High-power-handling ultra-compact acousto-optic modulators using one-dimensional topological interface states on thin-film lithium tantalate

Ultra-Compact, High-Power Acousto-Optic Modulator Design Leveraging Topological States in Thin-Film Lithium Tantalate

Context and Motivation

Integrated photonic devices increasingly demand high-efficiency, compact acousto-optic (AO) modulators with robust power-handling for dense integration and stable operation in high-power optical environments. Existing bulk-type AO modulators based on materials like quartz and lithium niobate (LN) are limited by their size and compatibility with photonic integrated circuits (PICs). While thin-film lithium niobate (TFLN) has enabled devices with excellent modulation metrics and efficient surface acoustic wave (SAW) generation, its photorefractive instability and susceptibility to optical damage under high power restrict performance, particularly in non-suspended architectures.

Thin-film lithium tantalate (TFLT) emerges as a promising alternative due to its mature manufacturing ecosystem, lower birefringence, suppressed parasitic effects, and higher optical damage threshold. The paper "High-power-handling ultra-compact acousto-optic modulators using one-dimensional topological interface states on thin-film lithium tantalate" (2604.13925) proposes and experimentally demonstrates an integrated AO modulator on TFLT, utilizing 1D topological interface states to achieve ultra-compact footprint, high modulation efficiency, and unprecedented power-handling capabilities.

Device Architecture and Physical Principles

The modulator design is anchored in a 1D topological photonic crystal (1D-TPC) nanobeam cavity, inspired by the Su-Schrieffer-Heeger (SSH) model. The cavity is formed by joining two distinct photonic crystal lattices with quantized Zak phases, yielding a robust, spectrally isolated topological boundary state (TBS) within the bandgap. This configuration provides high optical confinement and immunity to structural perturbations.

The device is fabricated on a commercial X-cut TFLT wafer (600 nm lithium tantalate atop 9 μm oxide), featuring a ridge waveguide with deep etch and precise air-hole lattices. An interdigital transducer (IDT) launches Rayleigh SAWs, efficiently modulating the localized cavity mode via the photoelastic and piezoelectric effects. The SAWs propagate along the crystal surface, with the primary acoustic strain component optimized at 0.54 GHz, maximizing photon-phonon overlap and minimizing substrate leakage at higher frequencies.

Experimental Implementation and Characterization

The device achieves a footprint of $130 \times 120$ μm² and a half-wave voltage-length product ($V_{\pi}L$) of $0.491$ V·cm for the 120 μm modulation region. Transmission experiments reveal a sharp cavity resonance at $1543.5$ nm with an extinction ratio of 17 dB, insertion loss of 1.53 dB, and an intrinsic quality factor $Q$ of 4244. The IDT exhibits an acoustic $Q$ of 413 at 0.544 GHz, with 83.1% RF power loading.

Microwave-to-optical conversion (S21 response) peaks at 0.544 GHz under a 0 dBm RF drive, with a signal-to-noise ratio exceeding 25 dB. The device demonstrates modulation efficiency comparable to top-tier non-suspended TFLN devices and substantially smaller footprint than both Mach-Zehnder interferometer (MZI) and microring-based AO modulators, which require ancillary components that inflate size.

High-Power Modulation and Stability Evaluation

In high-power experiments, stable AO modulation is sustained at on-chip optical powers up to 28 dBm (630.9 mW), eclipsing prior integrated AO devices by nearly two orders of magnitude in power handling. The resonance wavelength remains unwavering across input powers, while linewidth broadening is attributed to nonlinearities (multi-photon absorption, nonlinear refractive effects) inherent to LT under intense field enhancement.

AO modulation metrics ($V_{\pi}L$, S21) remained unchanged with power scaling, confirming suppression of photorefractive effects and robust mechanical and thermal stability. Continuous 30-minute tests at 28 dBm indicated wavelength drift <0.6 dB and SNR maintained above 25 dB, validating long-term modulatory integrity.

The superior power-handling arises from TFLT’s intrinsic material properties—higher photon-induced displacement resistance, stronger Ta–O bonds, and lower defect densities. The device’s true optical damage threshold exceeds the maximum tested power, limited by the experimental laser/EDFA saturation.

Implications and Future Directions

This study closes a critical gap in AO modulator technology, integrating topological photonic confinement with high-power operation on a scalable, manufacturable TFLT platform. The demonstrated combination of efficiency (0.49 V·cm), minimal area, and high optical power tolerance directly enables robust, multifunctional microwave-to-photonics transduction for photonic signal processing, lidar, optical computing, and quantum information modules.

The architectural advantages of TFLT and topological cavity engineering foresee further reductions in $V_{\pi}L$ (potentially <0.1 V·cm), supporting nonlinear photonic circuits and high-capacity analog photonic links. Enhanced piezoelectric and cavity designs could realize broadband modulation and further diminish thermal/noise limitations, driving adoption in AI accelerators, satellite communication, and integrated photonics domains.

Conclusion

The paper establishes the first experimental demonstration of high-performance, ultra-compact, high-power-handling AO modulation in thin-film lithium tantalate. By engineering topologically protected cavity states, the device achieves efficient, bias-stable modulation at optical powers far in excess of conventional platforms. TFLT-based topological architectures provide a scalable, robust solution for next-generation integrated photonic systems requiring microwave-to-photonic conversion in demanding, high-power environments, setting the stage for continued advances in photonic integration and signal processing (2604.13925).