  Using circular dichroism-angle resolved photoemission spectroscopy
(CD-ARPES), we report a study of the effect of angular momentum transfer
between polarized photons and topological surface states on the surface of
highly bulk insulating topological insulator Bi2Te2Se. The photoelectron
dichroism is found to be strongly modulated by the frequency of the helical
photons including a dramatic sign-flip. Our results suggest that the observed
dichroism and its sign-flip are consequences of strong coupling between the
photon field and the spin-orbit nature of the Dirac modes on the surface. Our
studies reveal the intrinsic dichroic behavior of topological surface states
and point toward the potential utility of bulk insulating topological
insulators in device applications.
