Layer-dependent dielectric permittivity of topological insulator Bi2Se3 thin films

Dielectric properties and their evolutions are fundamental to reveal the optical responses of topological insulators and to guide the design of related devices. Here, we systematically investigate the layer-dependent dielectric permittivity of Bi2Se3 over an ultra-broad spectral range (0.73-6.43 eV) by combining the spectroscopic ellipsometry, energy-loss function (ELF), critical point (CP) analysis, and density function theory. Results demonstrate that the epsilon-near-zero point of the real permittivity and the ELF peak exhibit blue shifts, indicating the resonance frequencies of surface plasmon modes move towards the higher energy, due to the enhanced hybridization between the opposite surfaces. The joint density of states dominates the layer-dependent increase in imaginary permittivity as the thickness increases. We also find that the CP transition positions in band structures are layer-independent, and the CP center energies exhibit red shifts resulted by the reduced surface state gap with the thickness increasing.