Optical conductivity of atomic hexagonal boron nitride layers

We present an analytic calculation of the optical conductivity of atomically thin hexagonal boron nitride (h  -BN) layers in the spectral range where this quantity is expected to be dominated by π-electronπ-electron transitions. The approach, which we rely on in this study, is entirely within an independent-particle description of optical response and is based on the tight-binding representation for the π-electronπ-electron energy bands of a single h-BN sheet. The most significant result of our calculation is an explicit closed form, in terms of elementary functions, we have found for the real part of the optical conductivity of the h-BN sheet. This allows the optical absorbance of a few-layer h  -BN films to be easily evaluated within the independent-layer approximation. The absorbance is found to be spectrally quite flat (at a level close to zero) up to the threshold frequency (in ultraviolet) corresponding to the onset of resonant interband transitions. Slightly above the threshold, the absorbance exhibits a pronounced sharp symmetric peak reaching ∼20%∼20% in magnitude. This feature as well as the overall spectral profile of the calculated optical absorbance is in good qualitative agreement with experiment.