  The surface bound electronic states of three-dimensional topological
insulators, as well as the edge states in two-dimensional topological
insulators, are investigated in the presence of a circularly polarized light.
The strong coupling between electrons and photons is found t o give rise to an
energy gap as well as a unique energy dispersion of the dressed states,
different from both graphene and conventional two-dimensional electron gas
(2DEG). The effects of electron-photon interaction, barrier height and width on
the electron tunneling through a p-n junction and on the ballistic conductance
in topological insulators are demonstrated by numerical calculations. A
critical energy for an incident electron to tunnel perfectly through a barrier
is predicted, where electrons behave as either massless Dirac-like or massive
Schrodinger-like particles above or below this threshold value. Additionally,
these effects are compared with those in zigzag graphene nanoribbons and a
2DEG. Both the similarities and the differences are demonstrated and explained.
