  Silicene (a monolayer of silicon atoms) is a topological insulator, which
undergoes a topological phase transition to a band insulator under an external
electric field. The spin polarization is unique and opposite at the K and K'
points due to the spin-orbit coupling. Accordingly, silicene exhibits a strong
circular dichroism with respect to optical absorption, obeying a certain
spin-valley selection rule. It is remarkable that this selection rule is
drastically different between these two types of insulators owing to a band
inversion taking place at the phase transition point. Hence we can tell
experimentally whether silicene is in the topological or band insulator phase
by circular dichroism. Furthermore the selection rule enables us to excite
electrons with definite spin and valley indices by optical absorption.
Photo-induced current is spin polarized, where the spin direction is different
between the topological and band insulators. It is useful for future
spintronics applications.
