On-Chip Electro-optically Tunable Narrow Linewidth Brillouin Microlasers Implemented in Thin Film Lithium Niobate

On-chip narrow linewidth microlasers with real-time wavelength tunability are highly desirable for various applications, including precision metrology, quantum technology, and coherent information processing. Although significant progress has been made by various groups in recent years [M. Li et al., Nat. Commun. 13, 5344 (2022)NCAOBW2041-172310.1038/s41467-022-33101-6; V. Snigirev et al., Nature (London) 615, 411 (2023)NATUAS0028-083610.1038/s41586-023-05724-2; X. Zhang et al., Appl. Phys. Lett. 124, 131101 (2024)APPLAB0003-695110.1063/5.0195628; S. Gundavarapu et al., Nat. Photonics 13, 60 (2019)NPAHBY1749-488510.1038/s41566-018-0313-2], realizing such microlasers, especially within compact microresonators with a small footprint, remains a challenge. In this Letter, we overcome these hurdles and demonstrate on-chip electro-optically tunable Brillouin microlasers in compact lithium niobate on insulator microdisks with diameters of 31.5 and 117.0  μm by utilizing cross-polarized stimulated Brillouin scattering configuration. A quasicontinuum band of bound shear mechanical modes inside the suspended microdisk is revealed, allowing feasible phase matching of stimulated Brillouin lasing. We achieve efficient cross-polarized optomechanical coupling and Brillouin lasing via the significant photoelastic tensors of lithium niobate (e.g., p_{41}=-0.151). This approach yields a 118-Hz short-term integral linewidth, a 0.16-Hz intrinsic linewidth, and a comparatively low threshold power of 3.15 mW. A real-time electro-optic tuning with a tuning efficiency of ∼93.1  kHz/V is also achieved, further showcasing potential of the lithium niobate on insulator platform for next-generation tunable photonic systems.