  The interaction between light and novel two-dimensional electronic states
holds promise to realize new fundamental physics and optical devices. Here, we
use pump-probe photoemission spectroscopy to study the optically-excited Dirac
surface states in the bulk-insulating topological insulator Bi2Te2Se, and
reveal optical properties that are in sharp contrast to those of bulk-metallic
topological insulators. We observe a gigantic optical life-time exceeding 4
micro-sec for the surface states in Bi2Te2Se, whereas the life-time in most
topological insulators such as Bi2Se3 has been limited to a few picoseconds.
Moreover, we discover a surface photo-voltage in topological materials, a shift
of the chemical potential of the Dirac surface states, as large as 100 mV. Our
results demonstrate a rare platform to study charge excitation and relaxation
in energy and momentum space in a two dimensional quantum system.
