Polarization-controlled third harmonic generation by quasi-bound states in the continuum in silicon nanopillar metasurfaces

[Display omitted] •The nonlinear optical efficiencies of dielectric nanostructures are generally low.•A dielectric metasurface composed of Si nanorods is designed.•Quasi BICs with high quality-factor are obtained by breaking the symmetries of the metasurface.•More than three-orders-of-magnitude enhancement of THG is achieved.•The polarization-controlled characteristic of the enhanced THG is studied. All-dielectric metasurfaces have received widespread attention due to their low intrinsic loss compared to the metallic counterparts. However, relatively high radiation loss and low quality (Q) factor of the dielectric nanostructures may significantly degrade their nonlinear optical efficiency. Here, we demonstrate that silicon-nanopillar-based metasurfaces supporting quasi bound states in the continuum (BICs) can greatly enhance the efficiency of third harmonic generation (THG). We show that symmetry-protected BICs form at the Γ-point of doubly degenerate band or nondegenerate band can be selectively transformed into quasi BIC through symmetry breaking of our silicon metasurfaces. Benefiting from the high Q and resonance enhancement by the quasi BICs, the THG conversion efficiencies of the silicon metasurface reaches 4.3 × 10-4 with a peak pump intensity of 1.3 GW/cm2, which can be four orders higher than that of a silicon planar film with the same thickness. In addition, the THG intensity is polarization-dependent for the single quasi BIC while it is polarization-independent for the doublet quasi BIC. Our results provide a new strategy for the design of high-efficiency silicon-based optical nonlinear devices.