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 −0.7 in the visible to −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 e∞⊥ = 4.95 and e∞∥ = 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.