  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 $3.66\times10^{-2}$ (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.
