biostudies-literatureLi XMinistry of Science and Technology of the People’s Republic of ChinaNational Natural Science Foundation of ChinaNatural Science Foundation of Guangdong ProvinceMinistry of Science and Technology of the People's Republic of China (Chinese Ministry of Science and Technology)Guangdong Department of Science and TechnologyMinistry of Education of the People's Republic of China (Ministry of Education of China)Natural Science Foundation of Guangdong Province (Guangdong Natural Science Foundation)National Natural Science Foundation of China (National Science Foundation of China)Ministry of Education of the People’s Republic of China317https://www.ebi.ac.uk/biostudies/studies/S-EPMC9622896biostudies-literatureUnknown11(1)Integrated photonics provides unprecedented opportunities to pursue advanced nonlinear light sources with low-power consumptions and small footprints in a scalable manner, such as microcombs, chip-scale optical parametric oscillators and integrated quantum light sources. Among a variety of nonlinear optical processes, high-efficiency second harmonic generation (SHG) on-chip is particularly appealing and yet challenging. In this work, we present efficient SHG in highly engineerable semi-nonlinear waveguides consisting of electron-beam resist waveguides and thin-film silicon nitride (SiN)/lithium niobate (LN). By carefully designing octave-separating bound states in the continuum (BICs) for the nonlinear interacting waves in such a hybrid structure, we have simultaneously optimized the losses for both fundamental frequency (FF) and second harmonic (SH) waves and achieved modal phasing matching and maximized the nonlinear modal overlap between the FF and SH waves, which results in an experimental conversion efficiency up to 4.05% W^-1cm^-2. Our work provides a versatile and fabrication-friendly platform to explore on-chip nonlinear optical processes with high efficiency in the context of nanophotonics and quantum optics.Light, science & applicationsEfficient second harmonic generation by harnessing bound states in the continuum in semi-nonlinear etchless lithium niobate waveguides.PMC96228962022B151502006722lgqb3262035017118744372021YFA1400803Li XLiu JWei DMa JLiu SChen BHuang PfalseEfficient second harmonic generation by harnessing bound states in the continuum in semi-nonlinear etchless lithium niobate waveguides.Integrated photonics provides unprecedented opportunities to pursue advanced nonlinear light sources with low-power consumptions and small footprints in a scalable manner, such as microcombs, chip-scale optical parametric oscillators and integrated quantum light sources. Among a variety of nonlinear optical processes, high-efficiency second harmonic generation (SHG) on-chip is particularly appealing and yet challenging. In this work, we present efficient SHG in highly engineerable semi-nonlinear waveguides consisting of electron-beam resist waveguides and thin-film silicon nitride (SiN)/lithium niobate (LN). By carefully designing octave-separating bound states in the continuum (BICs) for the nonlinear interacting waves in such a hybrid structure, we have simultaneously optimized the losses for both fundamental frequency (FF) and second harmonic (SH) waves and achieved modal phasing matching and maximized the nonlinear modal overlap between the FF and SH waves, which results in an experimental conversion efficiency up to 4.05% W^-1cm^-2. Our work provides a versatile and fabrication-friendly platform to explore on-chip nonlinear optical processes with high efficiency in the context of nanophotonics and quantum optics.2022-01-01T00:00:00Z2022 Nov2025-04-19T02:35:38.362Z2025-04-07T12:59:38.229ZS-EPMC96228963631630610.1038/s41377-022-01017-x