Observation of spontaneous Brillouin cooling

Abstract

A novel mechanism for cooling tiny mechanical resonators is now demonstrated. Inelastic scattering of light from phonons in an electrostrictive material attenuates the Brownian motion of the mechanical mode. Although bolometric- and ponderomotive-induced deflection of device boundaries are widely used for laser cooling, the electrostrictive Brillouin scattering of light from sound was considered an acousto-optical amplification-only process1,2,3,4,5,6,7. It was suggested that cooling could be possible in multi-resonance Brillouin systems5,6,7,8 when phonons experience lower damping than light8. However, this regime was not accessible in electrostrictive Brillouin systems1,2,3,5,6 as backscattering enforces high acoustical frequencies associated with high mechanical damping1. Recently, forward Brillouin scattering3 in microcavities7 has allowed access to low-frequency acoustical modes where mechanical dissipation is lower than optical dissipation, in accordance with the requirements for cooling8. Here we experimentally demonstrate cooling via such a forward Brillouin process in a microresonator. We show two regimes of operation for the electrostrictive Brillouin process: acoustical amplification as is traditional and an electrostrictive Brillouin cooling regime. Cooling is mediated by resonant light in one pumped optical mode, and spontaneously scattered resonant light in one anti-Stokes optical mode, that beat and electrostrictively attenuate the Brownian motion of the mechanical mode.