Visible Brillouin-quadratic microlaser in a high-Q thin-film lithium niobate microdisk

Narrow-linewidth lasers at short/visible wavelengths are crucial for quantum and atomic applications. However, such lasers are often accessible in bulky tabletop systems and remain scarce in on-chip photonic platforms. Here, we report an on-chip visible Brillouin-quadratic microlaser in a 117-μm-diameter thin-film lithium niobate (TFLN) microdisk via dispersion engineering. Enabled by the ultra-high optical Q factor of 4.0×10⁶, high optical mode density, small mode volume, strong photon-phonon interaction and high second-order nonlinearity of the TFLN microdisk, simultaneous generation of Stokes Brillouin lasing (SBL) with 10.17 GHz Brillouin shift and its second harmonic generation (SHG) are demonstrated, exhibiting a low threshold of only 1.81 mW. Further measurement confirms the intrinsic narrow-linewidths of both the SBL ( ~ 254 Hz) and the SHG ( ~ 864 Hz) along with a normalized frequency-double efficiency of 3.61%/mW. This demonstration of an on-chip ultra-narrow linewidth bi-chromatic Brillouin laser paves the way for on-chip quantum information processing and precise metrology.