Angularly tunable second harmonic generation from a structural-symmetry-broken silicon metasurface with bound states in the continuum

Second-harmonic generation (SHG) is usually suppressed in centrosymmetric materials such as silicon, due to their intrinsic inversion symmetry. Here, we report a significantly enhanced SHG from an amorphous silicon metasurface with symmetry-broken unit cells through accidental bound states in the continuum (BICs). Our experiments show a 40-fold increase of SHG compared to a flat silicon film. The accidental BIC mode can be modified by tuning the incident angle of the pump laser beam from 35° to 50°. Accordingly, a SHG output is achieved from 370.0 nm to 382.5 nm. Our results demonstrate an intriguing strategy for enhancing SHG by adjusting the incident angle of the fundamental beam.