Thin film lithium niobate on sapphire for integrated mid-infrared modulator

Domain: chemistry
Published: 2026-02-21
Editorial outcome: major_revision

Abstract
The mid-infrared spectrum, spanning from 3 to 14 µm, holds great promise for molecular spectroscopy and free space optical communication, benefiting from strong molecular absorption and reduced atmospheric attenuation. While progress in MIR photonics has accelerated due to improved sources and detectors, integrated low-loss, high-performance modulators remain limited. In order to address this gap, we demonstrate a broadband, high speed lithium niobate on sapphire Mach Zehnder electro optic modulator operating from 3.95 to 4.5 µm. The device shows a 3 dB bandwidth above 20 GHz, 17 dB extinction ratio, and V
π
L=22 V⋅cm, with optical output power at the half milliwatt level. We demonstrate 10 Gbit s−1 data transmission and a 70 GHz broad frequency comb, uniquely combining integration, low propagation loss, extinction ratio and high-speed operation.

Introduction
Mid-infrared (MIR) photonics requires the development of highly efficient platforms to fully exploit the unique advantages of this spectral region. The MIR spectrum, spanning 3–14μm, offers significant benefits for free-space optical applications1,2, including high atmospheric transparency, reduced scattering from micrometric aerosols, and enhanced resilience to atmospheric turbulence3–5. Furthermore, many environmentally relevant molecules exhibit strong absorption features in the MIR that are up to an order of magnitude stronger than in the near-infrared, enabling more precise and sensitive detection6,7. In this context, efficient electro-optic modulators play a crucial role in both domains. External modulators, in particular, enable high modulation depths and can handle significantly higher optical power than directly modulated sources, making them highly advantageous for achieving strong, high-quality modulated signals in the mid-infrared8. The MIR spectral range also offers several benefits for spectroscopy, for which precise control over the phase and amplitude of mid-infrared light is required. Amplitude modulation enables lock-in detection to improve sensitivity and suppress background signals9,10, and it can also be used to probe very fast phenomenon11 and delicate samples12. For example, Tamamitsu et al. use a 0.1ns (10GHz) MIR pulse to confine heat diffusion and enhance photothermal efficiency, although it is currently generated by a bulky OPO system. Phase modulation, on the other hand, enables wavelength modulation spectroscopy12 and the generation of frequency-comb light sources13, providing powerful tools for high-resolution and broadband spectroscopy. Achieving such performance in the MIR requires modulators that combine strong electro-optic efficiency, low propagation loss, and broadband operation, which remain challenging due to material and fabrication limitations in this wavelength range.

There have been many efforts towards amplitude modulation in the MIR, each with their own advantages and drawbacks. The most common demonstrations have been with direct modulation of quantum interband cascade lasers (QCLs/ICLs)14–16. QCLs operate most reliably in the long-wave infrared (3.8–14μm) range. While RF packaging requirements can introduce thermal constraints, QCLs can be directly modulated at high speeds. Promising optical 3dB bandwidths upto 3GHz have been demonstrated17,18, but the modulation depth remains relatively low (approximately 10dB) due to strong nonlinearities at high drive currents, along with the need for large current swings and high electrical power. ICL-based schemes have demonstrated gigahertz-scale performance19, benefiting from less demanding thermal management requirements, although they do suffer from limited output power. Moreover, direct modulation of QCLs/ICLs inherently causes simultaneous amplitude and phase modulation, limiting their use in some applications. Alternative approaches, such as external free-space Stark-effect modulators, have achieved comparable bandwidths20–22 and extinction ratios up to 3dB23. These devices are particularly attractive because they rely on a refractive index modulation mechanism rather than carrier absorption, enabling easier free-space coupling and potentially lower insertion losses. However, their scalability remains limited due to the high alignment sensitivity inherent to free-space configurations. In the integrated domain, III-V-based and group-IV carrier-injection or depletion modulators24 can operate at longer wavelengths (up to 8μm) but generally exhibit high optical losses. Despite their impressively low voltage-length product (V π L), these devices suffer from significant free-carrier absorption. Recent demonstrations include implementations on Ge-on-Si platforms24 and silicon-on-insulator structures25, which exhibit relatively high propagation losses due to lattice-mismatch defects between Ge and Si, and absorption in the silicon dioxide substrate, respectively. A newer technology relies on graded-index Ge-on-Si designs26, which minimize defects and thus reduce losses, but still suffer from shallow modulation depths despite being very promising in terms of wavelength accessibility. Hybrid Silicon/Lithium Niobate transfer-printed modulators27,28 have extended lithium niobate operation into the mid-infrared, demonstrating good performance near 3.8μm and showing promise for integrated spectroscopic sensing. However, fabrication complexity and limited electro-optic overlap still constrain their performance. Finally, lithium niobate on insulator (LNOI) modulators, despite their proven success in the near-infrared, are intrinsically limited to operation below approximately 3.8μm29 due to strong absorption in the buried silicon dioxide (SiO 2) layer30. On the other hand, phase modulation in the mid-infrared has so far been demonstrated in only a few platforms, including optical parametric oscillators (OPOs) based on periodically poled bulk lithium niobate31,32, Stark-effect devices using intersubband transitions33,34, and graphene-based structures35. While OPOs provide high optical power, they rely on bulky tabletop components and have very high power consumption. As already discussed, carrier depletion or injection modulators can achieve performance approaching a π-phase shift with lower (V π L) but suffer high optical losses24,25. Lithium niobate (LiNbO 3 or LN) presents a promising alternative for the 3–5μm wavelength range, offering low propagation loss, a wide transparency window (extending beyond 4.5μm), and a strong second-order non-linearity χ2, with a large Pockels coefficient up to 33pm/V at 1300nm36. This enables a change of the refractive index via an externally applied electric field. Conventional bulk LN waveguides are based on titanium in-diffusion and suffer from weak mode confinement and large bend radii, which limit both nonlinear efficiency and potential for scalable integration37. The advent of the LNOI platform has enabled high-confinement waveguides and compact, high-performance modulators in the C-band reaching bandwidths above 100GHz with V π L below 1V38,39. However, the LNOI material stack includes a buried SiO 2 layer, which absorbs strongly beyond 3.4μm30, hence limiting MIR operation. This limitation can be addressed by the lithium niobate on sapphire (LNOS) platform40, which replaces the SiO 2 substrate with sapphire, thereby extending the transparency window up to 4.5μm. LNOS preserves the electro-optic advantages of lithium niobate while enabling low-loss, high-confinement waveguides in the MIR. Moreover, sapphire offers several attractive properties, including strong acousto-optic effects, low radio frequency loss, and excellent thermal management41. We demonstrate an integrated amplitude modulator in the MIR, operating near a wavelength of 4μm. The modulators presented in this work are based on a traveling-wave Mach-Zehnder interferometer architecture implemented on the LNOS platform. Light is split by a multi-mode interferometer (MMI) into two parallel waveguide arms positioned within the gap of a ground-signal-ground coplanar microwave line, where tightly confined optical and radio-frequency fields propagate in the same direction. The differential electric field across the arms induces an antisymmetric phase shift via the Pockels effect, enabling amplitude modulation through interference at the output MMI junction. Our modulator exhibits a half-wave voltage-length product V π L of 22 V⋅cm, while achieving a 3dB bandwidth of higher than 20GHz with an extinction ratio of 17.1dB and an insertion loss of 14.1dB. To the best of our knowledge, these values of loss, speed, and extinction ratio have not previously been achieved in a single modulator operating in the MIR42. We also compare the performance of two device geometries based on different LN film thicknesses, namely 0.9μm and 1.5μm. The device demonstrates a strong modulation depth and excellent signal integrity at high frequencies, delivering an output optical power exceeding a usable half milliwatt. We demonstrated operation up to 4.5μm, although increasing propagation loss limits performance at longer wavelengths. In this work, we present high-quality fabrication on the LNOS platform, achieving low-loss MIR modulators. The process is particularly challenging due to the sapphire substrate’s transparency and insulating nature, which hinder charge dissipation and alignment, as well as the need for deep lithium niobate etching (>400nm) to ensure strong optical confinement at longer MIR wavelengths. These results represent an advancement in integrated mid-infrared photonics towards new capabilities in non-linear mid-infrared optics, coherent sensing, high-resolution spectroscopy, and free-space optical communication.

Results
Fabrication and design of the lithium niobate on sapphire platform
As a schematic of the device layout is shown in Fig. 1a. The chips were fabricated from commercially available 0.9μm- and 1.5µm-thin film x-cut LNOS wafers. Photonic waveguides were patterned using electron-beam lithography and etched via argon-based reactive ion etching. The fabricated waveguides feature a top width of 4μm with an etch depth of 400nm for the 0.9μm-thick film, and a top width of 2.5μm with an etch depth of 920nm for the 1.5μm-thick film. Notably, our fabrication process allows for deep etching up to 1μm. The achievable etch depth is limited by fabrication constraints, such as the thickness of the photoresist etch mask. In both cases, the sidewall angle is approximately 65°, resulting from the anisotropic physical etching process43. These designs ensure single-mode operation in transverse electrical (TE) polarization at a wavelength of 4μm, corresponding to the target operating range of the modulator as shown in Fig. 1b. Transverse magnetic modes are supported only for the 1.5-μm-thin film due to the thickness of the LN film. Moreover, with this thickness, the optical mode is significantly more confined within the lithium niobate, with a mode area of 6.2μm2 compared to 8.89μm2 for the 0.9μm film, where the mode also tends to leak into the sapphire substrate. The electro-optic electrodes were patterned using direct laser writing, followed by electron beam evaporation and lift-off of gold. The 900nm-thick electro-optic electrodes minimize high-frequency losses, enabling high-speed modulation. Thermo-optic electrodes were defined by electron-beam lithography, followed by another evaporation and gold lift-off. They were designed with a thickness of 100nm to ensure efficient Joule heating while maintaining structural integrity and preventing electrode burnout. The facet of the device was diced, and the waveguide facets were mechanically polished to minimize in- and out-coupling losses, followed by focused ion beam (FIB) milling to further refine the facet quality, as shown in Fig. 1c. A thermo-optic phase shifter (TOPS) is included on one arm of the MZM as seen in Fig. 1d, in order to set the relative phase between the two arms and ensure that the device operates at the quadrature point. The TOPS voltage set point corresponds to the condition where the optical output power of the interferometer is half of its maximum value, ensuring operation at the quadrature point for optimal electro-optic modulation efficiency. This condition was also verified by observing the modulator output on an oscilloscope and confirming symmetric modulation behavior with respect to the applied voltage.

MZM static performance
Figure 2 illustrates the experimental setup used for modulator characterization. The platform was tested at a wavelength of 4μm using a Fabry-Pérot QCL, configured in a custom-built external cavity to ensure single-mode emission with a tunability of approximately 150 nm and an output power of around 20mW. Details on the external cavity can be found in the Appendix B. Two DFB QCLs operating at 4.3μm and 4.5μm were additionally used to broaden the study of the device performance. A half-wave plate placed after the QCL rotates the polarization to enable efficient coupling into the fundamental TE mode of the waveguide. The beam is coupled into a multimode mid-infrared-compatible InF 3 lensed fiber and then focused onto the waveguide facet, coupling into the fundamental TE mode of the waveguide. At the output, light is collected by a second InF 3 lensed fiber and either directly injected into an optical spectrum analyzer (OSA), power meter, or collimated and focused onto a mid-infrared photodetector using a fiber collimator and a high-numerical-aperture germanium lens.

To evaluate the modulator’s performance, several tests were performed. The optical path difference between the two arms is thermally tunable via the integrated TOPS, which was characterised by varying the applied electrical power and monitoring the corresponding change in the MZM output power. A half-wave power (P π) of 1.42W was extracted, as shown in Fig. 3a. The high power requirement stems from the combined effects of a smaller phase shift per unit temperature change at MIR wavelengths, strong thermal dissipation through the sapphire substrate, and the need to keep the thermal electrode sufficiently separated from the waveguide to prevent plasmonic loss of the guided light. To extract the half-wave voltage (V π), the MZM was driven with a 100 kHz triangular waveform from an arbitrary function generator and amplified using a high-voltage amplifier. The resulting optical signal was detected by a mercury cadmium telluride (MCT) detector and recorded on an oscilloscope. The measured response is presented in Fig. 3b. The presented modulator exhibits a V π L of 22.4 V⋅cm, with an electrode length of 0.8 cm and a gap of 10.5μm, operating at a wavelength of 4μm. The measured extinction ratio reaches 17.1dB, highlighting the fabrication quality of the device. The modulated output power of the device reaches 350μW, enabling straightforward characterization with a power meter, without the need for a mechanical chopper.

As shown in Fig. 3c, simulations were performed to minimize optical propagation loss while maintaining efficient modulation performance, characterized by the half-wave voltage-length product (V π L), for both thin-film thicknesses. The simulated optical mode loss (see top panel of Fig. 3c) increases significantly when the electrode spacing (G) is reduced below 12μm and 8μm for the 0.9μm and 1.5μm films, respectively, due to plasmonic absorption. The thicker 1.5μm film provides improved optical confinement within the waveguide, whereas optical confinement in the 0.9μm film is reduced, leading to a larger mode overlap with the sapphire substrate, which does not contribute to the modulation process. This improved confinement allows the electrodes to be placed closer to the waveguide without incurring high optical loss. Specifically, gaps of 13.2μm and 15.6μm were used for the 0.9μm thin film, while gaps between 10 and 12μm were selected for the 1.5μm thin film. Bottom panel of Fig. 3c presents both simulated and experimental results for the achieved V π L. Simulations show good agreement with the experimental results for both the 0.9μm and the 1.5μm devices. With the 1.5μm film, a minimum of 18.4 Vcm was obtained for the smallest-gap modulator, while the 0.9μm film yielded 31.4 V⋅cm. The modulator with the lowest V π exhibited moderate output power of approximately 150μW due to increased plasmonic loss caused by a slight electrode misalignment during fabrication. This justified the choice to present results from the modulator with the highest optical output power instead. As shown by comparison with the simulations, conservative design choices were made to ensure low optical loss, tolerance to fabrication variations, and a clear demonstration of the viability of the proposed device architecture, especially given the moderate efficiency of typical mid-infrared detectors.

As shown on the top panel of Fig. 3d, we demonstrate the operational bandwidth of the MZM from 3.95μm to 4.5μm by evaluating the V π L as a function of wavelength. This corresponds to an impressive 500nm bandwidth. The bandwidth could potentially extend beyond this wavelength range; however, this region could not be characterized due to the lack of available monomode optical sources at those wavelengths in our laboratory. The slight increase in the value of V π L with wavelength is due to reduced confinement of the optical mode and the lower induced phase shift at longer wavelengths. As shown in the bottom panel of Fig. 3d, the waveguide loss was evaluated at different wavelengths using mid-infrared grating couplers. Additional information on the grating couplers can be found in the Appendix C. The loss is measured to be 1dB/cm at a wavelength of 3.7μm, and progressively increases to 5.3dB/cm at 4.5μm. The loss at the designed operating wavelength of 4μm was measured to be roughly 1.1dB/cm. The losses are hypothesized to be dominated by sidewall roughness44,45 and could be further reduced through improved fabrication techniques, as theoretical studies suggest that losses approaching 0.1dB/cm may be achievable40. Moreover, adopting a multimode waveguide geometry could further reduce these losses, since the optical mode becomes less sensitive to scattering at the waveguide boundaries, although this would reduce the efficiency of the presented modulator46. Finally, we measured a total fiber-to-fiber insertion loss of 14.1dB at 4μm, 17.5dB at 4.3μm, and around 24.0dB at 4.5μm, which includes coupling losses, waveguide propagation losses, and additional plasmonic losses due to the electrodes. The lower loss at 4μm results from the waveguide being optimized at this wavelength in terms of top width and mode confinement. As the wavelength increases beyond the design point, the mode becomes less confined within the waveguide, leading to higher insertion loss and higher propagation loss. Using the intercept from the cutback measurement with facet coupling, we can remove the coupling contributions and evaluate the intrinsic device loss, including both waveguide and plasmonic electrode losses, to be at most 4dB at 4μm and 5dB at 4.3μm. The insertion loss from facet coupling has not been evaluated at 4.5μm, but is expected to be higher. It is important to note that the present design is optimized for operation at a wavelength of 4μm. The performance at other wavelengths, particularly in terms of insertion and propagation losses, can be improved by optimizing the device geometry for each specific wavelength.

MZM high speed performance
The high-speed electro-optic performance of the modulators was characterized using S 21, which represents the small-signal transfer function from the electrical input to the electrical output of a photodetector monitoring the modulated optical signal. Several devices with varying film thickness and electrode lengths of L=0.4 cm, L=0.5cm, L=0.66cm, and L=0.8cm, and gaps were evaluated. The travelling wave electrodes were externally terminated with a 50 Ω RF load to avoid reflection at the end of the electrode. Additional details on the RF design are provided in the Methods and Appendix E. Electro-optic bandwidth measurements were carried out using a Vector Network Analyzer (VNA) in combination with an InAs/InAsSb type-II superlattice (T2SL) photodetector, which exhibits a bandwidth exceeding 20GHz. The detector demonstrates a high responsivity of over 0.8 A/W at 4μm when biased above 2V, as verified through FTIR measurements. Detailed characterization of the detector can be found in the Appendix C. The VNA output was amplified using a 30dB RF high-power amplifier with a saturation power of approximately 20dBm. The 0.9μm platform exhibited a maximum bandwidth of up to 16GHz, with a small dependence on electrode length, as shown in Fig. 4a. The observed increase in noise around 12.5GHz was attributed to the onset of antenna-like behaviour, whereby the detector picks up radiation from standing waves present in the RF cables and probes. The restricted bandwidth was attributed to a 50 Ω mismatch for the 0.9μm configuration. The 1.5μm platform was subsequently engineered with optimized signal and ground pad geometries to enhance radio-frequency performance. As shown in Fig. 4b, this design yields a nearly flat frequency response up to 20GHz, which is limited by both the VNA and the detector’s intrinsic 3dB bandwidth. The presented response has been normalized by the detector and amplifier chain, ensuring that the extracted 3dB bandwidth reflects the modulator’s intrinsic performance. No roll-off was observed up to 20GHz, suggesting that the actual 3dB bandwidth of the modulator likely exceeds this value. Further characterization at higher frequencies was limited by the absence of a higher bandwidth VNA source and the cutoff frequency of the detector.

To validate the applicability of our modulator, we conducted a free-space transmission experiment over half a meter using a two-level 10 Gbit/s pseudo-random bit sequence of length 215, generated by an arbitrary waveform generator (AWG) operating at a sampling rate of 20 GS/s. To evaluate the transmission quality, we present an eye diagram along with the corresponding bit error rate as shown in the Fig. 4c. The total transmitted bit sequence comprised approximately 106 bits. The primary limitation in achieving higher bit rates was the AWG sampling rate of 20 GS/s. Additionally, the implementation of higher-order modulation formats, such as 4-level or 8-level schemes, was constrained by the signal-to-noise ratio of our system.

Using our modulator, we successfully generated an optical frequency comb centered around the carrier frequency for three different modulation frequencies, as measured by the OSA with a resolution of 1.9GHz. As shown in Fig. 4d, the comb is centered at a wavelength of approximately 4μm, with symmetric sidebands clearly resolved, confirming the expected modulation behaviour. At 7GHz, based on the relative powers of the carrier and higher-order sidebands, we observed a maximum attenuation of 6.4dB per sideband peak, leading to a spectral bandwidth of around 70GHz, which reflects efficient energy transfer from the central mode into the sidebands. The energy transfer, highlighted by the slope of the comb, decreases with increasing frequency at 10GHz and 12GHz, which we attribute to the combined effects of a higher effective V π L at elevated frequencies and reduced electrical power delivery to the electrodes due to impedance mismatch. The measured spectrum is not limited by the OSA noise floor, which is around −70dBm. In fact, a broader comb could be obtained using a laser source with lower intrinsic amplitude noise. The relatively high noise level, around -50dBm in our measurements, arises from the use of the EC-QCL, which typically exhibit higher amplitude and frequency noise due to mechanical tuning elements and longer cavity lengths. For practical spectroscopic applications, a low-noise source such as a 4μm DFB QCL or an interband cascade laser would significantly enhance the signal-to-noise ratio and allow observation of broader comb spectra, with noise down to −60dBm and bandwidths exceeding 100GHz. The ability to tune the spacing between comb lines is a powerful tool for scanning absorption bands. These results demonstrate the modulator’s capability to generate rich frequency content through deep amplitude modulation, confirming its potential as a building block for mid-infrared frequency-comb generation and broadband spectroscopic systems.

Methods
Device fabrication
The chip was fabricated from a commercially available wafer with an x-cut (NGK Insulator, LTD), 900 or 1500-μm LN layer, and a sapphire substrate. Photonic waveguides were defined using a polymer resist with 100 kV electron beam lithography (Raith EBPG 5200+). They are then dry etched to a depth of 1μm using Ar+ plasma reactive ion etching (Oxford Instruments PlasmaPro 100). The remaining etch mask and the redeposited material after the etching process were removed using RCA-SCA1 cleaning and buffered oxide etch. Beyond removing redeposition, the RCA-SC1 also reduces sidewall roughness as it slightly etches the LN waveguide itself. After this, the chip is annealed at 500 ∘C under ambient conditions for 2 hours to heal any damage to the crystal caused by the dry etching process and decrease propagation losses. Thermo-optic phase shifters were then patterned by electron beam lithography to define a resist bilayer, followed by a 100 nm gold evaporation (Evatec 50) and a lift-off process. The electro-optic electrodes are then defined by direct laser writing (DWL 66+), followed by a 900 nm gold evaporation and another lift-off process. Finally, the chip was diced (Disco DAD 3221) to expose the waveguide facets, which were then mechanically polished (Allied Multiprep System 8") using diamond abrasive to minimize in- and out-coupling losses. To further improve facet quality, FIB milling (TFS Helios 5 UX) was performed as a final step.

Design of the MZM
The performance of passive components, including waveguides and MMIs, was estimated using an eigenmode expansion solver, Ansys Optics. Low-loss and single-mode operation was ensured through careful selection of waveguide width and etch depth. For the 0.9μm film, the chosen geometry consists of a waveguide top width of w ridge = 4μm and an etch depth of h ridge = 0.4μm, while for the 1.5μm film, the geometry is defined by w ridge = 2.5μm and h ridge = 0.92μm. The design was chosen to ensure single-mode optical confinement in the fundamental transverse electric mode. Electro-optic simulations using using the software COMSOL Multiphysics and Ansys Lumerical were used to evaluate modulation efficiency and optical field confinement, and optimize electro-optic and thermo-optic performance. Here, a compromise exists when choosing the gap between the waveguides and the electrodes. Reducing the gap increases the overlap between the optical and RF modes (resulting in a lower V π L). However, decreasing the gap beyond a certain point drastically increases the plasmonic losses, as shown in Fig. 3c (top and bottom panels). Building on the waveguide design optimized for single-mode operation and defined gap spacing, the widths and spacings of the transmission lines were optimized through RF-optical simulations to ensure 50 Ω impedance matching and velocity matching with the optical mode using COMSOL Multiphysics, thereby reducing reflections and enhancing the modulation bandwidth. Additional details on the electrode design are provided in Appendix E.

External cavity quantum cascade laser
To enable precise wavelength tuning around 4μm, an external cavity configuration in the Littrow geometry was implemented. The cavity consists of a QCL coupled to a ruled reflection grating (GR2550-30035) with a dispersion of 2.86 nm/mrad, mounted on a motorized precision rotation stage (PIRS-40). The first-order diffraction from the grating is fed back into the QCL to define the lasing wavelength, while a 50:50 beamsplitter positioned between the laser and the grating is used to extract the output power. Simultaneously, the zeroth-order reflection is used to monitor the injected signal spectrum in real time. The external cavity operates in continuous-wave mode with a tunable bandwidth of approximately 150 nm and an output power exceeding 20mW, enabling fine control of the emission wavelength via adjustment of the grating angle. The output of the external cavity is then coupled into an InF 3 lensed fiber (Le Verre Fluoré). A schematic of the external cavity and basic characterization can be found in Appendix B. For performance evaluation at 4.0μm and 4.3μm, distributed-feedback QCLs with maximum output powers of approximately 40mW and 60mW, respectively, were used.

Static characterization
Static characterization was performed using an arbitrary waveform generator (AFG-2125) in combination with a high-voltage amplifier (Thorlabs HVA200) to generate high-power triangular waveforms. The setup is capable of delivering a maximum electrical voltage of 200V. The optical output from the external cavity was injected into the system using the same experimental configuration described previously. To characterize the half-wave voltage V π and the extinction ratio, an MCT detector (Vigo PVM-2TE-10.6) was used to simultaneously recover the DC and AC electrical signal from the detected modulated optical signal. The signal was recorded using an oscilloscope (TBS 2000B) and subsequently processed with a custom Python script.

InAs/InAsSb type-II superlattice photodetector
InAs/InAsSb type-II superlattice photodetector were fabricated by the ShanghaiTech Material and Device Lab (SMDL). It was grown by molecular beam epitaxy (MBE) on an n-type GaSb substrate. The layer stack consisted of a 200 nm GaSb buffer, a 300 nm InAs/AlAsSb (1.5/1.5 nm) superlattice bottom contact, a 50 nm n-type InAsSb layer, and a 420 nm unintentionally doped InAsSb drift region. The absorption region comprised a 900 nm InAs/InAsSb (2.9/1 nm) type-II superlattice with a four-step graded doping profile. An AlAsSb/AlSb (1.22/0.61 nm) superlattice served as an electron barrier, capped by a 10 nm InAsSb top contact layer. Device mesas were defined using standard photolithography and inductively coupled plasma dry etching. Ohmic contacts were formed by Ti/Pt/Au deposition. The device surface was then passivated with SU-8, and coplanar waveguide pads were fabricated via air-bridging to provide a 50 Ω impedance suitable for high-frequency measurements. At room temperature and under a −1V bias, the device demonstrated a responsivity of approximately 0.6 A/W at 4.5μm. The cut-off wavelength was found to be around 5.5μm at room temperature. Temperature-dependent dark current measurements showed that the current density increased from 0.02 A/cm2 at 77 K to 3.94 A/cm2 at 300 K under a −1V bias. Arrhenius analysis indicated that the dark current is dominated by diffusion processes at high temperatures, with tunneling mechanisms becoming significant below 130 K. For high-speed characterization of the modulator, devices with a diameter of 20μm were used. To facilitate integration and simplify the experimental setup, the diced device was wire-bonded to a printed circuit board equipped with a standard SMA port, allowing for direct connection to measurement equipment. Both the on-chip and SMA-packaged photodetectors were evaluated using a Lightwave Component Analyzer system, which consisted of a 67 GHz VNA and a 1550 nm laser. The 3dB bandwidth of the unpackaged (on-chip) device is provided in references56,57, while the packaged device exhibited a 3-dB bandwidth of 20.3 GHz at a −5V bias.

High speed characterization
High-speed characterization was performed using a vector network analyzer (Keysight P5004A) with a maximum bandwidth of 20GHz. The output port of the VNA was connected to a modulator driver amplifier (AT Microwaves AT-BBLF-0020-3022B) with a bandwidth of approximately 20GHz and a saturation output power of around 20dBm. The optical coupling setup remained the same as in the static characterization. The output light is send through a germanium fiber collimator, then focused with a germanium lens with a high numerical aperture (NA=0.83) and a focal length of 1.873 mm onto the 20μm2 active area of the detector. The electrical output of the detector was then amplified using a 35dB low-noise amplifier (AT Microwaves AT-LNA-0043-3504Y) with a 3 dB bandwidth of 43.5 GHz, which was subsequently connected to the input port of the VNA. Both amplifiers and the detector were characterized independently, and their frequency responses were used to normalize the measured S 21 system response, allowing recovery of the intrinsic bandwidth of the modulator. For the transmission experiment, a 20 GS/s arbitrary waveform generator (ARBRider AWG-7204) was used in conjunction with a 30 GHz oscilloscope (Tektronix DPO77002SX). For frequency comb generation, the signal from a signal generator (Rohde & Schwarz SGS100A) was amplified using a high-power amplifier (Fairview Microwave FMAM513), and the output was measured with a Thorlabs OSA305 operating at a resolution of 1.9GHz.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors report a high-performance MIR MZM based on an LNOS platform. For this work to be considered for publication in a high-impact journal like Nature Communications, the following issues require careful attention and substantial revision.\n1.First, could the authors clarify the specific requirements for high-speed electro-optic modulators in application scenarios such as molecular spectroscopy detection, free-space optical communication, and MIR spectroscopy? For these applications, is high modulation efficiency more practical than high modulation speed?\n2.The authors should provide a more balanced and comprehensive evaluation of prior art. While it is reasonable to highlight the limitations of previous works to contextualize their own advances, a fair discussion should also acknowledge the pioneering role and significant contributions of those earlier studies to the field.\n3.The use of \"half-wave voltage of 22 V·cm\" in the abstract is incorrect, as the unit \"V·cm\" characterizes modulation efficiency, not the half-wave voltage itself. This same conceptual confusion appears on page 3, where \"Vπ L below 1 V\" is stated. The product Vπ·L is a metric for efficiency, and its value cannot be directly compared to a voltage (V). These should be corrected to avoid misunderstanding.\n4.The abstract states: \"Furthermore, we demonstrate full π-phase modulation in the MIR, representing a key milestone for integrated MIR photonics.\" Figure 3b shows that a π-phase shift is achieved within a bias voltage range of -14 V to 14 V. However, in Reference 19, a 0.75 π phase shift is achieved with a bias range of 0 V to 3.5 V, and in Reference 20, a π-phase shift is achieved with a bias range of 0 V to 1.2 V. Should the claim of achieving \"full π-phase modulation in the MIR as a key milestone\" be qualified with more specific context or limitations?\n5.The captions for Fig. 4c and 4b appear to be swapped.\n6.Equation 1 in the Discussion section is overly fundamental. Is it necessary to include it separately in this context?\n7.The caption for Fig. B2, \"Fabrication process of TFLN on sapphire,\" does not match the content of the figure.\n8.Could the authors attempt to subtract the S21 curve of the detector from the current S21 curve to further quantify the modulator's bandwidth?\n9.To significantly enhance the impact of this work, it is highly recommended that the authors include a demonstration of their high-speed MIR MZM in a practical application. Given the potential applications mentioned in the text, implementing a proof-of-concept experiment in areas such as MIR spectroscopy, gas sensing, or free-space data transmission would compellingly validate the device's utility and performance.", "reviewer_2": "Reviewer Report\nOriginal and Timely: 3 – Probably\nLasting Impact: No\nConvincing and Rigorous Data: Yes, but requires some revision\nInterest to the Community: Very Likely\nSufficient Information: No\nSatisfactory English: Yes\nAppropriate Title: Yes\nGood Abstract: Yes\nClear Figures: Yes\nAdequate References: No\nThis manuscript presents an integrated electro-optic modulator on a TFLN-on-sapphire (LNOS) platform, demonstrating low loss and high bandwidth operation in the mid-infrared (MIR) regime. Since MIR photonics remains a challenging field due to the lack of mature components such as lasers, detectors, and fibers, this work is technically well-executed and of potential interest to the community.\nHowever, similar integrated electro-optic modulators have already been reported. Therefore, to meet the publication standards of Nature Communications, the manuscript would benefit from additional experimental data and further discussion to clearly establish novelty and impact.\nBelow are detailed comments:\n1. Title clarity:\nSince a mid-IR modulator based on TFLN has already been demonstrated in the following paper, the title should be more specific, explicitly indicating that the work is based on the LNOS platform:\nHyeon Hwang, Kiyoung Ko, et al., “A wide-spectrum mid-infrared electro-optic intensity modulator employing a two-point coupled lithium niobate racetrack resonator,” APL Photonics 10, 016116 (2025).\n2. Overstated claim:\nThe sentence on page 4 —\n“To our knowledge, this represents the first photonic integrated MZM demonstrated in the MIR, surpassing previous demonstrated modulators in terms of speed and extinction ratio by nearly two orders of magnitude.”\n— should be removed, as it is not accurate in light of prior demonstrations.\n3. Insufficient references for MIR relevance:\nThe following statement is correct but lacks supporting citations:\n“In this context, phase and amplitude modulation are essential techniques for MIR spectroscopy – improving sensitivity, resolution, and signal-to-noise ratio.”\nPlease include appropriate references that emphasize the importance of MIR modulation and its applications.\n4. Spectral limitation and platform comparison:\nThe experiments were conducted only between 3.9–4.1 µm, covering a narrow range with limited molecular fingerprint information. To better demonstrate the potential broadband applicability of the platform, I suggest adding theoretical loss calculations for both LNOI and LNOS platforms across the 3–5 µm range (e.g., in Fig. 1). Additionally, including Q-factor measurements from ring resonators would help substantiate the claim of achieving a low-loss MIR platform.\n5. Inconsistency in extinction ratio data:\nIn Fig. 3b, the extinction ratio (ER) is labeled as 34.1 dB, but the data suggest a minimum transmission of ~0.1 (corresponding to ~10 dB ER). The same issue appears in Fig. E5. Please verify and clarify this discrepancy. It may also be more appropriate to plot the modulation data on a logarithmic (dB) scale to better visualize the claimed high ER performance.\n6. Missing references for wavelength-dependent modulation:\nThe following statement on page 9 should be supported by experimental references:\n“This behaviour arises from a fundamental limitation — the Pockels effect induces a change in refractive index Δn, but the resulting phase shift decreases at longer wavelengths, as described by:”\nI recommend citing the following works:\no Hixin Xue et al., “Full-spectrum visible electro-optic modulator,” Optica 10, 125–126 (2023).\no Hyeon Hwang, Kiyoung Ko, et al., “A wide-spectrum mid-infrared electro-optic intensity modulator employing a two-point coupled lithium niobate racetrack resonator,” APL Photonics 10, 016116 (2025).\n7. Discussion of the main limitation (high Vπ):\nAs discussed in the manuscript, the major limitation of the current platform is its high half-wave voltage (Vπ). It would be helpful to include additional information in the Supplementary Material regarding possible improvement limits through electrode design optimization. From a practical standpoint, it appears that the achievable enhancement may already be constrained by the inherent characteristics of MIR wavelengths.\nOverall evaluation:\nThis is a well-executed and potentially valuable study demonstrating MIR modulation on an LNOS platform. However, in its current form, the manuscript lacks sufficient distinction from prior work and requires additional data, clarifications, and references to convincingly justify publication in Nature Communications.", "reviewer_3": "In the manuscript \"Integrated thin film lithium niobate mid-infrared modulator\", the authors report a Mach-Zehnder electro-optic modulator (MZM) on lithium niobate on sapphire (LNOS) nanophotonic platform, using which they demonstrate phase and amplitude modulation in the mid-infrared (MIR), around 4-microns wavelength. They further demonstrate data-transmission and generation of an electro-optic (EO) comb in this spectral region.\nUpon reading this manuscript, I have the following questions and feedback for the authors:\n• Would it be possible for the authors to discuss in more detail the design considerations of their device, perhaps in section 2.1, or in a separate section in the appendix? I believe it would be helpful for the reader to understand how key figures of merit, such as the VπL and EO bandwidth, are dependent on the various design parameters, why particular waveguide dimensions and electrode dimensions were chosen, and if special attention was paid to group velocity matching and impedance matching. While some of these design considerations and simulations are interspersed with results, I think it’d be beneficial to have most of them collected in one place to provide a full picture of designing an MZM in the MIR, and to understand if any design optima may differ from those that are well-established in the near infrared.\n• The authors use a MIR-compatible multimode lensed fiber to couple light into the input waveguide. How do they ensure that they only have fundamental mode excitation in the waveguide?\n• Fig 3d has only four wavelengths characterized between 3.95 and 4.3 microns. Based on the QCL spectrum in appendix B, would it be possible to have measurements at more wavelengths in this range? It would be good to understand the apparent dispersive trends and ascertain if they arise from experimental noise or if they correspond to oscillations in the device efficiency.\n• The generation of MIR EO comb through the MZM is certainly of great interest due to its potential applications, however this portion of the manuscript appears to be lacking in some details, such as the driving voltage and observed bandwidth, as well as in discussions about ways to improve the bandwidth of the comb. Also, can multiple devices (varying waveguide thickness, electrode length, etc.) be tested for EO comb generation and the results compared?\n• From the transcript: “While the simulation shows good agreement with the experimental results for the 1.5 μm devices, a slight discrepancy is observed for the 0.9 μm case, likely due to a non-uniform gap length along the electrode.”; why would the gap between lithographically defined electrodes be nonuniform? Could it be due to improper metal liftoff? An optical microscope observation might provide some clarification.\n• Based on the propagation loss estimates from the cutback measurements, can the authors ascertain the excess loss through the MZM and if / how it depends on the waveguide and electrode designs?\nSome minor comments:\n• It appears from Fig. 1b that the waveguide width is 4 microns, while I believe the intention there is to show the wavelength?\n• The layout of the electro-optic electrode is not clear to me form figure 1d; perhaps the authors could change the color / contrast of the image to make the photonics components more visible in this image?\n• Typo in figure 3b caption, Vπ of 28 V (not Vcm)?\n• Figure 3c caption needs some clarification. I would also recommend that the authors plot the simulations as solid / dashed lines and reserve cross / dot for experimental data points.\n• Captions for Fig.4c&d appear to be interchanged.\n• Fig. B2 caption incorrect.\n• Please clarify the TOPS set point for the EO measurements.\nOverall, my impression is that while the authors have adequately demonstrated MZM in the LNOS platform in the MIR, if they could expand upon their demonstration and / or analysis of MIR EO-comb generation in this platform, it would substantially increase the contribution of their work and make it more suitable for publication in Nature Communications."}, "author_response": "Response to the Reviewers’ comments on the manuscript titled\n“Integrated thin film lithium niobate mid-infrared modulator”\nsubmitted to Nature Communications\nThank you very much for the opportunity to revise our manuscript, “Integrated thin\nfilm lithium niobate mid-infrared modulator.”\nWe would like to express our sincere gratitude to the Editor and Reviewers for their\nthorough evaluation of our manuscript. Below, we provide a detailed, point-by-point\nresponse to all the valuable suggestions and questions raised by the Reviewers. Along\nwith this resubmission, we have included a revised version of the manuscript that\nincorporates these constructive comments. We believe that the revisions and\nclarifications have substantially improved the quality of our work, bringing it to the high\nstandards of Nature Communications.\nIn particular, we have:\n• Corrected the points on the reference highlighted by the reviewers and improved\nthe state-of-the-art discussion to better contextualize our work. (Part Main)\n• Extended the experimental modulation measurements up to 4.5 µm, supported\nby new simulations illustrating the tunability of our device across the mid-\ninfrared range. (Part 2.2 – Fig 3.d)\n• Employed a mid-infrared grating coupler to precisely measure the waveguide\npropagation loss, demonstrating state-of-the-art loss of around 1.3 dB/cm at 4\nµm and reasonable loss at 4.5 µm (5.2 dB/cm). Included an experimental\nevaluation of the additional loss introduced by the plasmonic losses caused by\nthe electrodes. (Part 2.2 – Fig 3.c-d / Fig C3.b-d)\n• Extended the experimental demonstration of comb generation by providing\nadditional details and new measurements that expand the accessible frequency\nspacing. (Part 2.3 – Fig 4.d)\n• Included simulations and S11 measurements to validate the RF design\nmethodology of the devices. (Part Appendix E – Fig E5)\nWe also acknowledge the reviewers’ request for a more advanced demonstration of\nfrequency comb generation. This is indeed an exciting direction we are actively\ndeveloping in our laboratory. However, achieving a high-performance comb source\nrequires a different device design and characterization framework than the Mach–\nZehnder modulator presented in this work and goes beyond the scope of this work. We\ntherefore view the present manuscript as a foundational demonstration of a low-loss\nmid-infrared platform, showcasing its efficient Pockels effect and evaluating its low\nlosses in the MIR and related challenges. Demonstrating the feasibility and reliability of\na fabrication platform that extends lithium niobate, a leading material in integrated\nphotonics, into the mid-infrared for nonlinear optical applications (SPDC, DFG\nprocess…).\nWe believe the revised manuscript now substantially strengthens the work and clarifies\nits significance as a platform demonstration, while laying the groundwork for future\ndevelopments such as comb generation.\nReviewer #1 (Remarks to the Author):\nThe authors report a high-performance MIR MZM based on an LNOS platform. For this\nwork to be considered for publication in a high-impact journal like Nature\nCommunications, the following issues require careful attention and substantial\nrevision.\n1.First, could the authors clarify the specific requirements for high-speed electro-optic\nmodulators in application scenarios such as molecular spectroscopy detection, free-\nspace optical communication, and MIR spectroscopy? For these applications, is high\nmodulation efficiency more practical than high modulation speed?\nResponse: The specific requirements for mid-infrared electro-optic modulators indeed\ndepend strongly on the targeted application.\nFor free-space optical communication, high-speed modulation is essential to achieve\nhigh data rates while maintaining sufficient optical output power and extinction ratio to\nensure signal integrity and link robustness (Trichili et al., J. Opt. Soc. Am. B 37, A184–\nA201, 2020). In this context, the high-speed capability and low optical loss\ndemonstrated by our device are directly relevant, as they enable gigabit-per-second MIR\ndata transmission, as shown in our free-space link experiment.\nIn mid-infrared spectroscopy, many environmentally relevant molecules exhibit strong\nabsorption features in the MIR, up to an order of magnitude stronger than in the near-\ninfrared, enabling more precise and sensitive detection (Haas et al., Annu. Rev. Anal.\nChem. 9, 45–68, 2016). A system with a high extinction ratio allows clear discrimination\nbetween the measured signal and the noise floor under lock-in detection. Furthermore,\nhigh-speed modulation enables the observation of fast transient phenomena (Schultz\net al., Spectrochim. Acta A 188, 666–674, 2018) and helps protect samples from\ndamage while mitigating thermal effects (Tamamitsu et al., Nat. Photonics 18, 738–743,\n2024).\nPhase modulation, on the other hand, enables wavelength modulation spectroscopy\n(Schilt et al., Appl. Opt. 42, 6728–6738, 2003) and the generation of frequency combs,\nwhich are powerful tools for enhancing both the speed and sensitivity of spectroscopic\nmeasurements (Picqué & Hänsch, Nat. Photonics 13, 146–157, 2019). In this context,\nmodulation efficiency and speed are crucial, as they determine the achievable\nconversion efficiency into comb lines, the line spacing, and consequently the comb\nbandwidth.\nThus, the present demonstration of a high-speed, low-loss MIR modulator establishes\nthe core capability of the lithium niobate on silicon (LNOS) platform. It provides a\nfoundation upon which optimized designs can be tailored toward either high-speed\ncommunication or highly efficient spectroscopic applications, depending on the\ntargeted use case.\nThese points have been clarified in the revised manuscript to better highlight the\nimportance of a highly efficient, high-speed modulator for mid-infrared\napplications.\nChange in the manuscript:\n- “Furthermore, many environmentally relevant molecules exhibit strong\nabsorption features in the MIR that are up to an order of magnitude stronger than\nin the near-infrared, enabling more precise and sensitive detection [6, 7]” Part\nMain\n- “In this context, efficient electro-optic modulators play a crucial role in both\ndomains. External modulators, in particular, enable high modulation depths and\ncan handle significantly higher optical power than directly modulated sources,\nmaking them highly advantageous for achieving strong, high-quality modulated\nsignals in the mid-infrared.[8] The MIR spectral range also offers several benefits\nfor spectroscopy, whereby precise control over the phase and amplitude of mid-\ninfrared light is required. Amplitude modulation enables lock-in detection to\nimprove sensitivity and suppress background signals [9, 10], and it can also be\nused to probe very fast phenomenon [11] and delicate samples [12]. For\nexample, Tamamitsu et al. use a 0.1 ns (10 GHz) MIR pulse to confine heat\ndiffusion and enhance photothermal efficiency, although it is currently generated\nby a bulky OPO system. Phase modulation, on the other hand, enables\nwavelength modulation spectroscopy [12] and the generation of frequency-comb\nlight sources[13], providing powerful tools for high-resolution and broadband\nspectroscopy” – Part Main\n2.The authors should provide a more balanced and comprehensive evaluation of prior\nart. While it is reasonable to highlight the limitations of previous works to contextualize\ntheir own advances, a fair discussion should also acknowledge the pioneering role and\nsignificant contributions of those earlier studies to the field.\nResponse: We agree with your observation and have implemented a more balanced\ndiscussion that acknowledges the pioneering work in mid-infrared modulator\ntechnology.\n3.The use of \"half-wave voltage of 22 V·cm\" in the abstract is incorrect, as the unit\n\"V·cm\" characterizes modulation efficiency, not the half-wave voltage itself. This same\nconceptual confusion appears on page 3, where \"Vπ L below 1 V\" is stated. The product\nVπ·L is a metric for efficiency, and its value cannot be directly compared to a voltage (V).\nThese should be corrected to avoid misunderstanding.\nResponse: We agree that Vπ·L represents the half-wave voltage–length product, which\nis a metric of modulation efficiency rather than the half-wave voltage itself. In our\nmanuscript, we intended to refer to the half-wave voltage–length product rather than\nonly the half-wave voltage. The text has been revised accordingly to avoid confusion.\n4.The abstract states: \"Furthermore, we demonstrate full π-phase modulation in the\nMIR, representing a key milestone for integrated MIR photonics.\" Figure 3b shows that a\nπ-phase shift is achieved within a bias voltage range of -14 V to 14 V. However, in\nReference 19, a 0.75 π phase shift is achieved with a bias range of 0 V to 3.5 V, and in\nReference 20, a π-phase shift is achieved with a bias range of 0 V to 1.2 V. Should the\nclaim of achieving \"full π-phase modulation in the MIR as a key milestone\" be qualified\nwith more specific context or limitations?\nResponse: We agree that the statement in the abstract should be better qualified to\nclarify the specific context and significance of our result. In the revised manuscript, we\nhave rephrased this sentence to emphasize that, to the best of our knowledge, no other\nintegrated device operating in the mid-infrared has simultaneously achieved full π-\nphase modulation, low propagation loss, and high-speed performance.\nWhile References 19 and 20 report lower Vpi values, these devices are based on free-\ncarrier electro-absorption mechanisms rather than the Pockels effect, and therefore\nsuffer from strong free-carrier absorption, which leads to substantial optical loss.\nFor instance, in Ref. 19, a 0.75π phase shift is achieved at 3.5 V, but the modulator exhibits\napproximately 3.85 dB loss excluding MMI and waveguide losses due to free-carrier\nabsorption alone, and an extinction ratio of 13 dB. Similarly, in Ref. 20, although a π-\nphase shift is reached with a low drive voltage (1.2 V), the reported excess loss exceeds\n22 dB, mainly caused by carrier-induced absorption in the waveguide.\nMoreover, the reported waveguide propagation losses in Ref. 20 are 11.5 dB/cm at 3.72\nµm and 15.0 dB/cm at 3.88 µm, increasing with wavelength largely due to high material\nabsorption by the SiO2 buried oxide layer.\nIn contrast, our work demonstrates a Pockels-based integrated LiNbO₃ modulator with\nlow propagation loss, high speed and broad MIR operation up to 4.5 µm, achieving full\nπ-phase modulation without introducing significant excess absorption. This\ncombination of integration, efficiency, and low loss, all in the mid-infrared regime,\nconstitutes the key milestone highlighted in our manuscript.\nThe statement has been revised to clarify that, to the best of our knowledge, no\nintegrated device currently demonstrates simultaneously the same combination of\nperformance metrics achieved in our work. While previous reports have shown\ncomparable phase shifts, they either lack full integration, operate at lower speeds, or do\nnot meet all key metrics concurrently. We have also revised the manuscript to\nmention in more detail the current state-of-the-art in MIR modulators.\nChanges in the manuscript:\n- “Furthermore, we demonstrate full π-phase modulation in the MIR, uniquely\ncombining integration, low propagation loss, extinction ratio and high-speed\noperation, representing a key milestone for MIR photonics. These results\nestablish a pathway toward energy-efficient, high-performance mid-infrared\nphotonic systems for applications in telecommunications, sensing, and\nquantum technologies.” Part Abstract\n- “On the other hand, phase modulation in the mid-infrared has so far been\ndemonstrated in only a few platforms, including optical parametric oscillators\n(OPOs) based on periodically poled bulk lithium niobate [31, 32], Stark-effect\ndevices using intersubband transitions [33, 34], and graphene-based structures\n[35]. While OPOs provide high optical power, they rely on bulky tabletop\ncomponents and have very high- power consumption. As already discussed,\ncarrier depletion or injection modulators can achieve performance approaching\na π-phase shift with lower VπL but suffer from extinction ratios around 15 dB and\nhigh optical losses [24, 25]” Part Main\n- “To our knowledge, these values of loss, speed, and extinction ratio have not\npreviously been achieved in a single integrated MZM operating in the MIR [20],\nalbeit with a higher VπL.” Part Main\n5.The captions for Fig. 4c and 4b appear to be swapped.\nResponse: We have corrected the caption in the manuscript (Fig. 4c, Fig. 4b)\n6.Equation 1 in the Discussion section is overly fundamental. Is it necessary to include\nit separately in this context?\nResponse: We agree that Equation 1 presents a fundamental relationship; however, we\nbelieve it remains relevant in this context. In the revised manuscript, we have\nclarified its purpose by emphasizing that it not only explains the performance\ndegradation compared to similar Pockels-based telecom C-band modulators but\nalso provides a basis for extrapolating the device behavior outside the measured\nwavelength range. Specifically, the equation helps predict the decrease of\nperformance for higher wavelength and the improved performance can be expected at\nshorter wavelengths, which could not be experimentally verified due to the lack of an\nappropriate optical source.\nChanges in the manuscript:\n- “Consequently, increasing the wavelength by a factor of approximately 2.5\nresults in a 3–4× increase in the intrinsic Vπ L value.[24] This relationship not only\nexplains the observed performance degradation compared to C-band\nmodulators but also provides a framework for extrapolating the device behavior\nbeyond the experimentally measured wavelength range. Specifically, it indicates\nthat improved performance can be expected at shorter wavelengths, which\ncould not be experimentally verified due to the absence of a suitable optical\nsource.” Part Discussion\n7.The caption for Fig. B2, \"Fabrication process of TFLN on sapphire,\" does not match the\ncontent of the figure.\nResponse: We have corrected the caption in the manuscript.\n8.Could the authors attempt to subtract the S21 curve of the detector from the current\nS21 curve to further quantify the modulator's bandwidth?\nResponse: The detector’s S21 response has already been subtracted, and the\npresented curve therefore represents the modulator’s corrected bandwidth. The\ndetector S21 corresponds to its intrinsic response, after removing the contributions\nfrom the amplifier and modulator. The requested clarifications have been\nincorporated into the revised manuscript.\nChanges in the manuscript:\n- “The data has been normalized by the detector and amplifier responses to most\naccurately represent the intrinsic electro-optic bandwidth of the modulators.”\nCaption of Fig.4\n- “The presented response has been normalized by the detector and amplifier\nchain, ensuring that the extracted 3 dB bandwidth reflects the modulator’s\nintrinsic performance.” Part 2.3\n9.To significantly enhance the impact of this work, it is highly recommended that the\nauthors include a demonstration of their high-speed MIR MZM in a practical application.\nGiven the potential applications mentioned in the text, implementing a proof-of-\nconcept experiment in areas such as MIR spectroscopy, gas sensing, or free-space data\ntransmission would compellingly validate the device's utility and performance.\nResponse: We fully agree that demonstrating a practical application would further\nhighlight the potential of our high-speed MIR Mach–Zehnder modulator. In our current\nwork, we have already demonstrated free-space transmission at 10 Gbit/s (0.5 m\ndistance), which provides first proof of the device’s capability for mid-infrared data links\n(Fig. 4.c). Furthermore, we demonstrate effective comb generation for potential\nspectroscopy applications. (Fig. 4.d)\nImplementing a complete proof-of-concept system for applications such as mid-\ninfrared spectroscopy, gas sensing, or long-distance free-space communication would\nrequire specialized setups and instrumentation beyond the scope of the present study.\nSuch demonstrations would also demand additional expertise and complex\nexperimental infrastructures, and are therefore better suited for dedicated follow-up\nwork.\nWe believe that the current results already establish the innovative low loss platform,\nhigh-speed performance and practical potential of our device, laying a solid foundation\nfor future studies which are more application-oriented.\nWe thank the reviewer for their valuable feedback. In response, we have incorporated\nadditional data and clarifications that directly address the raised concerns and further\nstrengthen the manuscript.\nReviewer #2 (Remarks to the Author):\nReviewer Report\nOriginal and Timely: 3 – Probably\nLasting Impact: No\nConvincing and Rigorous Data: Yes, but requires some revision\nInterest to the Community: Very Likely\nSufficient Information: No\nSatisfactory English: Yes\nAppropriate Title: Yes\nGood Abstract: Yes\nClear Figures: Yes\nAdequate References: No\nThis manuscript presents an integrated electro-optic modulator on a TFLN-on-sapphire\n(LNOS) platform, demonstrating low loss and high bandwidth operation in the mid-\ninfrared (MIR) regime. Since MIR photonics remains a challenging field due to the lack of\nmature components such as lasers, detectors, and fibers, this work is technically well-\nexecuted and of potential interest to the community.\nHowever, similar integrated electro-optic modulators have already been reported.\nTherefore, to meet the publication standards of Nature Communications, the\nmanuscript would benefit from additional experimental data and further discussion to\nclearly establish novelty and impact.\nResponse: We apologize if the comparison was not sufficiently clear in the previous\nversion. We are confident that our device marks a key distinction from previously\nreported technologies. Regarding the references, it is important to distinguish between\nthree categories of modulators:\n• LNOI-based modulators are intrinsically limited to operation below\napproximately 3.8 µm due to strong absorption in the buried silicon dioxide\nlayer. For example, in the very interesting work by Hwang et al. (“A wide-\nspectrum mid-infrared electro-optic intensity modulator employing a two-\npoint coupled lithium niobate racetrack resonator,” APL Photonics 10, 01501\n(2025)), the device is based on the lithium-niobate-on-insulator (LNOI)\nplatform, which benefits from mature fabrication processes but suffers from\nsignificant absorption in the buried SiO₂ layer in the mid-infrared, as\nillustrated in the following graph. As shown by these results, the propagation\nloss already reaches about 4 dB/cm at 3.8 µm, which severely limits operation\nbeyond this wavelength. And the absorption would increase significatively\nafter 3.8 µm as shown in the graph below.\nMoreover, while a 100 GHz modulation bandwidth was predicted, only a 1 kHz\nresponse was experimentally demonstrated. In contrast, our modulator is\nimplemented on the lithium-niobate-on-sapphire platform, which eliminates the\nSiO₂ absorption issue (see figure above) and enables low-loss operation well\nbeyond 3.8 µm. Also, achieving high-quality fabrication on the LNOS platform is\nparticularly challenging due to the optical transparency and insulating nature of\nthe sapphire substrate, which complicates alignment during lithography and\ncharge dissipation during processing. An additional fabrication challenge is the\ndeeper lithium niobate etching (>400 nm) required compared to the typical 200–\n400 nm used in most LNOI devices, to accommodate the longer operating\nwavelength of our MIR device. Nonetheless, the successful realization of a high-\nspeed MIR modulator on this platform demonstrates its strong potential for\nextending electro-optic modulation further into the mid-infrared, while also\nbenefiting from its exceptional nonlinear properties.\n• Other lithium niobate technology: Si/LN hybrid platform based on transfer\nprinting, as demonstrated by Xu et al. (Advanced Optical Materials, 2023). This\nmethod enables mid-infrared operation up to 3.8 µm and probably higher,\nachieving a modulation efficiency of 12.3 V·cm, an extinction ratio around 25 dB,\nand a modulation bandwidth above 5 MHz. While this technique shows\npromising performance, it involves relatively high fabrication complexity\nassociated with the heterogeneous integration process. Moreover, the reported\ndevices exhibit approximately 3.5 dB/cm higher propagation loss compared to\nour platform, which may limit overall system efficiency and scalability.\n• III–V-based carrier-injection modulators suffer from very high optical loss during\nmodulation because of free-carrier absorption. These devices have\ndemonstrated modulation bandwidths roughly two orders of magnitude lower\nthan those achievable through electro-optic modulation. (Li, Tiantian, et al. \"Ge-\non-Si modulators operating at mid-infrared wavelengths up to 8 μm.\" Photonics\nResearch 7.8 (2019): 828-836.),\nOur work represents the first electro-optic Mach–Zehnder modulator enabling efficient\n(extinction ratio), very high speed (bandwidth), and low-loss modulation beyond 3.8 µm.\nMoreover, this is the first fabricated electro-optic device on the lithium-niobate-on-\nsapphire platform, establishing a solid foundation for future developments.\nChange in the manuscript:\n- “Alternative approaches such as external free-space Stark-effect modulators\nhave achieved comparable bandwidths [20–22] and extinction ratios up to 3 dB\n[23]. These devices are particularly attractive because they rely on a refractive\nmodulation mechanism rather than carrier absorption, enabling easier free-\nspace coupling and potentially lower insertion losses. However, their scalability\nremains limited due to the high alignment sensitivity inherent to free-space\nconfigurations. In the integrated domain, III–V-based and group-IV carrier-\ninjection or depletion modulators [24] can operate at longer wavelengths (up to 8\nμm) but generally exhibit high optical losses. Despite their impressively low\nvoltage–length product (Vπ L), these devices suffer from significant free-carrier\nabsorption. Recent demonstrations include implementations on Ge-on-Si\nplatforms [24] and silicon-on-insulator structures [25], which exhibit relatively\nhigh propagation losses due to lattice-mismatch defects between Ge and Si, and\nabsorption in the silicon dioxide substrate, respectively. A newer technology\nrelies on graded-index Ge-on-Si designs [26], which minimize defects and thus\nreduce losses, but still suffer from shallow modulation depths despite being very\npromising in terms of wavelength accessibility. Hybrid Si/LN transfer-printed\nmodulators [27, 28] have extended lithium niobate operation into the mid-\ninfrared, demonstrating good performance near 3.8 μm and showing promise for\nintegrated spectroscopic sensing. However, fabrication complexity and limited\nelectro-optic overlap still constrain their performance. Finally, lithium niobate on\ninsulator (LNOI) modulators, despite their proven success in the near-infrared,\nare intrinsically limited to operation below approximately 3.8 μm [29] due to\nstrong absorption in the buried SiO2 layer [30].”\nBelow are detailed comments:\n1. Title clarity:\nSince a mid-IR modulator based on TFLN has already been demonstrated in the\nfollowing paper, the title should be more specific, explicitly indicating that the work is\nbased on the LNOS platform:\nHyeon Hwang, Kiyoung Ko, et al., “A wide-spectrum mid-infrared electro-optic intensity\nmodulator employing a two-point coupled lithium niobate racetrack resonator,” APL\nPhotonics 10, 016116 (2025).\nResponse: We thank the reviewer for this valuable recommendation and apologize for\nhaving initially omitted this important reference. The work by Hwang et al. (APL\nPhotonics 10, 016116 (2025)) has now been cited in the revised manuscript. The\ntitle has been changed accordingly.\nChange in the manuscript:\n- “Thin film lithium niobate on sapphire for integrated mid-infrared modulator”\nPart Title\n- “Finally, lithium niobate on insulator (LNOI) modulators, despite their proven\nsuccess in the near-infrared, are intrinsically limited to operation below approximately\n3.8 μm [29] due to strong absorption in the buried SiO2 layer [30].” Part Main\n2. Overstated claim:\nThe sentence on page 4 —\n“To our knowledge, this represents the first photonic integrated MZM demonstrated in\nthe MIR, surpassing previous demonstrated modulators in terms of speed and\nextinction ratio by nearly two orders of magnitude.”\n— should be removed, as it is not accurate in light of prior demonstrations.\nResponse: Thank you for the observation, that is correct. The sentence has been\nrevised for accuracy. As previously mentioned, earlier demonstrations relied on LNOI,\ncarrier-depletion mechanisms or Si/LN hybrid platforms, which, while functional,\nexhibit significant drawbacks such as high optical losses. The text has been modified\naccordingly to clarify this distinction.\nChange in the manuscript:\n- “Furthermore, we demonstrate full π-phase modulation in the MIR, uniquely\ncombining integration, low propagation loss, extinction ratio and high-speed operation,\nrepresenting a key milestone for MIR photonics. These results establish a pathway\ntoward energy-efficient, high-performance mid-infrared photonic systems for\napplications in telecommunications, sensing, and quantum technologies.” Part Abstract\n- Deleted the overstated claim on page 4\n3. Insufficient references for MIR relevance:\nThe following statement is correct but lacks supporting citations:\n“In this context, phase and amplitude modulation are essential techniques for MIR\nspectroscopy – improving sensitivity, resolution, and signal-to-noise ratio.”\nPlease include appropriate references that emphasize the importance of MIR\nmodulation and its applications.\nResponse: We have now included several references that highlight the critical role\nof amplitude and phase modulation in enhancing the performance of mid-infrared\nspectroscopic systems.\nChange in the manuscript:\n- “In this context, efficient electro-optic modulators play a crucial role in both domains.\nExternal modulators, in particular, enable high modulation depths and can handle\nsignificantly higher optical power than directly modulated sources, making them highly\nadvantageous for achieving strong, high-quality modulated signals in the mid-\ninfrared.[8] The MIR spectral range also offers several benefits for spectroscopy,\nwhereby precise control over the phase and amplitude of mid-infrared light is required.\nAmplitude modulation enables lock-in detection to improve sensitivity and suppress\nbackground signals [9, 10], and it can also be used to probe very fast phenomenon [11]\nand delicate samples [12]. For example, Tamamitsu et al. use a 0.1 ns (10 GHz) MIR\npulse to confine heat diffusion and enhance photothermal efficiency, although it is\ncurrently generated by a bulky OPO system. Phase modulation, on the other hand,\nenables wavelength modulation spectroscopy [12] and the generation of frequency-\ncomb light sources[13], providing powerful tools for high-resolution and broadband\nspectroscopy” – Part Main\n4. Spectral limitation and platform comparison:\nThe experiments were conducted only between 3.9–4.1 µm, covering a narrow range\nwith limited molecular fingerprint information. To better demonstrate the potential\nbroadband applicability of the platform, I suggest adding theoretical loss calculations\nfor both LNOI and LNOS platforms across the 3–5 µm range (e.g., in Fig. 1). Additionally,\nincluding Q-factor measurements from ring resonators would help substantiate the\nclaim of achieving a low-loss MIR platform.\nResponse: Our experiments were conducted in the 3.9–4.5 µm range, which\ncorresponds to the spectral region accessible with our available lasers. This range\nalready extends beyond the limit of LNOI platforms (typically < 3.8 µm) with very\nimportant molecules fingerprint (such as CO2, CO and N2O). Particularly, the strong\nabsorption band of CO2 around 4.26 µm and Co at ~4.5 µm as well as the strong\nabsorption of N2O around 4.5 µm is very important. (https://hitran.org/)\nWe chose to focus on this wavelength because it lies between two major absorption\nbands while remaining within the atmospheric transparency window, making it a good\ndemonstration for applications in both free-space communication and spectroscopy.\nRegarding the difference in loss between lithium niobate on sapphire and on silicon, we\nhave added a reference in our response (see Fig. 1 in this document), which is also cited\nin the main text. As already mentioned, using thin-film lithium niobate on sapphire can\nreduce the loss at 4 µm by at least one order of magnitude. These results were obtained\nin this paper (Mishra, Jatadhari, et al. Optica 8.6 (2021): 921-924.) by evaluating the\nabsorption loss for silica and sapphire bottom claddings with identical thin-film\ngeometries, based on bulk absorption data and mode overlap calculations with the\ncladding.\nWe fully agree that Q-factor measurements would offer valuable insight into the\nintrinsic propagation losses. However, ring resonator characterization in the mid-\ninfrared remains technically challenging due to the limited wavelength tunability of\navailable external cavity lasers, whose step size is too large to finely resolve narrow\nresonances. Instead, we have included new cutback measurements, employing a\ngrating coupler, which confirm low-loss waveguide performance of approximately\n1.3 dB/cm. A plot showing the waveguide loss as a function of wavelength has also\nbeen added (Fig. 3d and Fig. C3c), together with additional details on the mid-\ninfrared grating coupler. Overall, these additional experimental and simulated results\nfurther support the broadband, low-loss potential of the LNOS platform for high-speed\nMIR electro-optic applications.\nChange in the manuscript: Fig 3.d (Top left), Fig C3c (Top right) and Fig C3b (Bottom left)\n5. Inconsistency in extinction ratio data:\nIn Fig. 3b, the extinction ratio (ER) is labeled as 34.1 dB, but the data suggest a minimum\ntransmission of ~0.1 (corresponding to ~10 dB ER). The same issue appears in Fig. E5.\nPlease verify and clarify this discrepancy. It may also be more appropriate to plot the\nmodulation data on a logarithmic (dB) scale to better visualize the claimed high ER\nperformance.\nResponse: The apparent discrepancy arises because the extinction ratio in Fig. 3b (and\nFig. E5) was evaluated relative to the noise floor (shown as a dashed line in the plots),\nrather than solely from the visible signal minimum. The minimum transmission\nobserved in linear scale (~0.1) does not reflect the true extinction-limited value, as the\ncorrect evaluation must be made with respect to the detector’s noise floor (noise from\nthe detector without laser illumination). We have now modified the figure accordingly\nin the manuscript to plot it in dB and better represent the value of the extinction\nratio. (Fig. 3b)\nChange in the manuscript: Fig 3b\n6. Missing references for wavelength-dependent modulation:\nThe following statement on page 9 should be supported by experimental references:\n“This behaviour arises from a fundamental limitation — the Pockels effect induces a\nchange in refractive index Δn, but the resulting phase shift decreases at longer\nwavelengths, as described by:”\nI recommend citing the following works:\no Hixin Xue et al., “xue,” Optica 10, 125–126 (2023).\no Hyeon Hwang, Kiyoung Ko, et al., “A wide-spectrum mid-infrared electro-optic\nintensity modulator employing a two-point coupled lithium niobate racetrack\nresonator,” APL Photonics 10, 016116 (2025).\nResponse: We respectfully disagree that additional experimental citations are required\nfor this statement. The sentence describes a fundamental property of the Pockels\neffect, namely that the induced phase shift scales inversely with wavelength due to the\ndependence of the optical phase. This relationship arises directly from the standard\nelectro-optic phase modulation expression and does not depend on specific\nexperimental demonstrations. The provided formula already conveys this well-\nestablished physical dependence in classical electro-optic theory. However, in the\nrevised manuscript, we have clarified the purpose of the equation and more clearly\nemphasized the aspect we intended to illustrate, while also including the first\nreference you cited to show the increasing trend.\nChange in the manuscript:\n- “Consequently, increasing the wavelength by a factor of approximately 2.5\nresults in a 3–4× increase in the intrinsic Vπ L value.[46]”\n7. Discussion of the main limitation (high Vπ):\nAs discussed in the manuscript, the major limitation of the current platform is its high\nhalf-wave voltage (Vπ). It would be helpful to include additional information in the\nSupplementary Material regarding possible improvement limits through electrode\ndesign optimization. From a practical standpoint, it appears that the achievable\nenhancement may already be constrained by the inherent characteristics of MIR\nwavelengths.\nResponse: We agree that the relatively high half-wave voltage currently represents the\nmain limitation of our platform.\nAs correctly noted, the achievable enhancement is indeed constrained by the intrinsic\ncharacteristics of mid-infrared operation. The longer wavelengths require wider optical\nmodes and larger electrode spacing to minimize absorption losses, which intrinsically\nincreases Vπ. Nevertheless, several trade-offs can be considered. For example,\nreducing the electrode spacing would lower Vπ but at the cost of higher optical loss,\nwhile operating at shorter wavelengths would allow tighter electrode confinement and\nimproved modulation efficiency. To complement this analysis, we have performed\nadditional experimental evaluations of the losses as a function of the spacing\nbetween the waveguide and the electro-optic electrodes. (Fig 3c & Fig. C3.d)\nFurthermore, alternative device architectures are mentioned in the revised manuscript\nand could be employed to reduce the effective driving voltage. In particular, resonant\nconfigurations such as ring-assisted Mach–Zehnder modulators (e.g., Xue, Yu, et al.,\n“Breaking the bandwidth limit of a high-quality-factor ring modulator based on thin-film\nlithium niobate,” Optica 9, 1131–1137 (2022)) can significantly lower Vπ while\nmaintaining compact device footprints. This approach will be the focus of future\ndevelopments on our platform.\nChange in the manuscript:\nOverall evaluation:\nThis is a well-executed and potentially valuable study demonstrating MIR modulation on\nan LNOS platform. However, in its current form, the manuscript lacks sufficient\ndistinction from prior work and requires additional data, clarifications, and references\nto convincingly justify publication in Nature Communications.\nWe thank the reviewer for their valuable input and hope that, with the added material\nand revisions, we have addressed the concerns and strengthened the manuscript for\npublication.\nReviewer #3 (Remarks to the Author):\nIn the manuscript \"Integrated thin film lithium niobate mid-infrared modulator\", the\nauthors report a Mach-Zehnder electro-optic modulator (MZM) on lithium niobate on\nsapphire (LNOS) nanophotonic platform, using which they demonstrate phase and\namplitude modulation in the mid-infrared (MIR), around 4-microns wavelength. They\nfurther demonstrate data-transmission and generation of an electro-optic (EO) comb in\nthis spectral region.\nUpon reading this manuscript, I have the following questions and feedback for the\nauthors:\n• Would it be possible for the authors to discuss in more detail the design\nconsiderations of their device, perhaps in section 2.1, or in a separate section in the\nappendix? I believe it would be helpful for the reader to understand how key figures of\nmerit, such as the VπL and EO bandwidth, are dependent on the various design\nparameters, why particular waveguide dimensions and electrode dimensions were\nchosen, and if special attention was paid to group velocity matching and impedance\nmatching. While some of these design considerations and simulations are interspersed\nwith results, I think it’d be beneficial to have most of them collected in one place to\nprovide a full picture of designing an MZM in the MIR, and to understand if any design\noptima may differ from those that are well-established in the near infrared.\nResponse: While some design considerations were already included in the original\nmanuscript, we fully agree that a more comprehensive overview would benefit the\nreader. We have therefore expanded the description and added complementary\ndetails in the Appendix C and E. We provided an analysis of the losses as a function\nof the spacing between the electro-optic electrode (Fig.3c top Panel & Fig. C3d).\nWe have also included the simulations performed and discussions on the Appendix\nE on RF design of the electrode. It shows how we designed and verify that the\nelectro-optic electrodes were impedance-matched to 50 Ω (Fig. E5a) and that the\nelectrical S₁₁ measurements exhibited minimal reflections up to 20 GHz (Fig. E5b).\nChange in the manuscript: Captions and discussions linked to those figures:\n• The authors use a MIR-compatible multimode lensed fiber to couple light into the\ninput waveguide. How do they ensure that they only have fundamental mode excitation\nin the waveguide?\nResponse: The waveguide used in our experiments is designed to support only the\nfundamental TE/TM mode at the operating wavelength. Therefore, even though a\nmultimode lensed fiber is used for coupling, only the fundamental mode of the\nwaveguide can propagate, and higher-order modes. TE mode can then be differentiated\nfrom the TM mode based on the modulation strength of the modulator, the different\npolarizations use different elements of the LN Pockels tensor, with the TE mode being\nmodulated roughly three times more strongly than the TM mode.\n• Fig 3d has only four wavelengths characterized between 3.95 and 4.3 microns. Based\non the QCL spectrum in appendix B, would it be possible to have measurements at\nmore wavelengths in this range? It would be good to understand the apparent dispersive\ntrends and ascertain if they arise from experimental noise or if they correspond to\noscillations in the device efficiency.\nResponse: The observed increase of Vπ with wavelength primarily arises from the\ninduced phase shift decreases with increasing wavelength, consistent with the\nexpected wavelength dependence of the electro-optic effect. In addition, it also comes\nfrom the waveguide design, which is optimized for operation around 4.0 µm. As the\nwavelength increases beyond this point, the optical mode becomes less confined in the\nlithium niobate layer and more in the sapphire substrate, leading to reduced modulation\nefficiency and a corresponding increase in Vπ. We have added a simulation in Fig. 3d\n(top figure) that evaluates the variation of Vπ·L as a function of wavelength. The\nsimulated results show good agreement with the experimental data.\nIn the revised manuscript, we have also extended the characterization up to 4.5\nµm. The measurement over the 3.9–4.5 µm wavelength range remains constrained by\nthe discrete emission modes of the available DFB QCLs (4.3 µm and 4.5 µm) and by the\nlimited tuning bandwidth of our external-cavity QCL sources around 4 µm, which do not\nprovide single-mode operation between 4.1 µm and 4.3 µm.\nChange in the manuscript:\n• The generation of MIR EO comb through the MZM is certainly of great interest due to its\npotential applications, however this portion of the manuscript appears to be lacking in\nsome details, such as the driving voltage and observed bandwidth, as well as in\ndiscussions about ways to improve the bandwidth of the comb. Also, can multiple\ndevices (varying waveguide thickness, electrode length, etc.) be tested for EO comb\ngeneration and the results compared?\nResponse: In the revised manuscript, we have included a more complete\ndescription of the measurement conditions and the MIR electro-optic comb\ngeneration at different driving frequencies. The device used for comb generation is\nthe one exhibiting the lowest Vπ, which is the most relevant parameter for efficient\ncomb formation. The waveguide thickness has already been optimized to achieve low\npropagation loss, strong optical confinement, and single-mode operation for the TE\npolarization. Nevertheless, in future developments, the device length could be slightly\nincreased (relatively to the size of the chip) to further enhance the modulation efficiency\nand broaden the comb bandwidth.\nChange in manuscript:\n- “Using our modulator, we successfully generated an optical frequency comb\ncentered around the carrier frequency for three different modulation frequencies,\nas measured by the OSA with a resolution of 1.9 GHz. As shown in Fig. 4d, the\ncomb is centered at a wavelength of approximately 4 μm, with symmetric\nsidebands clearly resolved, confirming the expected modulation behavior. At 7\nGHz, based on the relative powers of the carrier and higher-order sidebands, we\nobserved a maximum attenuation of 6.4 dB per sideband peak, leading to a\nspectral bandwidth of around 70 GHz, which reflects efficient energy transfer\nfrom the central mode into the sidebands. The energy transfer, highlighted by the\nslope of the comb, decreases with increasing frequency at 10 GHz and 12 GHz,\nwhich we attribute to the combined effects of a higher effective Vπ L at elevated\nfrequencies and reduced electrical power delivery to the electrodes due to\nimpedance mismatch. The measured spectrum is not limited by the OSA noise\nfloor, which is around –70 dBm. In fact, a broader comb could be obtained using\na laser source with lower intrinsic amplitude noise. The relatively high noise level,\naround –50 dBm in our measurements, arises from the use of the EC-QCL, which\ntypically exhibits higher amplitude and frequency noise due to mechanical tuning\nelements and longer cavity lengths. For practical spectroscopic applications, a\nlow-noise source such as a 4 μm DFB QCL or an interband cascade laser would\nsignificantly enhance the signal-to-noise ratio and allow observation of broader\ncomb spectra, with noise down to –60 dBm and bandwidths exceeding 100 GHz.\nThe ability to tune the spacing between comb lines is a powerful tool for\nscanning absorption bands. These results demonstrate the modulator’s\ncapability to generate rich frequency content through deep amplitude\nmodulation, confirming its potential as a building block for mid-infrared\nfrequency-comb generation and broadband spectroscopic systems.”\n• From the transcript: “While the simulation shows good agreement with the\nexperimental results for the 1.5 μm devices, a slight discrepancy is observed for the 0.9\nμm case, likely due to a non-uniform gap length along the electrode.”; why would the\ngap between lithographically defined electrodes be nonuniform? Could it be due to\nimproper metal liftoff? An optical microscope observation might provide some\nclarification.\nResponse: We re-examined the electrode spacing with an SEM between the waveguide\nand the metal electrode, and the measurements confirmed that the spacing is uniform\nand consistent with the design within ±100 nm. Therefore, the previously stated\nhypothesis regarding a non-uniform gap was incorrect. The slight discrepancy\nbetween the simulation and the experimental results remains unexplained at this\nstage, and the corresponding sentence has been removed from the revised\nmanuscript.\n• Based on the propagation loss estimates from the cutback measurements, can the\nauthors ascertain the excess loss through the MZM and if / how it depends on the\nwaveguide and electrode designs?\nResponse: In the revised manuscript, we have included new data obtained from\ncutback measurements using grating coupler structures, which allow a more\nprecise evaluation of the waveguide propagation loss independent of coupling\nvariations. (Fig 3d, FigC3b, FigC3c). These additional results confirm the low-loss\nperformance of the passive waveguides on the LNOS platform.\nAs already mentioned, we have added a new figure analyzing the additional loss\ninduced by the electrodes as a function of electrode spacing and length. (Fig3c, Fig\nC3d). This provides a direct estimate of the excess loss in the MZM and illustrates the\ntrade-off between modulation efficiency and optical absorption from the metal\nelectrodes.\nChanges in the manuscript:\nSome minor comments:\n• It appears from Fig. 1b that the waveguide width is 4 microns, while I believe the\nintention there is to show the wavelength?\nResponse: The value indicated in Fig. 1b (4 µm) indeed refers to the waveguide width,\nnot the wavelength. We have clarified this point by changing the scale with a length\nof 2 µm to avoid confusion with the wavelength.\nChanges in the manuscript:\n• The layout of the electro-optic electrode is not clear to me form figure 1d; perhaps the\nauthors could change the color / contrast of the image to make the photonics\ncomponents more visible in this image?\nResponse: The contrast and color scheme of Fig. 1d have been improved in the\nrevised manuscript to make the electrode layout and photonic components more\nclearly visible.\nChange in the manuscript:\n• Typo in figure 3b caption, Vπ of 28 V (not Vcm)?\nResponse: The change has been implemented in the revised manuscript.\n• Figure 3c caption needs some clarification. I would also recommend that the authors\nplot the simulations as solid / dashed lines and reserve cross / dot for experimental\ndata points.\nResponse: The change has been implemented in the revised manuscript.\nChange in the manuscript:\n• Captions for Fig.4c&d appear to be interchanged.\nResponse: The captions for Figs. 4c and 4d were indeed interchanged in the original\nsubmission. This has been corrected in the revised manuscript.\n• Fig. B2 caption incorrect.\nResponse: The caption for Fig. B2 has been corrected in the revised manuscript.\n• Please clarify the TOPS set point for the EO measurements.\nResponse: The set point corresponds to the condition where the optical output power\nof the interferometer is half of its maximum value, ensuring operation at the quadrature\npoint for optimal electro-optic modulation efficiency. This condition was verified either\nby monitoring the static transmission curve or by observing the modulator output on an\noscilloscope and confirming symmetric modulation behavior with respect to the applied\nvoltage. We have clarified the method used to set the device at the\nquadrature point.\nChange in the manuscript: “The TOPS voltage set point corresponds to the condition\nwhereby the optical output power of the interferometer is half of its maximum value,\nensuring operation at the quadrature point for optimal electro-optic modulation\nefficiency. This condition was also verified by observing the modulator output on an\noscilloscope and confirming symmetric modulation behavior with respect to the applied\nvoltage.”\nOverall, my impression is that while the authors have adequately demonstrated MZM in\nthe LNOS platform in the MIR, if they could expand upon their demonstration and / or\nanalysis of MIR EO-comb generation in this platform, it would substantially increase the\ncontribution of their work and make it more suitable for publication in Nature\nCommunications.\nResponse: We thank the reviewer for this thoughtful comment and for recognizing the\nsignificance of our demonstration. We fully agree that extending this work toward\noptimized mid-infrared electro-optic comb generation on the LNOS platform would\nrepresent an exciting and impactful next step.\nHowever, the main objective of the present work is to establish and experimentally\nvalidate the fundamental operating principle of the LNOS platform by demonstrating a\nhigh-efficiency Mach–Zehnder modulator operating in the mid-infrared with a low-loss\nplatform. In this paper, we focus on showcasing the high-performance fabrication\nprocess, including the realization of deep etching (~1 µm) on a transparent sapphire\nsubstrate and the successful integration of electro-optic electrodes and thermal phase\nshifters on a state-of-the-art low-loss platform with propagation losses around 1.3\ndB/cm @ 4 µm. These results represent an important milestone, confirming the\nfeasibility of implementing complex active photonic components on this emerging\nplatform, while also enabling nonlinear processes such as spontaneous parametric\ndown-conversion and difference-frequency generation in the mid-infrared.\nWe fully acknowledge the reviewer’s suggestion regarding EO-comb generation. Indeed,\nresonant and non-resonant comb architectures are currently under active investigation\nin our group, and preliminary results have already been obtained. Nevertheless,\ndemonstrating and analyzing optimized EO-comb generation would require a\nsubstantially different device design and extensive discussion of cavity dynamics and\ndispersion engineering, which would go beyond the scope of this paper. We have\nnevertheless included the best possible comb that can be generated with this device\narchitecture to demonstrate the potential of this platform in this regard.\nWe therefore believe that the current manuscript appropriately focuses on\ndemonstrating the core capability and fabrication maturity of the LNOS platform, which\nlays the essential groundwork for future demonstrations of EO-comb generation and\nother advanced functionalities in the mid-infrared.\nResponse to the Reviewers’ comments on the manuscript titled “Thin film\nlithium niobate on sapphire for integrated mid-infrared modulator”\nsubmitted to Nature Communications\nThank you very much for the opportunity to revise our manuscript. We would like to\nexpress our sincere gratitude to the Editor and Reviewers for their evaluation of our\nwork, which has greatly improved its quality.\nBelow, we provide a detailed, point-by-point response to the suggestions and questions\nraised by the Reviewers. Along with this resubmission, we have included a revised\nversion of the manuscript that incorporates these constructive comments highlighted in\ngreen.\nReviewer #1 (Remarks to the Author):\nThe authors have adequately addressed my previous comments. Regarding the revised\nmanuscript, I would like to suggest one minor improvement. To better illustrate the\nexperimental configuration for the free-space transmission at 10 Gbit/s over a 0.5 m\ndistance, the addition of a figure showing a photograph or a link diagram of the actual\nsetup would be valuable. Such a depiction would greatly aid in the interpretation and\nvisualization of this specific experimental scenario.\nResponse: Thank you for the positive assessment. Based on your suggestion, we have\nnow implemented a schematic of the actual setup and included it in the revised\nmanuscript in Fig 4.c where we added a schematic representation of the free space\ntransmission setup on the top panel. This depiction should make the experimental\narrangement clearer and help readers visualize the specific free-space transmission\nscenario.\nReviewer #2 (Remarks to the Author):\nI appreciate the authors' detailed response and the revisions made to the manuscript. I\nunderstand the authors' position that a full demonstration of spectroscopy applications\nis beyond the scope of this work and that the manuscript aims to serve as a\n\"foundational demonstration\" of the LNOS platform in the mid-infrared.\nHowever, if the focus is shifted towards the fundamental aspects of the platform rather\nthan practical applications, the analysis of the optical loss mechanisms must be\nsignificantly more rigorous.\nSpecifically, I strongly suggest adding a fundamental analysis comparing the\nexperimental losses with the theoretical limits. For instance, based on Mishra,\nJatadhari, et al., Optica 8.6 (2021): 921-924, the calculated absorption loss of the LNOS\nplatform is approximately ~0.01 dB/cm at 3.5 µm and ~0.1 dB/cm at 4 µm.\nIn contrast, the propagation losses reported in this manuscript (e.g., ~1.3 dB/cm at 4\nµm) are significantly higher than these theoretical limits. This discrepancy suggests that\nthe current device does not fully exploit the intrinsic low-loss advantage of the LNOS\nplatform. In fact, such loss values might be comparable to what could be achieved on\nLNOI platforms in certain conditions, thereby weakening the argument for LNOS.\nTherefore, to justify the significance of this work as a \"foundational demonstration,\" the\nauthors must:\n1. Explicitly compare their measured losses with the theoretical absorption limits\ncalculated in literature (e.g., Mishra et al.).\n2. Provide a detailed analysis of the sources of the excess loss (e.g., scattering due to\nsidewall roughness vs. material absorption).\n3. Discuss why the current experimental values deviate from the theoretical potential\nand how this gap can be bridged in future work.\nResponse: We thank the reviewer for the thoughtful evaluation and for clearly\nunderstanding the purpose of the article. We agree that a detailed loss study is indeed\nvery valuable, especially for a foundational demonstration of the LNOS platform.\nAs the reviewer notes, theoretical absorption limits represent the ultimate bound on\nmaterial performance. However, in practical integrated photonic systems, losses are\nalmost always dominated by fabrication-related imperfections rather than intrinsic\nmaterial absorption. Even on the well-established LNOI platform, the theoretical loss\nlimits have not yet been experimentally achieved. As shown in the referenced work, the\nlowest loss demonstrated is around 1.3 dB/m at 1550 nm, whereas the theoretical limit\nfrom the material is evaluated to be 0.1 dB/m, an order of magnitude lower (Zhu X et al.,\n“Twenty-nine million intrinsic Q-factor monolithic microresonators on thin-film lithium\nniobate”, Photonics Research, 2024).\nIn our case, two primary mechanisms contribute to the excess loss:\n- Imperfections at the bonded interface between sapphire and lithium niobate.\nResidual stress or interfacial defects can introduce additional scattering or\nlocalized absorption that increases propagation loss. Because the thin-film\nlithium niobate we use is commercially supplied, we have no control or\nknowledge over its bonding process, making it difficult to directly verify the\ninterface quality.\n- Sidewall roughness arising from the lithography process, which is transferred\nduring the dry-etching step, also contributes to propagation loss. A\nrepresentative SEM image of our waveguide is included just below, showing the\nsidewall roughness, although additional process refinement is expected to\nreduce it even further.\nWe also appreciate the reviewer’s comment suggesting that LNOI might achieve losses\ncomparable to LNOS at 4 µm. To our knowledge, published LNOI waveguides operating\nbeyond 3.5 µm generally report losses significantly above 1 dB/cm, primarily due to\nabsorption in the SiO₂ BOX layer. To the best of our knowledge there is no demonstration\nshowing lower loss at 4 µm for LNOI.\nFor completeness, simulated material absorption in lithium niobate at 4 µm is below\n0.1 dB/cm, whereas absorption in silica for a thin film of 1.5 µm thickness is near 1\ndB/cm. However, as already mentioned, experimental losses in thin-film lithium niobate\noften differ significantly from theoretical predictions.\nA relevant reference for mid-infrared LNOI loss is Hwang H et al., “A wide spectrum mid-\ninfrared electro-optic intensity modulator employing a two-point coupled lithium\nniobate racetrack resonator”, APL Photonics, 2025. This paper reports simulated LNOI\nlosses near 1 dB/cm for single-mode operation, whereas the experimental extraction\nbased on the intrinsic quality factor yields approximately 4 dB/cm at 3.8 µm, despite the\nmore mature LNOI fabrication. The authors also note in this paper, consistent with our\nobservation, that “the optical losses from the bottom SiO₂ layer and the electrode\nincrease with wavelength”. Consequently, one expects the loss to continue increasing\nbeyond 3.8 µm. To conclude, although LNOS fabrication is far less mature, the loss on\nour platform is likely to be reduced by roughly 3 dB/cm compared to LNOI.\nFinally, increasing the waveguide width would reduce the propagation loss although this\ncomes at the cost of entering a multimode regime. Indeed, as shown in Zhang M et al.,\n“Monolithic ultra-high-Q lithium niobate microring resonator”, Optica 2017, the loss\ndecreases by roughly one order of magnitude when the structure transitions from a\nsingle mode waveguide to a multimode one as it weakens the interaction between the\noptical mode and the lossy boundaries of the waveguide.\nWe thank the reviewer again for raising these important points. We have added\ndiscussions and clarified the distinction between intrinsic material limits and\nfabrication-related constraints in this inaugural demonstration of the LNOS platform.\nChanges in the manuscript: “The losses here are dominated by sidewall roughness and\ncould be further reduced by improved fabrication techniques, as theoretical studies\nshow that losses could be reduced to as low as 0.1 dB/cm [40]. Moreover, moving to a\nmultimode waveguide geometry would allow these losses to be reduced even further\nbecause the optical mode becomes less sensitive to scattering at the waveguide\nboundaries [44]”\nReviewer #3 (Remarks to the Author):\nThe authors have adequately addressed my queries and improved the manuscript.\nResponse: We thank the reviewer for the positive assessment. We appreciate the\nhelpful feedback which has significantly improved the manuscript. Thank you again for\nthe thorough and constructive review.\nResponse to the Reviewers’ comments on the manuscript titled “Thin\nfilm lithium niobate on sapphire for integrated mid-infrared modulator”\nsubmitted to Nature Communications\nThank you for the opportunity to revise our manuscript. Below, we provide a detailed\nresponse to the suggestions and questions raised by the Reviewer, clarifying the\nfeasibility of the requested analysis. Along with this resubmission, we have included a\nrevised version of the manuscript with changes highlighted in purple.\nWe would first like to emphasize that the demonstrated platform already exhibits low\npropagation loss for state-of-the-art single mode mid-infrared waveguides, particularly\nconsidering the long operating wavelength and the strong mode confinement required\nfor single mode operation. As discussed in the manuscript, propagation losses could be\nfurther reduced by moving to a multimode waveguide configuration. However, such an\napproach would not be relevant for the device demonstrated here, where single mode\noperation is essential for efficient modulation.\nRegarding the question of accurately calculating or isolating propagation loss\nmechanisms, we believe it is important to distinguish between the physical origin of the\nlosses and the practical limitations of the available measurements.\nFirst, any loss originating from the lithium niobate and sapphire interface introduced\nduring the wafer level bonding process is not under our direct control, as the detailed\nprocess flow and the associated material and interface imperfections are not fully\naccessible to us. Since we rely on commercial wafer manufacturers, crystallographic\ndefects or interfacial layers must be accepted as intrinsic loss factors contributing to\ndegraded performance. These contributions are very difficult to isolate experimentally\nor to model quantitatively in a predictive manner.\nSecond, from a modeling perspective, losses associated with waveguide sidewall\nroughness are well understood to arise from Rayleigh scattering, with a characteristic\nwavelength dependence scaling as λ⁻⁴. This wavelength scaling is well established in\nthe literature, including the references cited by the reviewer, and provides a sound\nqualitative physical framework for interpreting the observed trends. However,\nconverting this scaling law into an accurate prediction of the absolute loss magnitude,\nthat could be included in the paper, would require detailed knowledge of sidewall\nroughness statistics, correlation lengths, thin film thickness variation, and fabrication\nspecific parameters. While we could provide an estimate of the scattering loss and\ninclude it in the manuscript, we are concerned that such a result would not be truly\nrepresentative of the actual losses.\nIn thin film lithium niobate on silicon photonics, remarkably low propagation losses\nhave been demonstrated due to precise control of lithography and etching processes,\nparticularly through argon ion milling. This technique has a smoothing character that\nenables sub nanometer sidewall roughness. In contrast, our longer operating\nwavelength requires significantly deeper etching into the film. This requirement,\ncombined with the transparent and insulating nature of our wafer stack, necessitates a\ndifferent and more complex lithography approach, which in turn leads to increased\nwaveguide sidewall roughness. For these reasons, we attribute the dominant loss\nmechanism in our devices to scattering, with an additional contribution arising from\ninterfacial defects at the lithium niobate sapphire boundary.\nFrom an experimental standpoint, the type of conclusive evidence requested by the\nreviewer would require extremely high precision propagation loss measurements\ntogether with very high fabrication reproducibility. In practice, the most accurate loss\ncharacterization is achieved using ring resonators and quality factor analysis. Such\nmeasurements are not currently accessible due to the lack of a suitable finely tunable\nnarrow linewidth source. Moreover, a simple wavelength sweep would not yield\nconclusive trends for this type of analysis. As the wavelength decreases, the effective\nmode index increases and the effective mode area shrinks, resulting in reduced modal\noverlap with the waveguide sidewalls and therefore a decrease in the wavelength\ndependence of the scattering cross section. A rigorous investigation would thus require\nsimultaneous variation of both wavelength and waveguide cross section, an effort that\nwe believe lies beyond the scope of this work. Another possible method would be to\ntune the resonance using a thermal phase shifter placed close to the ring. This\napproach was attempted but yielded unsuccessful results due to crosstalk from the\nthermal phase shifter, leading to nearly identical extracted values that were not\nconclusive enough to be presented in a publication.\nFinally, due to the reasons explained above, propagation losses were extracted using a\ncutback method. While this provides a robust estimate of the average loss, it lacks the\nprecision required for a detailed decomposition of individual loss mechanisms.\nUnavoidable variations in coupling efficiency arising from differences in alignment\nquality, facet quality, or grating couplers between waveguides introduce uncertainties\nthat limit the sensitivity of this method for the refined loss analysis being requested.\nThe only additional experimental approach that would be realistically accessible is a\nsystematic study of propagation loss as a function of waveguide width, to evaluate the\noverall lowest achievable loss. While this could provide indirect insight into scattering\nrelated losses, it would require dedicated design, fabrication, and characterization of a\nlarge set of structures, and would constitute a substantial new section focused\nspecifically on waveguide loss engineering. Such a study would also be only weakly\nrelated to the present manuscript, as the waveguide presented here is strictly single\nmode. We believe such a study would go beyond the scope of a standard revision and\nwould more naturally form the basis of a separate, focused publication on propagation\nlosses in this platform.\nRefining the fabrication process is currently the focus of our efforts, with the goal of\nachieving waveguide quality comparable to that routinely demonstrated in lithium\nniobate photonics on silicon at visible and near infrared wavelengths. Moreover, we\nbelieve that replotting the simulated material loss of lithium niobate on sapphire\ncompared to lithium niobate on silicon, as already presented in the referenced work by\nMishra et al. (Optica 2021), would not constitute a scientific novelty.\nAs discussed by the reviewer, we agree that attributing the losses to a specific origin\nwithout ambiguity requires additional investigation. For this reason, we have rephrased\nthe sentence regarding the origin of the losses in the manuscript, highlighted in purple.\nIn addition, we have cited the papers mentioned by the reviewer. The revised sentence\nnow reads:\n“The losses are hypothesized to be dominated by sidewall roughness [44, 45] and could\nbe further reduced through improved fabrication techniques, as theoretical studies\nsuggest that losses approaching 0.1 dB/cm may be achievable [40]. Moreover, adopting\na multimode waveguide geometry could further reduce these losses, since the optical\nmode becomes less sensitive to scattering at the waveguide boundaries, although this\nwould reduce the efficiency of the presented modulator [46]”\nWe hope this clarifies why a fully predictive loss calculation or a conclusive\nexperimental isolation of scattering loss would represent a significant undertaking.\nWhile such an investigation is interesting and relevant on its own, we believe that it goes\nbeyond the scope of demonstrating the capabilities of our modulator on this platform.\nWe thank the editor and the reviewers again for all their comments and for helping to\nimprove the quality of the manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have adequately addressed my previous comments. Regarding the revised manuscript, I would like to suggest one minor improvement. To better illustrate the experimental configuration for the free-space transmission at 10 Gbit/s over a 0.5 m distance, the addition of a figure showing a photograph or a link diagram of the actual setup would be valuable. Such a depiction would greatly aid in the interpretation and visualization of this specific experimental scenario.", "reviewer_2": "I appreciate the authors' detailed response and the revisions made to the manuscript. I understand the authors' position that a full demonstration of spectroscopy applications is beyond the scope of this work and that the manuscript aims to serve as a \"foundational demonstration\" of the LNOS platform in the mid-infrared.\nHowever, if the focus is shifted towards the fundamental aspects of the platform rather than practical applications, the analysis of the optical loss mechanisms must be significantly more rigorous.\nSpecifically, I strongly suggest adding a fundamental analysis comparing the experimental losses with the theoretical limits. For instance, based on Mishra, Jatadhari, et al., Optica 8.6 (2021): 921-924, the calculated absorption loss of the LNOS platform is approximately ~0.01 dB/cm at 3.5 µm and ~0.1 dB/cm at 4 µm.\nIn contrast, the propagation losses reported in this manuscript (e.g., ~1.3 dB/cm at 4 µm) are significantly higher than these theoretical limits. This discrepancy suggests that the current device does not fully exploit the intrinsic low-loss advantage of the LNOS platform. In fact, such loss values might be comparable to what could be achieved on LNOI platforms in certain conditions, thereby weakening the argument for LNOS.\nTherefore, to justify the significance of this work as a \"foundational demonstration,\" the authors must:\n1. Explicitly compare their measured losses with the theoretical absorption limits calculated in literature (e.g., Mishra et al.).\n2. Provide a detailed analysis of the sources of the excess loss (e.g., scattering due to sidewall roughness vs. material absorption).\n3. Discuss why the current experimental values deviate from the theoretical potential and how this gap can be bridged in future work.", "reviewer_3": "The authors have adequately addressed my queries and improved the manuscript."}, "author_response": "Response to the Reviewers’ comments on the manuscript titled “Thin film lithium niobate on sapphire for integrated mid-infrared modulator”\nsubmitted to Nature Communications\nThank you very much for the opportunity to revise our manuscript. We would like to express our sincere gratitude to the Editor and Reviewers for their evaluation of our work, which has greatly improved its quality.\nBelow, we provide a detailed, point-by-point response to the suggestions and questions raised by the Reviewers. Along with this resubmission, we have included a revised version of the manuscript that incorporates these constructive comments highlighted in green."}, {"round": 3, "reviewer_comments": {"reviewer_2": "It is acknowledged that the disparity between experimental measurements and calculated values for propagation loss arises from uncontrolled losses introduced during the wafer-level fabrication process. However, the exact role of scattering loss in this discrepancy requires precise formulation. The underlying physics dictates that Rayleigh scattering loss, which is characteristic of surface roughness, scales inversely with the fourth power of the wavelength (λ−4 ).\nBy referencing the three cited papers, one can accurately predict the magnitude of scattering loss or, alternatively, derive the wavelength-dependent scaling of this loss mechanism. The paper, \"Estimation of Propagation Losses for Narrow Strip and Rib Waveguides,\" is specifically recommended as a simplified, practical tool for loss estimation.\nCrucially, conclusive empirical evidence is required to definitively establish that scattering loss is the singular or dominant root cause of the observed performance limitations.\n1. \"Roughness-Limited Performance in Ultra-Low-Loss Lithium Niobate Cavities\" by Khalatpour et al.\n2. \"A theoretical analysis of scattering loss from planar optical waveguides\" by Payne and Lacey\n3. \"Estimation of Propagation Losses for Narrow Strip and Rib Waveguides\" by Lindecrantz and Hellesø"}, "author_response": "Response to the Reviewers’ comments on the manuscript titled “Thin film lithium niobate on sapphire for integrated mid-infrared modulator”\nsubmitted to Nature Communications\nThank you for the opportunity to revise our manuscript. Below, we provide a detailed response to the suggestions and questions raised by the Reviewer, clarifying the feasibility of the requested analysis. Along with this resubmission, we have included a revised version of the manuscript with changes highlighted in purple.\nWe would first like to emphasize that the demonstrated platform already exhibits low propagation loss for state-of-the-art single mode mid-infrared waveguides, particularly considering the long operating wavelength and the strong mode confinement required for single mode operation. As discussed in the manuscript, propagation losses could be further reduced by moving to a multimode waveguide configuration. However, such an approach would not be relevant for the device demonstrated here, where single mode operation is essential for efficient modulation.\nRegarding the question of accurately calculating or isolating propagation loss mechanisms, we believe it is important to distinguish between the physical origin of the losses and the practical limitations of the available measurements.\nFirst, any loss originating from the lithium niobate and sapphire interface introduced during the wafer level bonding process is not under our direct control, as the detailed process flow and the associated material and interface imperfections are not fully accessible to us. Since we rely on commercial wafer manufacturers, crystallographic defects or interfacial layers must be accepted as intrinsic loss factors contributing to degraded performance. These contributions are very difficult to isolate experimentally or to model quantitatively in a predictive manner.\nSecond, from a modeling perspective, losses associated with waveguide sidewall roughness are well understood to arise from Rayleigh scattering, with a characteristic wavelength dependence scaling as λ⁻⁴. This wavelength scaling is well established in the literature, including the references cited by the reviewer, and provides a sound qualitative physical framework for interpreting the observed trends. However, converting this scaling law into an accurate prediction of the absolute loss magnitude, that could be included in the paper, would require detailed knowledge of sidewall roughness statistics, correlation lengths, thin film thickness variation, and fabrication specific parameters. While we could provide an estimate of the scattering loss and include it in the manuscript, we are concerned that such a result would not be truly representative of the actual losses.\nIn thin film lithium niobate on silicon photonics, remarkably low propagation losses have been demonstrated due to precise control of lithography and etching processes, particularly through argon ion milling. This technique has a smoothing character that enables sub nanometer sidewall roughness. In contrast, our longer operating wavelength requires significantly deeper etching into the film. This requirement, combined with the transparent and insulating nature of our wafer stack, necessitates a different and more complex lithography approach, which in turn leads to increased waveguide sidewall roughness. For these reasons, we attribute the dominant loss mechanism in our devices to scattering, with an additional contribution arising from interfacial defects at the lithium niobate sapphire boundary.\nFrom an experimental standpoint, the type of conclusive evidence requested by the reviewer would require extremely high precision propagation loss measurements together with very high fabrication reproducibility. In practice, the most accurate loss characterization is achieved using ring resonators and quality factor analysis. Such measurements are not currently accessible due to the lack of a suitable finely tunable narrow linewidth source. Moreover, a simple wavelength sweep would not yield conclusive trends for this type of analysis. As the wavelength decreases, the effective mode index increases and the effective mode area shrinks, resulting in reduced modal overlap with the waveguide sidewalls and therefore a decrease in the wavelength dependence of the scattering cross section. A rigorous investigation would thus require simultaneous variation of both wavelength and waveguide cross section, an effort that we believe lies beyond the scope of this work. Another possible method would be to tune the resonance using a thermal phase shifter placed close to the ring. This approach was attempted but yielded unsuccessful results due to crosstalk from the thermal phase shifter, leading to nearly identical extracted values that were not conclusive enough to be presented in a publication.\nFinally, due to the reasons explained above, propagation losses were extracted using a cutback method. While this provides a robust estimate of the average loss, it lacks the precision required for a detailed decomposition of individual loss mechanisms. Unavoidable variations in coupling efficiency arising from differences in alignment quality, facet quality, or grating couplers between waveguides introduce uncertainties that limit the sensitivity of this method for the refined loss analysis being requested.\nThe only additional experimental approach that would be realistically accessible is a systematic study of propagation loss as a function of waveguide width, to evaluate the overall lowest achievable loss. While this could provide indirect insight into scattering related losses, it would require dedicated design, fabrication, and characterization of a large set of structures, and would constitute a substantial new section focused specifically on waveguide loss engineering. Such a study would also be only weakly related to the present manuscript, as the waveguide presented here is strictly single mode. We believe such a study would go beyond the scope of a standard revision and would more naturally form the basis of a separate, focused publication on propagation losses in this platform.\nRefining the fabrication process is currently the focus of our efforts, with the goal of achieving waveguide quality comparable to that routinely demonstrated in lithium niobate photonics on silicon at visible and near infrared wavelengths. Moreover, we believe that replotting the simulated material loss of lithium niobate on sapphire compared to lithium niobate on silicon, as already presented in the referenced work by Mishra et al. (Optica 2021), would not constitute a scientific novelty.\nAs discussed by the reviewer, we agree that attributing the losses to a specific origin without ambiguity requires additional investigation. For this reason, we have rephrased the sentence regarding the origin of the losses in the manuscript, highlighted in purple. In addition, we have cited the papers mentioned by the reviewer. The revised sentence now reads:\n“The losses are hypothesized to be dominated by sidewall roughness [44, 45] and could be further reduced through improved fabrication techniques, as theoretical studies suggest that losses approaching 0.1 dB/cm may be achievable [40]. Moreover, adopting a multimode waveguide geometry could further reduce these losses, since the optical mode becomes less sensitive to scattering at the waveguide boundaries, although this would reduce the efficiency of the presented modulator [46]”\nWe hope this clarifies why a fully predictive loss calculation or a conclusive experimental isolation of scattering loss would represent a significant undertaking. While such an investigation is interesting and relevant on its own, we believe that it goes beyond the scope of demonstrating the capabilities of our modulator on this platform.\nWe thank the editor and the reviewers again for all their comments and for helping to improve the quality of the manuscript."}]

Decision letter