Broadband Enhancement of Third Harmonic Generation Based on the Strong Coupling Between all-dielectric Metasurfaces and Epsilon-Near-Zero Modes

A hybrid metasurface incorporating amorphous silicon nanocubes on an ultrathin Indium Tin Oxide (ITO) film is designed to investigate the coupling among Mie resonance, quasi-bound states in the continuum (QBICs), and the epsilon-near-zero (ENZ) guided mode. A more efficient coupling is found to take place between the Mie-ENZ interaction than the QBIC-ENZ coupling, which aligns well with the critical coupling condition and is confirmed through the near-field analysis. The coupled mode theory analysis indicates the Mie-ENZ coupling belongs to the strong coupling regime and results in a large measured Rabi-splitting energy of 190 meV. Such hybrid metasurface leads to an ultra-broadband third harmonic (TH) enhancement with a measured conversion efficiency beyond 10⁻⁷ from 1230 to 1600 nm. Over than 10-fold TH enhancement is observed especially in the blue light range (410–450 nm) when comparing the THG signals between the coupled and uncoupled system. Studies pave the way for diverse applications such as broadband light sources, wide range dielectric spectroscopy, and high-resolution imaging with extended spectral coverage.