High-Q Nanophotonics over the Full Visible Spectrum Enabled by Hexagonal Boron Nitride Metasurfaces.

Abstract

All-dielectric optical metasurfaces with high quality (Q) factors have been hampered by the lack of simultaneously lossless and high-refractive-index materials over the full visible spectrum. In fact, the use of low-refractive-index materials is unavoidable for extending the spectral coverage due to the inverse correlation between the bandgap energy (and therefore the optical losses) and the refractive index (n). However, for Mie resonant photonics, smaller refractive indicesare associated with reduced Q factors and low mode volume confinement. Here, symmetry-broken quasiboundstates in the continuum (qBICs) areleveragedto efficiently suppress radiation losses from the low-index (n≈2) van der Waals material hexagonal boron nitride (hBN), realizing metasurfaces with high-Q resonances over the complete visible spectrum. The rational use of low- and high-refractive-index materials as resonator components is analyzed and the insights are harnessed to experimentally demonstrate sharp qBIC resonances with Q factors above 300, spanning wavelengths between 400and 1000nm from a single hBN flake. Moreover, the enhanced electric near fields are utilized to demonstrate second-harmonic generation with enhancement factors above 102 . Theseresults provide a theoretical and experimental framework for the implementation of low-refractive-index materials as photonic media for metaoptics.