Optical conductivity of ultrathin Floquet topological insulators

Abstract

In this paper, we theoretically investigate the topological aspects of a 3D ultrathin topological insulator in the presence of an irradiated off-resonant circularly-polarized light field. Based on the Floquet formalism we treat the light field as an external perturbation which introduces an effective energy term in the system Hamiltonian and hence, gives mass to the massless Dirac fermions at the surfaces of the Floquet topological insulators (FTIs). By controlling the strength of the off-resonant light field we manipulate the interplay between the hybridization energies of the top and bottom surface states (SSs), and the effective energies induced by the irradiated light field to open and close band gaps of the SSs Dirac electrons. This gives rise to topologically nontrivial and trivial phases. We calculate the longitudinal and transverse conductivities of the FTI thin film and show that these strongly depend on the strength of the irradiated light field. The behavior of conductivities signatures TQPTs between distinct topological insulating phases.