# Hybrid Integrated Photonics Using Bulk Acoustic Resonators

## TL;DR

This paper introduces a novel hybrid integrated photonic platform utilizing bulk acoustic resonators for high-frequency acousto-optic modulation, enabling fast, broadband, and low-power optical tuning on a chip.

## Contribution

First integration of high overtone bulk acoustic wave resonators on a photonic chip for microwave frequency modulation with high linearity and low crosstalk.

## Key findings

- Achieved broadband acoustic modulation at 4.1 and 8.7 GHz
- Demonstrated sub-nanosecond optical resonance tuning
- Enabled on-chip non-reciprocal device potential

## Abstract

Microwave frequency acousto-optic modulation is realized by exciting high overtone bulk acoustic wave resonances (HBAR resonances) in the photonic stack. These confined mechanical stress waves transmit exhibit vertically transmitting, high quality factor (Q) acoustic Fabry Perot resonances that extend into the Gigahertz domain, and offer stress-optical interaction with the optical modes of the microresonator. Although HBAR are ubiquitously used in modern communication, and often exploited in superconducting circuits, this is the first time they have been incorporated on a photonic circuit based chip. The electro-acousto-optical interaction observed within the optical modes exhibits high actuation linearity, low actuation power and negligible crosstalk. Using the electro-acousto-optic interaction, fast optical resonance tuning is achieved with sub-nanosecond transduction time. By removing the silicon backreflection, broadband acoustic modulation at 4.1 and 8.7 GHz is realized with a 3 dB bandwidth of 250 MHz each. The novel hybrid HBAR nanophotonic platform demonstrated here, allowing on chip integration of micron-scale acoustic and photonic resonators, can find immediate applications in tunable microwave photonics, high bandwidth soliton microcomb stabilization, compact opto-electronic oscillators, and in microwave to optical conversion schemes. Moreover the hybrid platform allows implementation of momentum biasing, which allows realization of on chip non-reciprocal devices such as isolators or circulators and topological photonic bandstructures.