  A global trend to miniaturization and multiwavelength performance of
nanophotonic devices drives research on novel phenomena, such as bound states
in the continuum and Mietronics, as well as the survey for high-refractive
index and strongly anisotropic materials and metasurfaces. Hexagonal boron
nitride (hBN) is one of promising materials for the future nanophotonics owing
to its inherent anisotropy and prospects of high-quality monocrystals growth
with atomically flat surface. Here, we present highly accurate optical
constants of hBN in the broad wavelength range of 250-1700 nm combining the
imaging ellipsometry measurements scanning near-field optical microscopy and
first-principle quantum mechanical computations. hBN's high refractive index,
up to 2.75 in ultraviolet (UV) and visible range, broadband birefringence of
0.7, and negligible optical losses make it an outstanding material for UV and
visible range photonics. Based on our measurement results, we propose and
design novel optical elements: handedness-preserving mirrors and subwavelength
waveguides with dimensions of 40 nm operating in the visible and UV range,
respectively. Remarkably, our results offer unique opportunity to bridge the
size-gap between photonics and electronics.
