μW-level green light generation via quasi- and modal-phase-matched lithium niobate waveguides

Nonlinear frequency conversion in thin-film lithium niobate (TFLN) is a promising method for visible light generation. In particular, efficient green light generation remains challenging due to the lack of direct gain media, making nonlinear frequency conversion a viable alternative. Here, we demonstrate an efficient approach that combines modal phase-matching (MPM) and quasi-phase-matching (QPM) in a single TFLN waveguide with a single poling period, enabling green light generation under continuous-wave (CW) pumping. Second harmonic generation (SHG) occurs between fundamental transverse electric (TE) modes, while third harmonic generation (THG) is phase-matched with the higher-order TE₂₀ mode, verified by mode profile measurements. Our device achieves normalized conversion efficiencies of 795%W^-1cm^-2 (SHG) and 2,775%W^-2cm^-4 (THG), with on-chip SHG power reaching the mW level and THG power in the μW range. This compact and scalable design offers an efficient green light source for applications in quantum optics, bio-imaging, and advanced display technology.