Moon, Sunghyun Kim, Hansol Lee, Junhyung Kim, Changhyun Choi, Minho Jang, Hyeong-soon Bae, Hyeonjun Hwang, Hyeon Seo, Min-kyo Jung, Hojoong Kwon, Hyounghan 2025-12-23T02:00:14Z 2025-12-23T02:00:14Z 2025-12-19 2025-12 1094-4087 https://pubs.kist.re.kr/handle/201004/153838 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. English Optical Society of America μW-level green light generation via quasi- and modal-phase-matched lithium niobate waveguides Article 10.1364/OE.577082 1 Optics Express, v.33, no.24, pp.50922 - 50936 Optics Express 33 24 50922 50936 Y scie scopus 001633755600002 2-s2.0-105022938700 Optics Optics Article EMISSION FIBER LASER 2ND-HARMONIC GENERATION CONVERSION EFFICIENCY