Efficient frequency conversion in dielectric metasurfaces supporting both surface lattice resonances and quasi-bound states in the continuum

For efficient coherent generation at various wavelengths from nonlinear metasurfaces, we need to generate multiple high- resonances with single metasurfaces. In this report, we theoretically show simultaneous generation of high- surface lattice resonances (SLRs) and quasi-bound states in the continuum (quasi-BICs) in single dielectric metasurfaces composed of symmetry-broken unit cells of GaP nanorods surrounded by silica glass. Quite interestingly, there are quasi-BIC with double asymmetry parameters and also the resonance with duality between SLR and symmetry breaking-induced quasi-BIC, in addition to simple SLRs and quasi-BICs. For different polarizations of incident waves, one can obtain multiple high- SLRs and quasi-BICs, which can be used for efficient generation of coherent radiations in the range from visible to mid-infrared by second-harmonic generation (SHG), sum-frequency generation (SFG), and difference-frequency generation (DFG). The numerical results show that mid-infrared radiation at around 14 μm can be directly generated from the dielectric metasurfaces by DFG with an efficiency of about 1.4 × 10 % for pump intensities of 20 MW/cm by pumping at the resonant wavelengths of SLR and quasi-BIC. Besides efficient frequency conversions, coexisting high- SLRs and quasi-BICs in dielectric metasurfaces can also be employed for various photonic applications such as sensitive sensing.