Hexagonal boron nitride nanophotonics: a record-breaking material for the ultraviolet and visible spectral ranges.

A global trend towards miniaturization and multiwavelength performance of nanophotonic devices drives research on novel phenomena, such as bound states in the continuum and Mietronics, as well as surveys for high-refractive index and strongly anisotropic materials and metasurfaces. Hexagonal boron nitride (hBN) is one of the promising materials for future nanophotonics owing to its inherent anisotropy and prospects of high-quality monocrystal growth with an atomically flat surface. Here, we present highly accurate optical constants of hBN in the broad wavelength range of 250-1700 nm combining imaging ellipsometry measurements, scanning near-field optical microscopy and first-principles quantum mechanical computations. hBN's high refractive index, up to 2.75 in the 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 ranges, respectively. Remarkably, our results offer a unique opportunity to bridge the size gap between photonics and electronics.

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics.

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy has been recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This issue inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Waals interaction. To do this, we made correlative far- and near-field characterizations validated by first-principle calculations that reveal a huge birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this remarkable anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.