High-Efficiency Acousto-Optic Modulation on Non-Suspended Thin-Film Lithium Tantalate

Abstract

Acousto-optic (AO) interactions provide a powerful interface between the microwave and optical domains, enabling functionalities such as optical switching, non-reciprocal propagation and efficient microwave-to-optical transduction. Integrated demonstrations to date have largely relied on thin-film lithium niobate (TFLN), which offers strong piezoelectric response and low optical loss performance. Here, we establish lithium tantalate on insulator (LTOI) as a scalable platform for integrated acousto-optics. LTOI combines intrinsically low birefringence, high optical damage threshold, strong electro-optic and Kerr nonlinearities, and superior acoustic quality factors with a mature high-volume manufacturing base. We demonstrate for the first time acousto-optic modulation on the LTOI platform. By exploiting the anisotropy of surface acoustic waves, we reveal a direct correlation between acousto-optic modulation efficiency and the electromechanical coupling coefficient of lithium tantalate. In particular, acoustic excitation along the crystal Z-axis enhances the higher-order R1 mode, yielding the highest modulation efficiency. Our Mach-Zehnder interferometers achieve a modulation efficiency of 0.68 $\mathrm{\mathbf{V \cdot cm}}$, while racetrack resonators reach 0.022 $\mathrm{\mathbf{V \cdot cm}}$ -representing, to the best of our knowledge, the lowest $\mathrm{V_πL}$ demonstrated in non-suspended ferroelectric platforms. This record performance directly enables microwave-to-optical conversion without suspended structures, establishing LTOI as a robust and scalable platform for integrated acousto-optics with broad applications in communications, signal processing, and quantum information technologies.

Figures (5)

• Figure 1: Design and simulation. (a) Schematic layouts of AO modulators. Light is coupled to/from the chip by inverse taper (green). An SAW is launched by the IDT (yellow), and the amplitude is enhanced in cavity formed by the reflectors. Cross-sectional renderings of the optical and acoustic fields are also shown. The spacing between the MZI arms is designed to match the anti-symmetric acoustic phase condition for push-pull modulation. (b) Obtained Y11 curve from FEM simulation of acoustic resonator on LTOI. The Y11 magnitude is normalized to 1 Siemens (0 dB = 1 S). (c) Static $k^2$ of X-cut LT, $k_{11}$ represents $k^2$ of Rayleigh wave, $k_{16}$ represents $k^2$ of shear horizontal wave. (d) Effective $k^2$ of Rayleigh type SAW R0 varies with propagation angle. (e) Effective $k^2$ of Rayleigh type SAW R1 varies with propagation angle. (f) Effective $k^2$ of Rayleigh type SAW SH0 varies with propagation angle.
• Figure 2: Characterization of the AO MZI. (a) Simplified experimental schematic. CTL, continuously tunable laser; PD, photodetector; The sensitivity of the PD is 22.5 $\mathrm{V/W}$; VNA, vector network analyzer. (b) Microscope image of the fabricated MZI AO device. (c) Optical transmission of the AO MZI. (d) Measured acoustic $\mathrm{S_{11}}$ and opto-acoustic $\mathrm{S_{21}}$ spectra.
• Figure 3: Characterization of the anisotropy of AO modulation in Lithium Tantalate. (a) Measured $\mathrm{S_{11}}$ and $\mathrm{S_{21}}$ spectrum of R0 mode when $\theta$ = -60$\degree$. (b) Measured $\mathrm{S_{11}}$ and $\mathrm{S_{21}}$ spectrum of R0 mode when $\theta$ = -30$\degree$. (c) Measured $\mathrm{S_{11}}$ and $\mathrm{S_{21}}$ spectrum of R0 mode when $\theta$ = 0$\degree$. (d) Measured $\mathrm{S_{11}}$ and $\mathrm{S_{21}}$ spectrum when $\theta$ = 30$\degree$. (e) Measured $\mathrm{S_{11}}$ and $\mathrm{S_{21}}$ spectrum when $\theta$ = 60$\degree$. (f) Measured $\mathrm{S_{11}}$ and $\mathrm{S_{21}}$ spectrum when $\theta$ = 90$\degree$. (g) Measured $\mathrm{V_\pi L}$ and $k^2$ varies with $\theta$ of R0 mode. (h) Measured $\mathrm{V_\pi L}$ and $k^2$ varies with $\theta$ of R1 mode.
• Figure 4: Characterization of the AO racetrack resonator. (a) Simplified experimental schematic. CTL, continuously tunable laser; PD, photodetector; VNA, vector network analyzer; OSC, oscilloscope. (b) $\mathrm{S_{11}}$ reflection spectrum of the acoustic resonator. (c) Measured acoustic $\mathrm{S_{11}}$ and opto-acoustic $\mathrm{S_{21}}$ spectra from the same setup as that employed for the MZI AO modulators. (d) Optical transmission varies with the detuning of racetrack cavity. (e) $\mathrm{S_{21}}$ varies with the detuning of racetrack cavity. (f) $V_\pi$ varies with the detuning of racetrack cavity.
• Figure 5: Demonstration of a microwave-photonic link. (a) Simplified experimental schematic. CTL, continuously tunable laser; PD, photodetector; SG, signal generator; OSC, oscilloscope; ESA, spectrum analyzer. (b) Measured acoustic $\mathrm{S_{11}}$ and opto-acoustic $\mathrm{S_{21}}$ spectra from the same setup as that employed for the MZI AO modulators. (c) Microwave spectrum of the PD output signal with a microwave power of 16 dBm applied to the interdigital transducer of acoustic resonator. The laser is blue-detuned. (d) Optical transmission of the racetrack cavity for different microwave powers. The emerging multiple dips represent the coupling of acousto-optic modulation sidebands into the optical cavity modes.