Design of a quasi-2D photonic crystal optomechanical cavity with tunable, large $x^2$-coupling

TL;DR

This paper designs a quasi-2D photonic crystal cavity with tunable, large quadratic optomechanical coupling, enabling enhanced control of light-mechanical interactions for potential quantum applications.

Contribution

It introduces a novel photonic crystal cavity design with tunable slot sizes that achieve large, controllable quadratic optomechanical coupling in the optical and mechanical frequency range.

Findings

Achieves a per-phonon x^2-coupling rate of 1 kHz.

Demonstrates tuning of cavity splitting to enhance coupling.

Shows strong interaction between optical supermodes and mechanical resonances.

Abstract

We present the optical and mechanical design of a mechanically compliant quasi-two-dimensional photonic crystal cavity formed from thin-film silicon in which a pair of linear nanoscale slots are used to create two coupled high-$Q$ optical resonances. The optical cavity supermodes, whose frequencies are designed to lie in the $1500$~nm wavelength band, are shown to interact strongly with mechanical resonances of the structure whose frequencies range from a few MHz to a few GHz. Depending upon the symmetry of the mechanical modes and the symmetry of the slot sizes, we show that the optomechanical coupling between the optical supermodes can be either linear or quadratic in the mechanical displacement amplitude. Tuning of the nanoscale slot size is also shown to adjust the magnitude and sign of the cavity supermode splitting $2J$, enabling near-resonant motional scattering between the two optical supermodes and greatly enhancing the $x^2$-coupling strength. Specifically, for the fundamental flexural mode of the central nanobeam of the structure at $10$~MHz the per-phonon linear cross-mode coupling rate is calculated to be $\tilde{g}_{+-}/2\pi = 1$~MHz, corresponding to a per-phonon $x^2$-coupling rate of $\tilde{g}'/2\pi=1$~kHz for a mode splitting $2J/2\pi = 1$~GHz which is greater than the radiation-limited supermode linewidths.