Strain Tuning of the Optical Absorbance in Topological Insulator Material Bismuth Selenide

Bismuth selenide is among the first materials discovered to be a three-dimensional topological insulator (TI) [Zhang et al., Nat. Phys. 5, 438 - 442 (2009)]. The topology of the bandstructure engenders electronic surface states located at the boundary of the material with linear dispersion within the bulk band gap. Similar to graphene TIs show great potential for optoelectronic applications [Zhang et al., PRB 82, 245107 (2010)]. We present calculations of the optical absorbance using a k.p model and Fermi's golden rule including all symmetry allowed strain terms to first order. The absorbance edge for a thin film can be effectively tuned by applying uniaxial strain, and for large strains a Van Hove singularity leads to a diverging band edge absorbance. Shear strain breaks the isotropy of the model leading to a polarization dependent absorbance. Remarkably the directional average is always found to increase, independent of the material parameters