Electro-optically tunable Brillouin microlasers on lithium niobate platform

Abstract: Miniature, monolithic, tunable sources of coherent radiation with narrow linewidths and low noise are highly sought after for photonic integration. Brillouin lasers, known for their intrinsically narrow linewidths, present an appealing solution. However, achieving the strong optomechanical coupling necessary for lasing in optical microresonators remains challenging due to the difficulty of simultaneously confining optical and acoustic modes with substantial modal overlap. Additionally, most Brillouin lasers lack tunability. Lithium niobate (LN) emerges as an excellent platform for integrated photonics, offering large photoelastic coefficients and electro-optic tunability. In this work, we demonstrate strong optomechanical interactions in a 117-um-diameter LN microdisk resonator, achieved through post-fabrication dispersion management and enhanced mode matching enabled by the large off-diagonal elements of the photoelastic tensor. This approach yields an impressive optomechanical coupling rate of up to 30 kHz per photon, resulting in a tunable, integrated Brillouin microlaser with an intrinsic linewidth of 118 Hz and a low lasing threshold of 3.15 mW -- a remarkable performance for such a compact device. Furthermore, we demonstrate electro-optic tuning of the microlaser with an efficiency of 43.2 kHz/V, showcasing the versatility of LN for next-generation tunable photonic systems.