  We analytically calculate the intrinsic longitudinal and transverse optical
conductivities of electronic systems which govern by a modified-Dirac fermion
model Hamiltonian for materials beyond graphene such as monolayer MoS$_2$ and
ultrathin film of the topological insulator. We analyze the effect of a
topological term in the Hamiltonian on the optical conductivity and
transmittance. We show that the optical response enhances in the non-trivial
phase of the ultrathin film of the topological insulator and the optical Hall
conductivity changes sign at transition from trivial to non-trivial phases
which has significant consequences on a circular polarization and optical
absorption of the system.
