Observation of spontaneous Brillouin cooling

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 process ^1-7. It was suggested that cooling could be possible in multi-resonance Brillouin systems ^5-8 when phonons experience lower damping than light ⁸. However, this regime was not accessible in electrostrictive Brillouin systems ^1-3,5,6 as backscattering enforces high acoustical frequencies associated with high mechanical damping ¹. Recently, forward Brillouin scattering ³ in microcavities ⁷ has allowed access to low-frequency acoustical modes where mechanical dissipation is lower than optical dissipation, in accordance with the requirements for cooling ⁸. 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.