  All-dielectric optical metasurfaces with high quality (Q) factors have so far
been hampered by the lack of simultaneously lossless and high refractive index
(RI) materials over the full visible spectrum. To achieve broad spectral
coverage, the use of low-index materials is, in fact, unavoidable due to the
inverse correlation between the band-gap energy (and therefore the optical
losses) and the RI. However, for Mie resonant photonics, smaller RIs are
associated with reduced Q factors and mode volume confinement. In this work, we
leverage symmetry-broken bound states in the continuum (BICs) to 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. In particular, we analyze the rational use
of low and high RI materials as resonator components and harness our insights
to experimentally demonstrate sharp BIC resonances with Q factors above 300,
spanning wavelengths between 400 nm and 1000 nm from a single hBN flake.
Moreover, we utilize the enhanced electric near-fields to demonstrate second
harmonic generation (SHG) with enhancement factors above 102. Our results
provide a theoretical and experimental framework for the implementation of low
RI materials as photonic media for metaoptics.
