Gigahertz-frequency acousto-optic phase modulation of visible light in a CMOS-fabricated photonic circuit

Domain: physics
Published: 2025-12-08
Editorial outcome: major_revision

Abstract
Optical phase modulators operating at visible wavelengths are essential components for photonic technologies such as those for quantum control, communications, and laser ranging. However, they remain challenging to implement in scalable, integrated platforms capable of handling high optical powers. Here we present a visible-light, gigahertz-frequency acousto-optic phase modulator, fabricated on a 200-mm wafer in a volume CMOS foundry, that supports greater than 500 mW of optical power at 730 nm. The device combines a piezoelectric transducer and a photonic waveguide within a single, wavelength-scale structure that confines both a propagating optical mode and an electrically excitable breathing-mode mechanical resonance. By tuning the device’s geometry to optimize the optomechanical interaction, we achieve modulation depths up to 4.85 rad with 80 mW of applied microwave power at 2.31 GHz in a 2-mm-long device. This corresponds to resonant modulation figures of merit of V \pi=1.32 V and V \pi \cdot L=0.26 V cm. To our knowledge, this is the lowest V \pi ever demonstrated in any acousto-optic phase modulator and represents a 15-fold reduction in V \pi and a 100-fold reduction in required microwave power relative to state-of-the-art modulators with high visible-wavelength power handling commonly employed in quantum control systems.

Introduction
Bulk acousto-optic technology has enabled high-extinction pulse carving, single-sideband frequency shifting, beam deflection, and tunable filtering1. However, the devices that provide these critical functions typically consume high power and operate at low frequencies. Recently, on-chip acousto-optics platforms have yielded fast optical switches2, microwave-to-optical transducers3–5, isolators6–8, beam deflectors9, and narrow-band filters10. These devices offer increasingly compact and energy-efficient versions of their traditionally large, high-power-consumption bulk analogs and facilitate higher frequency operation in the gigahertz range1,3,11. Also, because non-vanishing photoelastic coefficients exist regardless of a material’s crystal symmetries, acousto-optic devices can be implemented in a significantly broader class of materials than their electro-optic counterparts12. This flexibility allows for the use of very large scale integration-compatible materials such as silicon nitride (SiNx) that have desirable properties including wide transparency window13, low propagation loss14, high power-handling capability15, and compatibility with complementary metal oxide semiconductor (CMOS) fabrication processes16.

Demonstrations of high-frequency, integrated electro-optic17–20 and acousto-optic21–28 phase and frequency modulators with excellent performance have been implemented in a variety of material platforms. Yet to date, these operate with telecom-band light or else rely on materials that are both incompatible with CMOS foundry processes and incapable of supporting high optical power (see comparison in Supplementary Material Section 1)29. For instance, many of these modulators are based on lithium niobate, for which photorefractive effects set stringent limits on maximum optical power density, especially at visible wavelengths30 (Supplementary Material Section 2.2).

These limitations to power handling and mass manufacturability hinder the scalability of on-chip optical signal processors, which could otherwise allow for simultaneous control of many channels derived from a single, high-power laser. For example, trapped-ion and neutral-atom quantum computers will require thousands or even millions of channels with independent frequency control for each qubit31,32, and gate operations typically require tens of milliwatts of laser power incident on the qubit site33. This means integrated photonic control chips will need to handle watt-level power.

For these reasons and more13, many demonstrations of integrated components for quantum control have been implemented in platforms based on SiNx, which has high power handling at visible wavelengths and is CMOS-compatible. These demonstrations include multi-channel light-delivery systems34,35, chip-to-qubit free-space couplers with focusing diffraction gratings36, polarization rotators37, and reconfigurable Mach-Zehnder interferometers2. The addition of on-chip optical frequency control to this growing library of components would serve as a crucial resource for generating individually addressable, channelized optical signals for Raman gates, cooling beams, and state measurements, all of which require visible-light operation and gigahertz frequency shifts31.

In this work, we present the design and experimental realization of a CMOS-fabricated, resonant acousto-optic phase modulator implemented in a piezo-optomechanical aluminum nitride-SiNx platform. We obtain a strong optomechanical interaction by confining a propagating optical mode and a breathing-mode mechanical resonance to a single, wavelength-scale structure. A piezoelectric actuator embedded within the structure, combined with this optomechanical coupling, enables efficient electromechanically driven acousto-optic phase modulation. Our device harnesses this phase modulation to generate gigahertz-frequency sidebands on a 730-nm laser input. At a 2.31-GHz modulation frequency, we achieve a resonant figure of merit of V \pi=1.32 V with a 2-mm-long device (V \pi \cdot L=0.26 V cm). To the best of our knowledge, this is the lowest V \pi ever demonstrated with an integrated acousto-optic modulator, which is significant because applications like sideband generation typically require large modulation depths, and scaling shorter devices with low V \pi \cdot L to longer lengths entails substantial challenges (Supplementary Material Section 1.2). We demonstrate optical power handling in our platform’s waveguides that exceeds 500 mW (Supplementary Material Section 2.1), and we operate the device with up to 19 dBm of applied microwave power, at which point the modulation depth reaches 4.85 rad. The modulator has a bandwidth, which we define as the frequency range over which the modulation depth is maintained at least half of its maximum value, of 24 MHz around the center frequency of 2.31 GHz. Our device is uniquely suited for scaling optical systems operating with high power at visible wavelengths, such as those for underwater communication38, visible-light laser ranging (LiDAR)39, and quantum control31,40. This is because it employs SiNx waveguides known for their power handling at visible wavelengths, is fabricated on a 200-mm wafer in a CMOS foundry, has a 15x decrease in V \pi and a 100x reduction in required microwave power compared to the state-of-the-art (Supplementary Material Section 1), and is built in a process in which many functions such as high-speed switches2 and tunable narrow-band filters41 already exist. It further lays the foundation for a visible-light, acousto-optics platform capable of supporting a variety of components such as isolators and single-sideband frequency modulators.

Results
Device platform and working principle
Our acousto-optic modulator comprises a photonic waveguide situated directly above a piezoelectric actuator. A SiNx core enclosed by SiO 2 cladding forms the waveguide, and the actuator leverages the dominant d 33 and d 31 piezoelectric coefficients of an aluminum nitride (AlN) film by sandwiching it between two electrode layers (Fig. 1)42. The devices are fabricated in a volume CMOS foundry on a 200-mm wafer, which is then diced into around 120 individual chips (one of these chips is photographed in Fig. 1g, and a subset of the phase modulators it contains are micrographed in Fig. 1f). We utilize etched trenches and a release layer to physically isolate the structure from the wafer substrate and surrounding photonic and piezoelectric layers (Fig. 1e). This inhibits the principal mechanism by which mechanical energy can escape the device and consequently allows sharp mechanical resonances to exist. The AlN’s thickness of 450 nm is chosen to maximize its piezoelectric response while remaining amenable to etching43, and the 710-nm bottom cladding is designed to optimize the tradeoff between strain-overlap with the optical mode and metal-induced propagation losses41. Further details on layer thicknesses and the fabrication process are provided in Supplementary Material Section 11.

We obtain resonantly enhanced optomechanical coupling by confining a breathing-mode mechanical resonance and a propagating optical mode to the same wavelength-scale structure. The coupling results from the photoelastic effect, which describes strain-induced changes to a material’s refractive index, and the moving-boundary effect, which accounts for alterations to a waveguide’s cross section by mechanical displacements at the core-cladding interface44,45. These mechanically driven effects modify the optical mode’s effective refractive index by an amount \Delta n eff. The optical mode then accumulates a phase shift of \Delta \phi = \omega \Delta n eff L/c after propagating through a length, L, over which this index-modification is present, where \omega is the angular frequency of the optical mode and c is the speed of light in vacuum.

We electrically excite resonantly enhanced phase shifts by applying a sinusoidal microwave signal to a device’s piezoelectric transducer at a mechanical resonance frequency, \Omega/2\pi. The index modification then becomes \Delta n eff(t) = \Delta n 0 sin(\Omega t), which produces modulation of the form

E(t) = E 0 exp i[\omega t + \alpha sin(\Omega t)], (1)

where \alpha = \omega \Delta n 0 L/c is the modulation depth resulting from a sinusoidally varying index modification with amplitude \Delta n 0. A phase mismatch between the optical mode and mechanically induced index perturbation constrains the above expression for \alpha to the regime in which the optical traversal time of the modulator is much shorter than a period of the mechanical mode’s oscillation (Supplementary Material Section 3). The device presented in this work has a length of 2 mm that falls within this regime.

Device performance, measured in modulation depth imparted per volt of applied microwave signal, ultimately depends on the entire piezo-optomechanical transduction chain. First, microwave power is transferred from driving electronics to the on-chip device, where the effective voltage drop across the device depends on its electrical impedance matching with the microwave transmission line. Second, the piezoelectric transducer converts some fraction of the delivered microwave power to mechanical energy, the efficiency of which depends on the electromechanical coupling coefficient, k 2 46. Third, as discussed above, the optomechanical coupling between the two modes in question determines the change to the optical propagation constant per unit excited mechanical displacement. Finally, the quality factor, Q, of the excited mechanical mode increases the displacement and strain build-up in the structure and consequently determines the degree to which the modulation is resonantly enhanced.

Taking into account the above, we optimize our modulator’s design by simulating the field profiles of the device’s mechanical resonances and optical modes as a function of the cladding width, w 1, and SiNx width, w 2. From these field profiles, we calculate the photoelastic and moving boundary coupling integrals given by

\Delta \beta pe = \kappa \iint E* \cdot \Delta \epsilon \cdot E dA (2)

\Delta \beta mb = \kappa \oint [u \cdot n^ \Delta \epsilon 12 |E|| |2 - \Delta \epsilon 12 1 |D? |2] dl: (3)

In Eq. (2), the integral is evaluated over a cross-section of the device, E is the optical electric field profile, and \Delta \epsilon is the photoelastically induced perturbation to the permittivity tensor. In Eq. (3), the integral is evaluated along the boundary, n^ is the unit vector normal to the boundary, E|| is the electric field parallel to the boundary, D? is the electric displacement field perpendicular to the boundary, \Delta \epsilon 12 is the difference in permittivity between the SiO 2 and SiNx, and \Delta \epsilon 12 1 is the difference in reciprocal of the permittivity between the SiO 2 and SiNx. In both equations \kappa is a constant that includes normalization factors. The simulated field profiles of a strongly coupled mode-pairing are shown in Fig. 1b-c, and details of the coupling calculation, as well as simulations of k 2 are found in Supplementary Material Section 4. In principle, one should maximize the product of k 2, optomechanical coupling, microwave power transfer efficiency, and mechanical quality factor. However, Q is difficult to accurately simulate and also critically affects the device’s equivalent electrical impedance within a linewidth of a mechanical resonance47. In the absence of reliable models to predict the mechanical Q, we design our devices to mitigate radiative loss to the substrate and surrounding material.

Phase modulators are often used to generate gigahertz-frequency sidebands, and achieving maximum conversion efficiency, defined here as the fraction of optical power converted to one of the first-order sidebands, is thus an important performance benchmark48–50. By rewriting Eq. (1) with the Jacobi-Anger expansion as

E(t) = E 0 \sum n = -\infty \infty J n (\alpha) exp[i(\omega - n\Omega)t], (4)

one can see optical power in the first-order sideband is maximized when J 1(\alpha) itself is maximized, which occurs when \alpha = 1.84 rad and yields a conversion efficiency of 33.9%. A large Q is necessary to obtain maximum optical power in the first-order sideband while maintaining low-voltage operation because the constraints on device length imposed by the phase-matching condition render it impossible to indefinitely elongate the device to achieve larger phase shifts. However, some applications, particularly those for quantum control, require fast on/off time and do not necessarily favor high-Q operation51. This is because increasing the quality factor prolongs the ring-up and decay time, \tau = 2Q/\Omega, of the mechanical mode, which is the switching time of the modulator. Therefore, it is desirable to have strong enough optomechanical and electromechanical coupling so that a device can efficiently produce modulation depths near \alpha = 1.84 rad without necessitating ultra-high quality factors that preclude fast switching times and limit the bandwidth of operation.

Experimental results
We characterize the fabricated devices with the setup shown schematically in Fig. 2c. A laser source is coupled to a fiber interferometer whose upper arm contains the chip on which the devices are fabricated and whose lower arm contains a commercially available acousto-optic frequency shifter (AOFS). In the upper arm, light from the fiber is grating-coupled to and from a waveguide that routes an optical mode through a particular phase modulator device. Meanwhile, light in the lower arm is passed through the AOFS, which shifts its frequency by an amount \Delta/2\pi = 125 MHz. The arms are subsequently recombined on a 50/50 directional coupler and directed to a fast photodetector whose electrical output is connected to a radio-frequency spectrum analyzer (RFSA).

We supply microwave power from a vector network analyzer (VNA) at an angular frequency, \Omega, to the electrodes of the on-chip device (by means of the electrical pads and interconnects imaged in Fig. 2b and graphically depicted in Fig. 1e). This excites a sinusoidal phase modulation process and produces the sidebands of equation (4). These sidebands then interferometrically mix with the light of the lower arm to generate intensity modulation of the form I(t) = I 0 \sum n J n (\alpha) cos[(n\Omega + \Delta)t], where I 0 is a constant determined by the initial laser strength and the various loss mechanisms present in the setup (e.g., grating coupler insertion loss and waveguide propagation loss). An explanation for the AOFS’s presence in the lower arm and a detailed derivation of the intensity signal are provided in Supplementary Material Section 5.1. The microwave power at the angular frequencies n\Omega + \Delta can then be recorded on the RFSA as a function of the driving frequency, \Omega. Since these powers are proportional to J n 2(\alpha), they are proportional to the optical power produced by the on-chip phase modulator at the angular frequency \omega - n\Omega. The plot in Fig. 2a shows these powers for n = 0, 1, 2 when driven at a constant microwave power of 15 mW for the device with a cladding width of w 1 = 1.25 \mu m, core width of w 2 = 500 nm, and length of L = 2 mm. We observe resonantly enhanced modulation at several frequencies, including at 2.31 GHz, at which the optical power in both the first-order and second-order sidebands surpasses that of the remaining carrier, corresponding to appreciable modulation depths.

To precisely determine the modulation depth, we extract it from the ratio of the measured optical power of two distinct sidebands at the microwave frequencies m\Omega + \Delta and n\Omega + \Delta (details of this method and its verification found in Supplementary Material Section 5.2). The calculated modulation depth is plotted in Fig. 3a along with the microwave reflection coefficient, S 11, measured by the VNA. Note that this modulation-depth curve is akin to a typical electro-optic S 21 curve. Dips in the S 11 curve indicate those frequencies at which mechanical resonances are transduced, and they are labeled with their associated quality factors (which are calculated by a fit detailed in Supplementary Material Section 5.3). As expected, the locations of these resonant mechanical dips closely agree with the frequencies at which peaks in modulation depth occur. However, the presence of a strongly excited mechanical mode does not necessarily yield a large modulation depth because the degree to which a given mechanical mode induces changes to the effective refractive index is mediated by both the mechanical quality factor and the strength of the optomechanical coupling. This is evidenced by drive frequencies at which large dips in S 11 appear, yet correspond only to a weak or nonexistent rise in modulation depth. We also observe that the regions of resonantly enhanced modulation depth consist of doublet peaks (see inset of Fig. 3a). This is because the device’s continuous translational symmetry is broken by the periodically placed nanopillar supports along its length (as depicted in Fig. 1e). This discrete translational symmetry makes the device a relatively weak phononic crystal and gives rise to the doublets52.

For the 2.31 GHz resonance, we observe a linear dependence between the modulation depth and voltage amplitude, V, of the applied microwave signal until V \approx 1.91 V (Fig. 3c), past which the device malfunctions. We suspect that the ultimate malfunctioning is caused by dielectric breakdown of the AlN (Supplementary Material Section 5.2)53. Although a modulation depth of nearly 5 rad is sufficient for purposes such as sideband generation, some applications benefit from stronger modulation54, which future work could achieve by elongating the device to reduce V \pi.

Using V \pi = 1.32 V, obtained from a fit to the data in Fig. 3c, to convert from drive power to modulation depth, we theoretically calculate the conversion efficiency to each of the sidebands of a pure phase modulation process and compare this against our experimentally obtained values in Fig. 3b. Excellent agreement is observed until a drive power of 19 dBm, beyond which the device ceases to function. The theoretical calculation predicts slightly higher conversion efficiencies than those observed for the sidebands, but this is explained by frequency-dependent loss in the microwave cables connecting the photoreceiver to the RFSA (Fig. 2c).

We extract a mechanical quality factor of Q = 228 from a fit to the modulation depth data at the 2.31 GHz resonance, implying a fast switching time of 31.4 ns. We perform loss measurements on a ring resonator that has nearly the same cross-section as our modulator and which is fabricated on the same die as it. These measurements indicate the on-chip propagation losses are 0.55 dB cm-1 (Supplementary Material Section 7). The coupling loss is 9.61 dB per grating coupler, which can be improved by optimizing the grating-coupler design or by employing edge couplers. We also measure the on-resonance impedance of the device to be 149 \Omega, which corresponds to 74.3% microwave power coupling. In future work, impedance matching networks could be used to improve the microwave coupling efficiency55. We further check the stability of the device when driven with P 0 = 15 mW of incident microwave power on the resonance at 2.31 GHz, and we find that the average modulation depth remains relatively constant over a two-hour period (Supplementary Material Section 8). We note that during this experiment, the electrical power delivered to the device, which we define here as (1 - S 11) P 0 = 11.15 mW, was constant over the measurement.

Operation at other mechanical frequencies and optical wavelengths
The ability to accurately and precisely design our device to operate at desired, application-specific modulation frequencies would allow for its deployment in a variety of systems. For example, hyperfine-ground-state and Zeeman-split qubits have transition frequencies that depend on the atomic species or applied magnetic field, typically lying between 1 GHz and 13 GHz31,56. Consequently, optical control systems require modulation across this same range. Our device’s mechanical modes determine its operating frequencies, and they primarily depend on material properties, layer thicknesses, and layer widths. The latter of these provides a straightforward knob by which to tune the mechanical resonance frequency because the layer widths are lithographically controlled, and modifying them therefore avoids adjusting any finely tuned fabrication steps.

To demonstrate this capability, we perform piezoelectrically coupled mechanical simulations of our device and track the mechanical resonance frequencies of several mode families as the width of the device is swept from 500 nm to 1.5 \mu m in steps of 25 nm. We also fabricate and characterize the resonance frequencies of devices with four distinct widths: 800 nm, 950 nm, 1.10 \mu m, and 1.25 \mu m (Supplementary Material Section 6.3). Figure 4a shows these measured resonance frequencies superimposed onto the bands we extract from the simulations. The top oxide thickness is the only free parameter in our model because it is the least precisely controlled in the material stack. All other parameters use standard material parameters reported in the literature and the as-designed geometric parameters. The close agreement between the predictions of the simulation and our experimental results confirms the ability to design the device to operate at a given frequency.

The average amount by which the experimentally observed resonance frequencies differ from those predicted by simulation is 37 MHz. Although this amount is larger than the bandwidth of the modulator presented herein, we show in Supplementary Material Section 9 we can control the bandwidth by lithographically modifying the release structure of the devices. This enables us to adjust the expected yield of devices given the degree of precision our model and fabrication provide. To investigate the latter, we collect statistics on several fabricated devices from different dies and wafers and find the standard deviation in resonance frequency is 18 MHz and the standard deviation in V \pi is 0.10 V (Supplementary Material Section 10).

We further show through FEM simulations of the mechanical and optical modes, combined with calculations of the optomechanical coupling integrals (Eqs. (2) and (3)), that our modulator can perform efficiently over the entire visible wavelength range and into the near-infrared. Figure 4b shows the results of these simulations for two mechanical modes calibrated to the device performance experimentally measured at 730 nm (see Supplementary Material Section 6.1 for details). In practice, our fabricated devices are limited by the bandwidth of their grating couplers, so we experimentally confirm the simulations over the 12-nm range over which they efficiently operate (Fig. 4c). For applications requiring broadband optical operation, edge couplers with higher optical bandwidth could be employed57.

Methods
Device fabrication
The devices are fabricated on 200-mm wafers at Sandia National Laboratories. The process begins with a SiO 2 film on a silicon substrate. First, aluminum is deposited and patterned to form a metal routing layer. Next, amorphous silicon (a-Si) and tungsten are deposited and patterned to form a release layer and vertical electrical connections, respectively. Then, the bottom electrode, AlN, and top electrode are sequentially deposited and patterned to form the piezoelectric actuation layers. After this, the optical waveguiding layers are deposited and patterned, which include the lower SiO 2 cladding, SiNx core, and top SiO 2 cladding. With all these layers formed, patterned trenches are etched through all layers until the a-Si is reached. This serves the dual purpose of accessing the a-Si in order to perform the release etch while also generating the approximately micron-wide structures that comprise the phase modulators. Finally, the a-Si is selectively removed through these trenches with a xenon-difluoride etch. A figure depicting this process and more details, including layer thicknesses, is provided in Supplementary Material Section 11.

Measurement details
In order to characterize the phase modulation, the device is driven with a sinusoidal electrical signal produced by a VNA (Keysight P9374A) and delivered to the chip through a radio-frequency probe (FormFactor ACP40-GSG). The VNA also records the microwave reflection coefficient of the device, from which we obtain its equivalent electrical impedance, and the frequencies at which mechanical modes are efficiently excited. We then use laser light at a 730-nm wavelength (Sacher Lasertechnik LION series) to read out the on-chip phase modulation using a heterodyne interferometer (Fig. 2c). We use a high-speed photoreceiver (Newport 1554-A) to convert the optical output to microwave signals that are recorded with an RFSA (Rhode and Schwarz FSV3013). From these data, we extract the ratio, R mn, of the optical power in the mth and nth sidebands of the phase-modulation comb (Fig. 3d). We then fit the modulation depth, \alpha, using

R mn = [J m(\alpha) / J n(\alpha)] 2, (5)

where J m and J n are the mth and nth Bessel functions of the first kind, respectively. Mathematical details and further explanation of this technique are provided in Supplementary Material Section 5.

Simulations
Simulations of the device’s optical modes and mechanical modes are performed with COMSOL Multiphysics FEM software using the Wave Optics, Solid Mechanics, and Piezoelectricity modules.

Data availability
All data that support the conclusions of this study are included in the article and Supplementary Items. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This manuscript demonstrates a novel visible acousto-optic modulator leveraging CMOS-fabricated SiN photonic integrated circuits and AlN as the piezoelectric layer. Efficient modulation at 2.31 GHz with a modulation depth of 2.1 radians was achieved. The figure of merit, 0.26 V·cm, is not so impressive, especially considering that shorter wavelengths typically result in lower VπL. The authors should compare their results with Ref. [1] and [2] to better highlight the advantages of this work. Ref. [1] reported a VπL as low as 0.019 V·cm at 1.16 GHz. Ref. [2] demonstrated 0.03 V·cm at 0.84 GHz, with a phase shift per square root microwave power of 0.4 rad/√mW. Overall, this visible acousto-optic modulator presents promising potential for scalable qubit control. The manuscript includes detailed measurements of AO modulation and presents solid theoretical modeling. I believe this manuscript could continue under review at Nature Communications, provided the authors emphasize their innovations more clearly through a comprehensive comparison with other state-of-the-art AOMs and address the following points:\n1. The manuscript provides limited information about the AlN actuator design. For instance, why was 450 nm AlN chosen, the metal selection and 200-nm air gap? Optimization strategies should be discussed in detail.\n2. The measured on-chip and coupling losses should be included in the main text.\n3. What are the key factors for increasing the saturation voltage? This is crucial for achieving higher modulation depth and needs further discussion.\n4. The reported quality factor is 228. Are there design-level improvements—within the CMOS process described—that could help enhance the Q factor?\n5. Further increasing the operating frequency to match different types of qubits requires adjusting the thickness of the piezoelectric actuator, which may not be flexible to implement on this CMOS platform. How can precise frequency tuning be achieved to align with specific qubit transition frequencies in the future? A wider bandwidth could also be beneficial in this context. Any promising solutions should be discussed.\n[1] A. E. Hassanien, et al., \"Efficient and wideband acousto-optic modulation on thin-film lithium niobate for microwave-to-photonic conversion,\" Photonics Res. 9, 1182-1190 (2021).\n[2] L. Wan, et al., \"Highly efficient acousto-optic modulation using nonsuspended thin-film lithium niobate-chalcogenide hybrid waveguides,\" Light: Science & Applications 11, 145 (2022).", "reviewer_2": "In this paper entitled \"Gigahertz-Frequency Acousto-Optic Phase Modulation of Visible Light in a CMOS-Fabricated Photonic Circuit\", the authors have demonstrated an efficient, CMOS-compatible, visible-light, acousto-optic modulator (AOM) using mature MEMS and CMOS fabrication processes. By optimizing the geometry for strong optomechanical interaction, the 2-mm-long device achieves a modulation depth exceeding 2 rad with only 15 mW of microwave power at 2.31 GHz. Overall I find this work interesting and timing. I believe it has potential to be published ultimately at Nat. Communications. Before a final decision is made, I would like to ask the authors to address the following comments.\n1. One key advantage of piezoelectric AOMs is that, it employs resonant acoustic confinement to enhance photon-phonon coupling strength. However, it sacrifices the bandwidth, and the best modulation efficiency is achieved at a particular frequency (in this work, 2.31 GHz). For specific applications that operate at specific microwave frequency, e.g. atomic clocks and optic-to-microwave conversion with superconducting qubits, the resonance frequency of the piezoelectric AOMs must well align with the application frequency. I’d like to ask the authors to describe and discuss, how well they can control the AMO frequency to a specific value that matches the target application frequency? What determines the AMO resonant frequency (e.g. AlN thickness, material stress, waveguide width etc)? How does the AMO frequency vary over the entire 8-inch wafers? This description / discussion highlights the maturity of the technology for quantum applications, as well as the challenges to address in the future.\n2. When a device is made, is there any approach to tune the AOM frequency experimentally? How much is the tuning range?\n3. One issue I notice is that, when keeping injecting microwave power over a long time, the device will naturally get heated. How does this thermal issue affect the drift of operation point?\n4. Can the authors measure or calculate the power consumption when continuously operating the device at 2.31 GHz? Does this device, with mechanical effect, suffers from fatigue issues?\n5. It is still better to define a modulation bandwidth here.\n6. The manuscript will benefit if an SEM image showing the actually structure and material stack, like Fig 1a or 1d, is included.\n7. The readers may wonder, why use AlCu to make the electrode for AlN? Especially, Cu is poisonous to CMOS foundries.\n8. How the nanopillars supporting the structure are fabricated? Can the authors add description?\n9. What is the measured optical loss in the SiN waveguide?\nOther minor points:\n10. In the abstract, the authors state that \"by tuning the device's geometry... this corresponds to a modulation figure of merit of V ·L = 0.26 V·cm... that can be π straightforwardly extended to a wide range of optical wavelengths and modulation frequencies.\" This is partial as the low V ·L is achieved only at a specific π mechanical resonance frequency (2.31 GHz, Q=228). If the authors insist to have this statement in the abstract, they should add this info.\n11. The authors state that \"this represents a 15× decrease in V and a 100× decrease π in required microwave power compared to the commercial state-of-the-art.\" I suggest to add a table in the SI to quantitatively compare the current AOMs with other visible-light integrated modulators (e.g., LN-based and AlN-based platforms).\n12. I am somehow not convinced by the derivation of Supplementary Equations S3 to S8, as it assumes a traveling-wave modulator configuration. However, the acoustic wave propagates normally to the chip surface rather than as a guided traveling wave along the waveguide axis. Additionally, the n should specifically represent the group refractive index of light, not the effective refractive index. Can the authors carefully check?\n13. I would like to learn the authors’ opinion on the material choice. They use AlN, but are there better materials? For example, how about ScAlN with even larger k2?\n14. A recent review article on piezoelectric modulation on integrated photonics, [H. Tian et al. Piezoelectric actuation for integrated photonics. Adv. Opt. Photon. 16, 749 (2024)], seems to be relevant to this work.", "reviewer_3": "This manuscript presents an efficient, visible-light, gigahertz-frequency acousto-optic modulator fabricated on a 200 mm wafer in a volume CMOS foundry. The device integrates a piezoelectric transducer and a photonic waveguide within a single microstructure, enabling simultaneous optical and mechanical mode confinement. The authors demonstrate a modulation depth exceeding 2 rad with 15 mW applied microwave power at 2.31 GHz in a 2 mm-long device, achieving a modulation figure of merit of Vπ ·L = 0.26 V∙cm. Although the work makes a step forward an efficient high-frequency modulation efficiency in an acousto-optic modulator, the concept is not new. Additionally, there are several key concerns needed to be addressed:\n1) The manuscript does not sufficiently distinguish the current waveguide structure from the authors’ previous work (Ref. 43). In addition, a detailed comparison is required to fully compare the modulator performance metrics in terms of size, bandwidth, modulation depth and operation optical wavelength with recently reported works.\n2) The resonant enhancement relies on mechanical resonance, inherently restricting operation to fixed frequencies. This significantly limits tunability, a critical feature for practical applications. Additionally, Fig. 3(a) shows seemingly random resonance frequencies and inconsistent modulation depths, raising concerns about reproducibility and predictability. The cause is unclear in the manuscript.\n3) While simulations suggest operation across 400–1000 nm wavelengths and mechanical resonances spanning 1–5 GHz, experimental validation is lacking. Provide experimental results demonstrating performance at different wavelengths and microwave frequencies, as these are critical modulator parameters.\n4) In the introduction, a key advantage of the proposed modulator is that a high optical power of the incident optical signal is allowed. It seems in the experimental demonstation there is few evidence on this point.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to referees for the manuscript “Gigahertz-frequency acousto-optic modulation of visible light in a CMOS-fabricated photonic circuit”\nWe thank the referees for their careful reading of our manuscript and for their detailed and constructive reviews. Our manuscript has substantially improved because of their suggestions. In the following, we present a point-by-point response to each referee’s report. The original text of the referees’ comments is given in italicized black, our replies are given in blue, and changes made to the manuscript are described in green.\n[... (Full rebuttal text provided in source) ...]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thanks to the authors for their thorough response to the questions raised in my initial review. I believe the manuscript has been significantly improved based on these revisions, and I recommend the manuscript for publication.", "reviewer_2": "My review report PDF file is uploaded below", "reviewer_3": "I appreciate the authors’ efforts to address my previous concerns. After carefully reviewing the revised manuscript, I find that several issues still need to be addressed.\nThe revised manuscript claims that the device is uniquely suited for scaling optical systems operating with high power at visible wavelengths such as those for underwater communication, visible light laser ranging (LiDAR). However, the proposed phase modulator exhibits a relatively narrow modulation bandwidth, which appears to be a significant limitation for high-speed communications and frequency-modulated continuous-wave (FMCW) LiDAR applications. The authors should clarify why the proposed device remains competitive under these conditions.\nThe introduction states that integrated photonic control chips will need to handle watt-level optical power. Although in the discussion optical power handling that exceeds 500mW is discussed, no direct evidence is provided in the revised manuscript to demonstrate that the proposed chip can operate under watt-level optical signals. I recommend including supporting data or analysis to substantiate this claim.\nBased on my understanding, the authors' previous work (P. R. Stanfield et al., Optics Express 27, 28588 (2019)) used a similar silicon nitride waveguide structure and piezoelectric transducer. It would be highly beneficial to include a cross-sectional view of both devices side by side to facilitate a clearer comparison. This would help readers better appreciate the novelty and significance of the current work relative to the earlier study.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to referees for the manuscript “Gigahertz-frequency acousto-optic modulation of visible light in a CMOS-fabricated photonic circuit”\nWe thank the referees for their careful reading of our revised manuscript and for their detailed and constructive reviews. In the following, we present a point-by-point response to each referee’s report. The original text of the referees’ comments is given in italicized black, our replies are given in blue, and changes made to the manuscript are described in green.\n[... (Full rebuttal text provided in source) ...]"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have adequately addressed my further comments. I have no more question. I believe the current version can be accepted as it is.", "reviewer_3": "The authors have addressed my concerns and I think the manuscript is ready for the publication.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter