infrared electro-\nModulator [5]\noptic modulators\nUltrafast tunable lasers using lithium niobate integrated\nLight Source [6]\nphotonics\nIntegrated lithium niobate photonics for sub-ångström\nSpectrometer [7]\nsnapshot spectroscopy\nVisible-to-ultraviolet frequency comb generation in lithium\nLight Source [8]\nniobate nanophotonic waveguides\nVersatile Lithium Niobate Platform for\nPlatform (integrated this\nPhotoacoustic/Thermoelastic Gas Sensing and\nsource and detector) work\nPhotodetection\nIn summary, the originality of this work lies in establishing a lithium-niobate-based multifunctional platform in which three fundamentally different sensing mechanisms, photoacoustic sensing, thermoelastic photodetection, and light-induced thermoelastic sensing, together with a mid-infrared QCL source are realized within a single, strongly multi-physics-coupled LN architecture and validated in a compact PCB-level module. This creates a complete and coherent progression from material properties to device concept and ultimately to a practically deployable hybrid sensing system. Thereby defining a new multimodal sensing framework tailored to the LN integrated photonics ecosystem.\nWe believe that this material-level and system-level advance go beyond incremental performance improvements and is well aligned with the expectations of Nature Communications for interdisciplinary work that opens genuinely new research directions. To make these points clearer to the reader, we have revised the Introduction and Discussion sections to more explicitly highlight these unique scientific and technological contributions. We have also added Table 1r to situate our work within the broader context of state-of-the-art LN integrated photonic devices.\nReferences\n[1] Siddharth A. et al. Ultrafast tunable photonic-integrated extended-DBR Pockels laser. Nat. Photon. 19, 709-717 (2025).\n[2] Wei C. et al. Programmable multifunctional integrated microwave photonic circuit on thin-film lithium niobate. Nat. Commun. 16, 2281 (2025).\n[3] Zhang M. et al. Broadband electro-optic frequency comb generation in a lithium niobate microring resonator. Nature 568, 373-377 (2019).\n[4] Yan C. et al. Cascaded nonlinear down-conversion in poling-free lithium niobate nanophotonic waveguides. Nat. Commun. 16, 9987.\n[5] Renaud D. et al. Sub-1 Volt and high-bandwidth visible to near-infrared electro-optic modulators. Nat. Commun. 14, 1496 (2023).\n[6] Snigirev V. et al. Ultrafast tunable lasers using lithium niobate integrated photonics. Nature 615, 411-417 (2023).\n[7] Yao Z. et al. Integrated lithium niobate photonics for sub-ångström snapshot spectroscopy. Nature 646, 567-575 (2025).\n[8] Wu T. H. et al. Visible-to-ultraviolet frequency comb generation in lithium niobate nanophotonic waveguides. Nat. Photon. 18, 218-223 (2024).\nIn the abstract replace the text:\n“By levering LN's strong piezoelectric and thermoelastic properties, along with an optimized fork-shaped design, our platform achieves high sensitivity and selectivity across a broad spectral range—from visible to long-wave infrared wavelengths.”\nwith the following:\n“Leveraging the intrinsic multi-physics nature of ferroelectric lithium niobate, the proposed LN multi-functional platform (LN-MFP) unifies photoacoustic spectroscopy, light-induced thermoelastic spectroscopy, and thermoelastic photodetection within a single chip, while operating over a broad spectral range spanning from the visible to the mid-infrared.”\nAnd substitute the abstract text:\n“This compact, integrated, multi-functional architecture significantly reduces system complexity and footprint, paving the way for portable, scalable sensing systems in environmental monitoring, point-of-care diagnostics, and on-site chemical analysis, marking a key advancement in integrated photonics.”\nwith:\n“This compact, hybrid, multi-functional architecture significantly reduces system complexity and footprint compared with conventional benchtop systems and is intrinsically compatible with the rapidly developing lithium niobate integrated photonics ecosystem. The LN-MFP establishes a lithium niobate–based multimodal sensing framework and provides a core sensing building block for future all-lithium-niobate spectroscopic chips for environmental monitoring, point-of-care diagnostics, and on-site chemical analysis.”\nAdd the following paragraph in the main text Page 4 Line 104 at the beginning of the section:\nDesign and Fabrication of the Lithium Niobate Multi-Functional Platform\n“The design philosophy of the LN-MFP fundamentally differs from that of conventional quartz- or Si-based PAS/LITES devices. Lithium niobate is a ferroelectric crystal with 3m point-group symmetry and spontaneous polarization, leading to piezoelectric coefficients and electromechanical coupling factors more than one order of magnitude larger than those of quartz (for example, d ≈ 25 pC/N for LN versus d₁₁ ≈ 2.3 pC/N for quartz). At the same time, 22 LN exhibits strong electro-optic, nonlinear-optical, and pyroelectric effects that are absent in quartz and most MEMS materials. These intrinsic properties mean that LN-based sensors cannot be obtained by simply scaling or copying established quartz tuning fork designs; instead, the fork geometry, electrode layout, and fabrication process in this work are specifically engineered to harness LN’s multi-physics coupling. As a result, a single LN-MFP chip can simultaneously operate as a high-efficiency acoustic transducer for PAS, a thermoelastic detector, and a platform for LITES, thereby enabling multimodal operation that is not available in conventional quartz or MEMS devices.”\nReplace the following text on main article Page 20 Line 429:\n“The development of the lithium niobate multi-functional platform represents a significant breakthrough in integrated spectroscopic sensing technologies. By harnessing the multifunctional properties of LN and incorporating a resonant fork-shaped design, we have successfully integrated photoacoustic spectroscopy, light induced thermoelastic spectroscopy, and photodetection into a single compact device. Experimental results reveal high sensitivity and selectivity across a broad spectral range, achieving detection limits suitable for real-world applications in environmental monitoring and industrial process control. This platform effectively addresses the integration challenges associated with conventional spectroscopic systems, providing a scalable and versatile solution that enhances performance while significantly reducing size and complexity, and system requirements.”\nwith the following:\n“Furthermore, by co-packaging the LN-MFP chip with a mid-infrared QCL chip and the associated drive and readout electronics on a compact PCB, we demonstrate that the LN-MFP can be seamlessly interfaced with semiconductor laser sources using standard microelectronic assembly techniques. Experimental results reveal high sensitivity and selectivity across a broad spectral range, achieving detection limits suitable for real-world applications in environmental monitoring and industrial process control. Even though the current implementation is at the PCB level rather than monolithic, this hybrid QCL–LN architecture already reduces the footprint and alignment complexity of the spectrometer and establishes the LN-MFP as a practical building block for future fully integrated lithium-niobate spectroscopic chips.\nIn summary, we have established a lithium-niobate-based multi-functional platform (LN-MFP) that unifies three fundamentally different sensing mechanisms, photoacoustic spectroscopy, light-induced thermoelastic spectroscopy, and thermoelastic photodetection, within a single ferroelectric crystal chip. The LN-MFP exploits the strong piezoelectric, pyroelectric, electro-optic, and photoelastic responses of lithium niobate to achieve efficient acoustic-to-electric and thermoelastic transduction, while operating over a spectral range extending from the visible to the mid-infrared. Beyond the device level, we demonstrate a PCB-level co-packaged LN-MFP–QCL module capable of mid-infrared gas sensing, thereby validating that the LN-MFP can serve as a practical sensing front-end compatible with semiconductor laser sources and standard microelectronic assembly. Together with recent progress in LN-based lasers, frequency combs, and high-speed modulators, this multifunctional sensing platform fills a key gap in the LN photonics landscape and points toward future “all-lithium-niobate” spectroscopic chips in which light generation, modulation, and multimodal detection are realized within a single material system. We therefore view the main advance of this work not as an incremental improvement in a single performance metric, but as the introduction of a new LN-centric material and system-level paradigm for integrated spectroscopic sensing.”\nAdditionally, the term “on-chip integration”, especially in the abstract, may lead readers the impression of monolithic or wafer-scale integration. In this work, however, the demonstrated QCL-LN configuration is based on PCB-level co-packaging rather than true chip-scale integration. I recommend clarifying this distinction in both the abstract and the main text to avoid overstating the integration level. In addition, it would be helpful for the authors to more explicitly describe what the key novelty and strengths of this work are, although the integration remains at the PCB level. Providing a clear justification of the scientific value and technological impact of this work, despite not achieving on-chip integration, would strengthen the manuscript’s positioning.\nWe thank the reviewer for this important comment and agree that our present implementation corresponds to PCB-level co-packaging of a discrete QCL chip with the LN-MFP chip, rather than to monolithic or wafer-scale integration. In the original submission we used the expressions “on-chip integration” and “on-chip sensor” to emphasize that the sensing functions (photoacoustic spectroscopy, light-induced thermoelastic spectroscopy, and photodetection) are monolithically implemented on the lithium niobate chip. However, as the reviewer correctly points out, this wording may inadvertently give the impression that the QCL source is also already integrated on the same chip.\nTo avoid any possible overstatement of the integration level, we have carefully revised the terminology in both the abstract and the main text:\nIn addition, following the reviewer’s suggestion, we now more clearly articulate the novelty and strengths of the work despite the current integration level:\n• First, at the device level, the LN-MFP monolithically integrates photoacoustic spectroscopy, light-induced thermoelastic spectroscopy, and photodetection on a single lithium-niobate chip and operates over a broad spectral range from the visible to the long-wave infrared. This multifunctionality and bandwidth, enabled by the intrinsic material properties of LN and the fork-shaped resonant design, remain the central scientific contribution of the work.\n• Second, at the system level, the revised text emphasizes that co-packaging the LN-MFP chip with a mid-infrared QCL chip and on-board drive/readout electronics on a compact PCB demonstrates a realistic, industry-compatible hybrid architecture. Even though the light source is not yet monolithically integrated, this PCB-level QCL–LN module already reduces system footprint and alignment complexity compared with conventional benchtop spectrometers and validates the LN-MFP as a practical building block for future fully integrated lithium-niobate spectroscopic chips.\nTo make these points explicit to the reader, we have (i) revised the last part of the abstract and (ii) added clarifying sentences to the concluding paragraph of the main text that highlight the scientific value and technological impact of the current PCB-level implementation while clearly positioning it as an intermediate, but important, step toward monolithic QCL–LN integration.\nThe manuscript revisions are listed below.\nIn the abstract we replace the following sentences:\n“To further enhance on-chip integration, we implement a custom packaging solution featuring a printed circuit board with transimpedance amplification and a quantum cascade laser chip, enabling direct wire bonding to the LN-MFP. Exploiting a 4.6 µm quantum cascade laser chip, we successfully demonstrate carbon monoxide detection via second-harmonic measurements, highlighting the LN-MFP’s potential for fully integrated spectroscopic applications. This compact, integrated, multi-functional architecture significantly reduces system complexity and footprint, paving the way for portable, scalable sensing systems in environmental monitoring, point-of-care diagnostics, and on-site chemical analysis, marking a key advancement in integrated photonics.”\nwith the text:\n“To demonstrate system-level integration in a practical form factor, we implement a custom packaging solution in which the LN-MFP chip and a 4.6 µm quantum cascade laser (QCL) chip are co-mounted on a printed circuit board together with transimpedance amplification, forming a hybrid PCB-level QCL–LN module with direct wire bonding to the LN-MFP. Using this co-packaged module, we successfully demonstrate carbon monoxide detection via second-harmonic measurements, thereby validating that the LN-MFP can operate as the core sensing element of a compact hybrid QCL–LN spectroscopic system and outlining a clear route toward future fully integrated on-chip implementations. This compact, hybrid, multi-functional architecture significantly reduces system complexity and footprint compared with conventional benchtop systems and is intrinsically compatible with the rapidly developing lithium niobate integrated photonics ecosystem. The LN-MFP establishes a lithium niobate–based multimodal sensing framework and provides a core sensing building block for future all-lithium-niobate spectroscopic chips for environmental monitoring, point-of-care diagnostics, and on-site chemical analysis.”\nWe changed the subsection title of main article on Page 18 Line 401:\n“On-chip LN-MFP Sensor”\nto\n“PCB-level Co-packaged LN-MFP Sensor Module”\nOn Page 18 Line 402 we substitute the following text:\n“Based on the abovementioned multifunctional capabilities, we introduced an designed on-chip packaging approach and evaluated its performance. As the photograph shown in Fig. 5a, ….”\nwith the following text:\n“Based on the abovementioned multifunctional capabilities, we next implemented a PCB-level co-packaged LN-MFP sensor module and evaluated its performance. As shown in the photograph in Fig. 5a, ………”\nOn Page 19 Line 424 we substitute the following text:\n“Figure 5d presents the 2f signal of 8000 ppm CO in N measured using the packaged LN-MFP-QCL chip, demonstrating the platform’s strong potential for fully integrated on-chip spectroscopic applications.”\nwith the text:\n“Figure 5d presents the 2f signal of 8,000 ppm CO in N₂ measured using the PCB-level co-packaged LN-MFP-QCL module, demonstrating that the LN-MFP can serve as a compact and efficient sensing element in a hybrid QCL-LN configuration and highlighting a realistic pathway toward future fully integrated on-chip spectroscopic systems.”\nReplaced Figure 5 caption:\n“Fig. 5 | On chip LN-MFP sensor. a Photograph of the packaged on-chip sensor. b Optical-microscope image of the coupling region showing the QCL facet and the LN-MFP prong gap. c Schematic diagram of the free-space optical coupling. d 2f photoacoustic spectrum recorded from 8,000 ppm CO in N₂.”\nwith:\n“Fig. 5 | PCB-level co-packaged LN-MFP-QCL sensor module. a Photograph of the packaged PCB-level hybrid sensor module. b Optical-microscope image of the coupling region showing the QCL facet and the LN-MFP prong gap. c Schematic diagram of the free-space optical coupling. d 2f photoacoustic spectrum recorded from 8,000 ppm CO in N₂.”\nIn the main article Page 20 Line 429, we replaced the following text:\n“The development of the lithium niobate multi-functional platform represents a significant breakthrough in integrated spectroscopic sensing technologies. By harnessing the multifunctional properties of LN and incorporating a resonant fork-shaped design, we have successfully integrated photoacoustic spectroscopy, light induced thermoelastic spectroscopy, and photodetection into a single compact device. Experimental results reveal high sensitivity and selectivity across a broad spectral range, achieving detection limits suitable for real-world applications in environmental monitoring and industrial process control. This platform effectively addresses the integration challenges associated with conventional spectroscopic systems, providing a scalable and versatile solution that enhances performance while significantly reducing size and complexity, and system requirements.”\nwith:\n“Furthermore, by co-packaging the LN-MFP chip with a mid-infrared QCL chip and the associated drive and readout electronics on a compact PCB, we demonstrate that the LN-MFP can be seamlessly interfaced with semiconductor laser sources using standard microelectronic assembly techniques. Experimental results reveal high sensitivity and selectivity across a broad spectral range, achieving detection limits suitable for real-world applications in environmental monitoring and industrial process control. Even though the current implementation is at the PCB level rather than monolithic, this hybrid QCL–LN architecture already reduces the footprint and alignment complexity of the spectrometer and establishes the LN-MFP as a practical building block for future fully integrated lithium-niobate spectroscopic chips.”\n2. Although the authors provide additional details regarding the free-space alignment between the QCL and the LN-MFP, the optical coupling efficiency is still not reported. Since key performance metrics such as NNEA and MDL depend on the optical power actually delivered to the sensing region, the absence of a coupling-efficiency estimate introduces uncertainty in the quantitative interpretation of the results. Please clarify how the NNEA values were calculated rigorously.\nWe thank the reviewer for raising this important point regarding optical coupling efficiency.\nBased on the structural parameters provided in both the initial manuscript and the previous revision, we have now conducted an optical simulation using Zemax to estimate the coupling efficiency from the QCL chip to the sensing region between the LN-MFP prongs.\nIn the simulation, we assume a conical Gaussian beam emission from the QCL chip, with a divergence angle of ~70°. The chip to LN-MFP distance is approximately 100 µm, as shown in the Fig. 1r a. While the lateral divergence is constrained by the gap between the two prongs of the LN-MFP, the vertical divergence extends beyond the LN-MFP plane (500 µm thickness), resulting in partial optical loss as shown in the Fig. 1r b. This out-of-plane component does not contribute to the detection process. Based on this configuration, Zemax modeling estimates the effective optical coupling efficiency to be ~29%.\na\nb\nFig. 1r | Optical coupling between the QCL chip and the LN-MFP. a Schematic diagram of the free-space optical coupling. b Optical simulation by ZEMAX.\nTherefore, we have added the following clarification to the revised manuscript on Page 19, Line 420:\n“The optical coupling efficiency was estimated to be ~29%.”\nAs correctly noted by the reviewer, the effective optical power used in the NNEA calculation corresponds to this coupled power rather than the total QCL output. To clarify this, we have included a rigorous explanation of the NNEA evaluation in the revised Supplementary Section 9, Page 13 Line 294：\n“9. The calculation of NNEA\nTo quantitatively assess the performance of the LN-MFP–based spectroscopic sensor, the normalized noise-equivalent absorption (NNEA) was evaluated for the different gas channels.\nThe NNEA was calculated according to the following expression:\nNNEA= 𝛼min 𝑃𝑒𝑓𝑓 / √ENBW (2)\nwhere 𝑃 is the effective optical power delivered to the sensing region, 𝛼 denotes the 𝑒𝑓𝑓 min minimum detectable absorption coefficient, and ENBW is the equivalent noise bandwidth of the detection system. The ENBW was determined by the settings of the lock-in amplifier. In our experiments, a time constant of 1 s and a filter slope of 12 dB/oct resulted in an equivalent noise bandwidth of 0.25 Hz.”\nWe hope this detailed response and new supplementary material fully address the reviewer’s concern. Many thanks for the great improvement again.\nReviewer #2:\nThe authors have made significant revision in the manuscript and well answered most of my questions. Here I have only a few minor comments for authors' consideration and clarification.\n(1) I appreciate the authors added N₂ background signals in Fig. 2 for direct comparison, but there is significant non-zero signal in (b) and (c). Please explain and mention whether it is an issue for applications.\nWe thank the reviewer for this insightful comment. The baseline may arise from system-induced offsets, including the residual electronic background of the lock-in amplifier, imperfections in laser intensity modulation, and thermo-mechanical background excitation of the LN-MFP at resonance. However, this baseline offset does not affect the actual sensing performance or practical operation, as the measurements rely on differential 2f signal amplitudes relative to the baseline. As long as the baseline remains stable over time, even if it is non-zero, it can be effectively compensated through standard data processing and baseline-correction procedures.\n(2) The authors made detailed response to the previous question Q3 regarding comparison to LITES. I notice this part has been added to supplemental, but nothing is mentioned in the main text. Please inform readers this part is discussed in the supplemental information.\nFollowing this comment, the following sentence has been added to the main article Page 18 Line 385:\n“A comparison with state-of-the-art LITES is provided in the Supplementary Section 10.”\n(3) In the previous question Q5, the Table 3s compares pyroelectric detectors. Is it possible to include photovoltaic detector in the table as well, considering its wide use in spectroscopy.\nThanks for the constructive suggestion. The photovoltaic detectors were added to the revised Table 3s of the supplementary Page 15 Line 326:\nTable 3s | Photodetector using LN-MFP and pyroelectric/photovoltaic sensors.\nCharacterized Response\nNEP Responsivity\nDetector type Category wavelength time\n(W/√Hz) (V/W)\nrange\nLN-MFP Pyroelectric 450−9770 nm 5.65×10-8 1 s 373\nQTF [12] Pyroelectric 1540 nm / 1 s 47.4\nLiNbO [13] Pyroelectric / 1.623×10-7 1.9 s /\nSb Te −Bi Te [16] Pyroelectric 600−700 nm 8.0 × 10−9 341 μs 38\n2 3 2 3\nHgCdTe [17] Photovoltaic 4.8 μm 1.7×10-13 13 ns /\nGraphene [18] Photovoltaic 1550 nm 4.54×10-12 / 210\nLN-MFP: lithium niobate multi-functional platform; QTF: quartz tuning fork.\n[17] Wang, Y. et al. Fast uncooled mid-wavelength infrared photodetectors with heterostructures of van der Waals on epitaxial HgCdTe. Adv. Mater. 34, 2107772 (2022).\n[18] Molaei-Yeznabad, A. & Abedi, K. Optimal design of graphene-based plasmonic enhanced photodetector using PSO. Sci. Rep. 14, 15291 (2024).\nReviewer #3:\nThe authors have carefully revised the manuscript accroding to my comments. I am conent with the revisions and suggesting acceptance now.\nMany thanks for the great improvement."}]

Decision letter