# Dueling Dynamical Backaction in a Cryogenic Optomechanical Cavity

## TL;DR

This paper investigates a cryogenic optomechanical cavity where radiation pressure and photothermal forces compete, revealing novel self-oscillation behavior and potential for ground-state cooling.

## Contribution

It demonstrates the coexistence of opposing dynamical backaction forces in a silicon optomechanical cavity and models their nonlinear interplay.

## Key findings

- Photothermal force induces self-oscillations contrary to radiation-pressure expectations.
- Hysteretic response fitted with a nonlinear model reveals cavity properties.
- Device operates in a regime suitable for quantum ground-state cooling.

## Abstract

Dynamical backaction has proven to be a versatile tool in cavity optomechanics, allowing for precise manipulation of a mechanical resonator's motion using confined optical photons. In this work, we present measurements of a silicon whispering-gallery-mode optomechanical cavity where backaction originates from opposing radiation pressure and photothermal forces, with the former dictating the optomechanical spring effect and the latter governing the optomechanical damping. At high enough optical input powers, we show that the photothermal force drives the mechanical resonator into self-oscillations for a pump beam detuned to the lower-frequency side of the optical resonance, contrary to what one would expect for a radiation-pressure-dominated optomechanical device. Using a fully nonlinear model, we fit the hysteretic response of the optomechanical cavity to extract its properties, demonstrating that this non-sideband-resolved device exists in a regime where photothermal damping could be used to cool its motion to the quantum ground state.