Brillouin lasing in coupled silica toroid microcavities

Stimulated Brillouin scattering (SBS) is a well-known nonlinear process in which two optical waves interact via an acoustic wave. The acoustic wave is generated via the electrostriction process. SBS has recently received a lot of attention because it can be employed for low-noise lasers, microwave synthesizers, slowing light and light storage. Whispering-gallery mode (WGM) microcavities with high Q factors and small mode volumes enable us to generate SBS with low threshold power. In this research, it is necessary to match the free spectral range (FSR) to the Brillouin frequency shift (tens of GHz) by using mm-scale cavities or to prepare high-order transverse WGMs that separate the Brillouin frequency shift to generate SBS [1-3]. With both approaches, it is very difficult to fabricate microcavities for SBS because we need to control the size of the microcavities precisely. The coupled cavities enable mode separation to be freely tuned, and this allows us to avoid the need for precise control of the cavity size. Coupled silica toroid microcavities have already been used to amplify optomechanical oscillation [4], but this approach has yet to be expanded to Brillouin lasing. In this work, we demonstrated SBS in coupled silica toroid microcavities experimentally for the first time.