Observation of Spontaneous Brillouin Cooling

TL;DR

This paper demonstrates for the first time that forward Brillouin scattering in microresonators can be used to cool mechanical vibrations, expanding the understanding of acousto-optical interactions beyond traditional amplification.

Contribution

The study experimentally shows Brillouin cooling in a microresonator, revealing a new regime of operation for Brillouin processes that enables mechanical cooling.

Findings

Demonstrated Brillouin cooling in a microresonator.

Identified two regimes: amplification and cooling.

Showed optical pump and scattered light attenuate mechanical motion.

Abstract

While radiation-pressure cooling is well known, the Brillouin scattering of light from sound is considered an acousto-optical amplification-only process. It was suggested that cooling could be possible in multi-resonance Brillouin systems when phonons experience lower damping than light. However, this regime was not accessible in traditional Brillouin systems since backscattering enforces high acoustical frequencies associated with high mechanical damping. Recently, forward Brillouin scattering in microcavities has allowed access to low-frequency acoustical modes where mechanical dissipation is lower than optical dissipation, in accordance with the requirements for cooling. Here we experimentally demonstrate cooling via such a forward Brillouin process in a microresonator. We show two regimes of operation for the Brillouin process: acoustical amplification as is traditional, but also for the first time, a Brillouin cooling regime. Cooling is mediated by an optical pump, and scattered light, that beat and electrostrictively attenuate the Brownian motion of the mechanical mode.