<i>μ</i> W-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 <i>μ</i>W range. This compact and scalable design offers an efficient green light source for applications in quantum optics, bio-imaging, and advanced display technology.