Enhanced sum-frequency generation from etchless lithium niobate empowered by dual quasi-bound states in the continuum

The miniaturization of nonlinear light sources is central to the integrated photonic platform,driving a quest for high-efficiency frequency generation and mixing at the nanoscale.In this quest,the high-quality(Q)resonant dielectric nanostructures hold great promise,as they enhance nonlinear effects through the resonantly local electromagnetic fields overlapping the chosen nonlinear materials.Here,we propose a method for the enhanced sum-frequency generation(SFG)from etcheless lithium niobate(LiNbO_(3))by utilizing the dual quasi-bound states in the continuum(quasi-BICs)in a one-dimensional resonant grating waveguide structure.Two high-Q guided mode resonances corresponding to the dual quasi-BICs are respectively excited by two near-infrared input beams,generating a strong visible SFG signal with a remarkably high conversion efficiency of 3.66×10^(-2)(five orders of magnitude higher than that of LiNbO_(3)films of the same thickness)and a small full-width at half-maximum less than 0.2 nm.The SFG efficiency can be tuned via adjusting the grating geometry parameter or choosing the input beam polarization combination.Furthermore,the generated SFG signal can be maintained at a fixed wavelength without the appreciable loss of efficiency by selectively exciting the angle-dependent quasi-BICs,even if the wavelengths of input beams are tuned within a broad spectral range.Our results provide a simple but robust paradigm of high-efficiency frequency conversion on an easy-fabricated platform,which may find applications in nonlinear light sources and quantum photonics.

High-Q micro-ring resonators and grating couplers for silicon-on-insulator integrated photonic circuits

An ultra-small integrated photonic circuit has been proposed,which incorporates a high-quality-factor passive micro-ring resonator（MR） linked to a vertical grating coupler on a standard silicon-on-insulator（SOI） substrate.The experimental results demonstrate that the MR propagation loss is 0.532 dB/cm with a 10μm radius ring resonator,the intrinsic quality factor is as high as 202.000,the waveguide grating wavelength response curve is a 1 dB bandwidth of 40 nm at 1540 nm telecommunication wavelengths,and the measured fiber-to-fiber coupling loss is 10 dB.Furthermore,the resonator wavelength temperature dependence of the 450 nm wide micro-ring resonator is 54.1 pm/℃.Such vertical grating coupler and low loss MR-integrated components greatly promote a key element in biosensors and high-speed interconnect communication applications.