Magnetic toroidal dipole resonances for efficient THz generation in dielectric metasurfaces

Nonlinear dielectric metasurfaces represent a rapidly advancing family of optical devices characterized by high nonlinear conversion efficiency with an ultrathin form factor. Here, we report on terahertz (THz) emission through difference-frequency generation (DFG) in an Aluminum Gallium Arsenide (AlGaAs) metasurface supporting magnetic toroidal dipolar resonances at the two pump wavelengths, which nonlinearly interact with each other. The simultaneous excitation of these high-Q resonances, together with the dispersive behavior of the nonlinear susceptibility, enables an exceptionally strong DFG response. Conversion efficiencies around 8 THz exceed those of an unstructured AlGaAs slab of the same thickness by over nine orders of magnitude. These results highlight the potential of DFG metasurfaces as compact and highly efficient platforms for THz generation.