  Using time- and angle-resolved photoemission, we investigate the ultrafast
response of excited electrons in the ternary topological insulator (Bi$_{1
x}$Sb$_{x}$)$_2$Te$_3$ to fs-infrared pulses. We demonstrate that at the
critical concentration $x$=0.55, where the system becomes bulk insulating, a
surface voltage can be driven at room temperature through the topological
surface state solely by optical means. We further show that such a photovoltage
persists over a time scale that exceeds $\sim$6 $\mu$s, i.e, much longer than
the characteristic relaxation times of bulk states. We attribute the origin of
the photovoltage to a laser-induced band-bending effect which emerges near the
surface region on ultrafast time scales. The photovoltage is also accompanied
by a remarkable increase in the relaxation times of excited states as compared
to undoped topological insulators. Our findings are relevant in the context of
applications of topological surface states in future optical devices.
