  Topological insulators (TIs) have attracted much attention due to their
spin-polarized surface and edge states, whose origin in symmetry gives them
intriguing quantum-mechanical properties. Robust control over the chemical
potential of TI materials is important if these states are to become useful in
new technologies, or as a venue for exotic physics. Unfortunately, chemical
potential tuning is challenging in TIs in part because the fabrication of
electrostatic top-gates tends to degrade material properties and the addition
of chemical dopants or adsorbates can cause unwanted disorder. Here, we present
an all-optical technique which allows persistent, bidirectional gating of a
(Bi,Sb)2Te3 channel by optically manipulating the distribution of electric
charge below its interface with an insulating SrTiO3 substrate. In this fashion
we optically pattern p-n junctions in a TI material, which we subsequently
image using scanning photocurrent microscopy. The ability to dynamically write
and re-write mesoscopic electronic structures in a TI may aid in the
investigation of the unique properties of the topological insulating phase. The
optical gating effect may be adaptable to other material systems, providing a
more general mechanism for reconfigurable electronics.
