Observation of spontaneous Brillouin cooling

Although bolometricand 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 process1–7. It was suggested that cooling could be possible in multiresonance Brillouin systems5–8 when phonons experience lower damping than light8. However, this regime was not accessible in electrostrictive Brillouin systems1–3,5,6 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. Spontaneous Raman and Brillouin scattering are common to almost any media. Incident photons are annihilated in these processes, giving rise to scattered photons at redder Stokes or bluer anti-Stokes frequencies. These scattering events lead to the creation or annihilation of phonons, respectively. Although previous research in optomechanics has used bolometric forces9–11, centrifugal radiation pressure12–18 and optical gradient force19–21 to excite mechanical motion of device boundaries, it is only recently that electrostrictive Brillouin scattering5–7 was demonstrated in microcavities. As electrostriction is one aspect of radiation pressure, extensive work in radiation pressure cooling13–18 raises the question of whether it is possible to cool phonon modes by means of a Brillouin process, similar to the scattering process available in bulk media1. For achieving Brillouin anti-Stokes cooling, the heating Stokes line needs to be eliminated. However, as low acoustic frequencies separate the pump line from the Stokes and anti-Stokes lines, filtering out the Stokes line against the anti-Stokes line requires a rapid transmission change over an extremely small frequency difference, which is not easily available in bulk materials. Here we transform the energy flow direction in spontaneous Brillouin scattering using two discrete optical resonances separated by the mechanical-resonance frequency. This allows the Brillouin cooling process to gain efficiency and selectivity by pumping at the lower resonance and emitting anti-Stokes photons at the upper resonance. This is different from systems with a single optical resonance, where coolingwas achieved by detuning the pumpbelow the resonance by a distance equal to the frequency of the mechanical mode22. Furthermore, in contrast to boundary-deforming