Dispersion properties in the visible range of carrier concentration of topologically protected Bi1-xSex films revealed by spectroscopic ellipsometry

Optical properties evolution of Bi1-xSex films with different compositions were investigated by spectroscopic ellipsometry (SE). A significant dispersion of penetration depth of Bi0.38Se0.62 films was observed, which would lead to a varying carrier concentration with the different wavelength because of the topologically protected surface state. To describe the special properties of topological insulators, dispersive plasma energy was introduced into traditional dielectric function model. Optical properties of Bi0.38Se0.62 film were acquired by this modified model and the topologically protected surface state could be represented from the dispersion properties of free carrier concentration with the smaller plasma energy versus the deeper penetration depth. We demonstrated that SE is a useful tool for characterizing the properties of the topological insulators.