  Recent studies suggest that an exemplary Kondo insulator SmB6 belongs to a
new class of topological insulators (TIs), in which non-trivial spin-polarized
metallic states emerge on surface upon the formation of Kondo hybridization gap
in the bulk. Remarkably, the bulk resistivity reaches more than 20 Ohm cm at 4
K, making SmB6 a candidate for a so-called bulk-insulating TI. We here
investigate optical-pulse responses of SmB6 by pump-and-probe photoemission
spectroscopy. Surface photovoltage effect is observed below ~90 K. This
indicates that an optically-active band bending region develops beneath the
novel metallic surface upon the bulk-gap evolution. The photovoltaic effect
persists for >200 microsec, which is long enough to be detected by electronics
devices, and could be utilized for optical gating of the novel metallic
surface.
