Low-index dielectric metasurfaces supported by metallic substrates for efficient second-harmonic generation in the blue-ultraviolet range.

Despite the great importance of high-index materials for dielectric nanophotonics, their optical functionalities are significantly limited for diverse photonic applications and thus, the usability of low-index materials should be explored. This work proposes the use of metallic substrates for low-index dielectric metasurfaces for significantly enhancing the local field and their optical responses. Plasmon-assisted dipole resonances mainly contribute to field enhancement in dielectric nanoparticles comprising the metasurfaces, where the intensity enhancement increases on decreasing the index of the nanoparticles when supported by metallic substrates. Another challenge with the current high-index materials is strong optical losses in the blue-ultraviolet range, which limit their practical applications such as harmonic generations in this spectral range. For a pump with a peak intensity of about 3.4 GW cm-2, a metasurface of lithium niobate nanodisk array supported by a gold substrate generates second harmonic at 400 nm with an efficiency of about 5 × 10-2%, which is one order of magnitude higher than the previously reported efficiency of harmonic generation in this range. The results presented in this work promise the significant extension of the current nonlinear nanophotonics, which is limited to high-index semiconducting materials.