Natural optical anisotropy of h-BN: Highest giant birefringence in a bulk crystal through the mid-infrared to ultraviolet range

The giant birefringence of layered h-BN was demonstrated by analyzing the interference patterns in reflectance and transmittance measurements in the mid-infrared to the deep ultraviolet energy range. The refractive index for polarization perpendicular to the $c$ axis is much higher than the refractive index for polarization parallel to the $c$ axis, and it displays a strong increase in the ultraviolet range that is attributed to the huge excitonic effects arising from the unique electronic structure of h-BN. Thus, h-BN is shown to exhibit a giant negative birefringence that ranges from $\ensuremath{-}0.7$ in the visible to $\ensuremath{-}2$ in the deep ultraviolet close to the band gap. The electronic dielectric constants for polarization perpendicular and parallel to the $c$ axis were determined to be ${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}^{\ensuremath{\perp}}=4.95$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\infty}}^{\ensuremath{\parallel}}=2.86$, respectively. The anisotropy we find in high-quality h-BN is significantly larger than proposed in previous experimental studies but in excellent agreement with ab initio calculations.