Hexagonal boron nitride (hBN) is a van der Waals material with exceptional optical properties, making it a cornerstone for advanced photonic and quantum applications. Its wide bandgap, strong phonon-polariton response, and hyperbolic dispersion distinguish it from conventional dielectrics and semiconductors. These characteristics enable unique functionalities in deep-UV optics, quantum light sources, and nanophotonic systems.
One of the most notable features of hBN is its ultraviolet (UV) transparency. With a bandgap exceeding 6 eV, hBN exhibits high transparency in the deep-UV range, below 200 nm. This property is rare among solid-state materials, as most oxides and nitrides absorb strongly in this spectral region. The transparency arises from the absence of electronic transitions within this energy range, allowing hBN to serve as an ideal optical material for deep-UV lenses, windows, and coatings. Experimental studies using UV-Vis spectroscopy confirm negligible absorption coefficients below 6 eV, making hBN suitable for applications such as UV lithography and solar-blind photodetectors.
Beyond its electronic transitions, hBN exhibits strong phonon-polariton modes in the mid-infrared (IR) region. Phonon-polaritons are hybrid quasiparticles resulting from the coupling of photons with optical phonons. In hBN, the anisotropic crystal structure leads to hyperbolic dispersion, where the dielectric permittivity is negative along one crystal axis and positive along another. This property allows for the propagation of hyperbolic phonon-polaritons (HPhPs), which exhibit ultra-confined electromagnetic fields with wavelengths significantly shorter than free-space photons. Near-field microscopy techniques, such as scattering-type scanning near-field optical microscopy (s-SNOM), have been used to visualize these polaritons, revealing their highly directional propagation and sub-diffractional confinement. The Reststrahlen band of hBN, spanning approximately 12 to 15 µm, is where these effects are most pronounced, enabling applications in super-resolution imaging and subwavelength light manipulation.
The hyperbolic dispersion of hBN also facilitates novel nanophotonic phenomena. Unlike isotropic materials, hyperbolic media support high-k wavevectors, enabling enhanced light-matter interactions at the nanoscale. This property is exploited in hyperlenses, which break the diffraction limit by converting evanescent waves into propagating waves. Experimental demonstrations using hBN-based hyperlenses have achieved sub-100 nm resolution in the mid-IR range. Additionally, hBN’s low optical losses compared to metallic hyperbolic materials make it advantageous for polariton waveguides and resonant cavities.
In quantum photonics, hBN is a host for bright, room-temperature quantum emitters. These emitters, often attributed to defects or impurities within the hBN lattice, exhibit narrowband photoluminescence in the visible to near-IR range. Cathodoluminescence (CL) spectroscopy has been instrumental in identifying these emitters, revealing zero-phonon lines with linewidths as narrow as a few meV. The quantum emitters in hBN demonstrate high brightness and photostability, making them promising candidates for single-photon sources in quantum communication and computing. Furthermore, the ability to integrate these emitters with photonic structures, such as plasmonic nanocavities or dielectric resonators, enhances their emission properties through Purcell effects.
As a cladding layer in nanophotonics, hBN provides several advantages. Its atomically smooth surface minimizes scattering losses, while its chemical inertness protects underlying materials from degradation. In plasmonic systems, hBN acts as a spacer or encapsulation layer, reducing ohmic losses in metallic nanostructures. For example, in graphene plasmonics, hBN substrates and overlayers significantly improve plasmon propagation lengths by screening charge impurities and surface phonons. The low dielectric loss of hBN at telecommunication wavelengths also makes it suitable for hybrid photonic-integrated circuits, where it can isolate optical modes while maintaining low crosstalk.
Experimental techniques such as electron energy-loss spectroscopy (EELS) and Fourier-transform infrared (FTIR) spectroscopy have been employed to probe hBN’s optical responses with high spatial and spectral resolution. EELS measurements in transmission electron microscopes reveal the energy-momentum dispersion of hBN’s phonon-polaritons, confirming their hyperbolic nature. FTIR spectroscopy, combined with polarized light, provides insights into the anisotropic dielectric functions of hBN, which are critical for designing polaritonic devices.
In summary, hBN’s optical properties span a broad spectral range, from the deep-UV to the mid-IR, driven by its wide bandgap, hyperbolic phonon-polaritons, and anisotropic dispersion. These attributes enable diverse applications, including UV optics, quantum emitters, and nanophotonic components. Advanced characterization techniques continue to uncover new phenomena in hBN, solidifying its role as a versatile material for next-generation optical technologies.