Bound states in the continuum (BIC) to achieve highly efficient frequency conversion using high quality-factor (high-Q) metasurfaces have been demonstrated using symmetry-broken structures with high robustness; however, the breaking-symmetry tactics are typically limited to one of the dimensions of the structures, which restricts the nonlinearity with BIC. In this work, we present a new metasurface structure in the form of an array of unit cells composed of two identical nano-bars with two mirror-symmetric corners cut into each nano-bar to break this limit. By using the high refractive index and large third-order nonlinearity of amorphous silicon (a-Si), we demonstrate ultra-high theoretical Qs up to ~ 2×10^5. Owing to the large nearfield enhancement in the meta-atoms, we observe optical Kerr effect in efficient third harmonic generation from the a-Si BIC metasurfaces via different levels of pump power, which paves the way for variational quasi-BIC for switchable nonlinear generation.
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