Nearly vertically emitted nonlinear light sources with high efficiencies enabled by high-Q nearly-degenerate polygon modes

Vertically emitted integrated light sources, typically constrained to a narrow bandwidth, play a pivotal role in optical displays, telecommunications, integrated quantum information processing, and terahertz photonics. Achieving transversely emitted nonlinear light sources with high efficiency is essential for extending the integrated coherent emission wavelength and facilitating the interface between free space optics and photonic integrated circuits. However, this remains challenging due to the difficulty in transverse phase matching. In this work, nearly vertically emitted light signal is observed in a thin-film lithium niobate (TFLN) microresonator by exciting high-Q nearly-degenerate square modes. Such modes are formed in the TFLN microresonator by introducing weak perturbation by a tapered fiber, significantly enhancing Rayleigh backscattering of the forward pump light, and simultaneously facilitating nonlinear interactions with high mode field overlap factors. Under pumping the high-Q square mode with a unidirectionally injected continuous-wave laser at 1555 nm, bi-directionally propagating stimulated Raman scattering (SRS) signals are generated at 1721 nm, achieving threshold powers as low as 0.43 mW and high conversion efficiencies up to 32%. Furthermore, a nearly vertically emitted nonlinear light source at 817 nm is demonstrated via transverse-phase matched sum-frequency generation between the pump and SRS signals in the counterpropagating configuration, attaining a conversion efficiency of 0.272%/mW. This transverse-phase matching nonlinear interaction in counterpropagating configuration not only effectively extends the spectral coverage of vertically emitted integrated light sources, but also advances the development of the integrated classical and quantum light sources.