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Bound states in the continuum (BICs), a concept from quantum mechanics, areubiquitous physical phenomena where waves will be completely locked insidephysical systems without energy leaky. Such a physical phenomenon in opticswill provide a platform for optical mode confinement to strengthen local fieldenhancement in nonlinear optics. Here we utilize an optical system consistingof asymmetric nanogratings and waveguide of thin film lithium niobate (LiNbO3)material to enhance second harmonic response near BICs. By breaking thesymmetry of grating periodicity, we realize strong local field confined insidewaveguide up to 25 times normalized to incident field (with dissymmetric factorof 0.2), allowing strong light-matter interaction in nonlinear material. Fromthe numerical simulation, we theoretically demonstrate that such an opticalsystem can greatly enhance second harmonic intensity enhancement of about 104compared with undersigned LiNbO3 film and conversion efficiency reaching1.53e-5 for dissymmetric factor=0.2 under illumination of 1.33 GW/(suqare cm).Surprisingly, we can predict that a giant enhancement of second harmonicconversion efficiency will exceed 8.13e-5 for dissymmetric factor=0.1 when theoptical system is extremely close to BICs. We believe that such an opticalsystem to trap local field inside is also accessible to promote the applicationof thin film lithium niobate in the field of integrated nonlinear optics.

Highly Efficient Second Harmonic Generation of Thin Film Lithium Niobate Nanograting near Bound States in the Continuum