Nonlinear optomechanical paddle nanocavities

Nonlinear cavity optomechanics is a promising platform for realizing experiments which reveal the quantum properties of nanomechanical structures. Here we introduce an optomechanical system combining strong nonlinear optomechanical coupling, low mass and large optical mode spacing. This nanoscale “paddle nanocavity” supports mechanical resonances with hundreds of fg mass which couple nonlinearly to optical modes with a single photon quadratic optomechanical coupling rate that is four orders of magnitude higher than competing systems. This coupling relies on strong interactions between the nanocavity optical field and mechanical excitations in a structure whose optical mode spectrum is highly non-degenerate. Nonlinear optomechanical readout of thermally driven motion in these devices should be observable for temperatures above 50 mK, and measurement of phonon shot noise is achievable. This work shows that strong nonlinear effects can be realized without relying on coupling between nearly degenerate optical modes, thus avoiding parasitic linear coupling present in two mode systems.