Low threshold Brillouin lasing with hybrid modes of coupled silica microtoroids

We experimentally demonstrated a stimulated Brillouin laser with hybrid modes, utilizing two coupled silica microtoroid cavities. The first cavity consists of paired modes with a frequency difference close, but not exactly equal, to its Brillouin frequency shift. The second cavity has a resonant mode that is close to one of the modes of the first cavity. The strong coupling between the two similar frequency modes induces mode splitting, resulting in the generation of a hybrid paired mode. This hybrid mode comprises one eigenmode from the first cavity and the super-modes of the coupled microtoroids. By finely tuning the coupling strength to match the frequency difference between the paired modes and the Brillouin shift, we achieve Brillouin lasing. Furthermore, the offset of the frequency shifting in the hybrid modes configuration is much smaller than the Brillouin frequency shift, significantly reducing coupling loss and enabling the realization of a lowthreshold Brillouin laser. We experimentally observed a lasing threshold as low as 0.45 mW, which is two orders of magnitude lower than that of the direct super-modes frequency matching method. This novel approach relaxes the strict requirement of exact frequency matching conditions for Brillouin lasing, making it an excellent platform for compact and ultra-low threshold Brillouin lasers.