# Lithium Niobate Piezo-optomechanical Crystals

## TL;DR

This paper demonstrates a lithium niobate photonic crystal resonator that efficiently couples light, mechanical motion, and microwaves, advancing integrated quantum photonics and optomechanics.

## Contribution

It introduces a high-Q lithium niobate device supporting optical, mechanical, and microwave coupling with significant optomechanical interaction and phonon lasing capabilities.

## Key findings

- Achieved optomechanical coupling rate g0/2π ≈ 120 kHz
- Demonstrated phonon lasing with cooperativity C > 1
- Measured quantum coupling efficiency η ≈ 10^{-8}

## Abstract

Demonstrating a device that efficiently connects light, motion, and microwaves is an outstanding challenge in classical and quantum photonics. We make significant progress in this direction by demonstrating a photonic crystal resonator on thin-film lithium niobate (LN) that simultaneously supports high-$Q$ optical and mechanical modes, and where the mechanical modes are coupled piezoelectrically to microwaves. For optomechanical coupling, we leverage the photoelastic effect in LN by optimizing the device parameters to realize coupling rates $g_0/2\pi\approx 120~\textrm{kHz}$. An optomechanical cooperativity $C>1$ is achieved leading to phonon lasing. Electrodes on the nanoresonator piezoelectrically drive mechanical waves on the beam that are then read out optically allowing direct observation of the phononic bandgap. Quantum coupling efficiency of $\eta\approx10^{-8}$ from the input microwave port to the localized mechanical resonance is measured. Improvements of the microwave circuit and electrode geometry can increase this efficiency and bring integrated ultra-low-power modulators and quantum microwave-to-optical converters closer to reality.