  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.
