Bismuth selenide is a prototypical topological insulator, which is a remarkable classof materials due to their linear dispersion relation owing to their time-reversal symmetry-
protected Dirac cones. Recently, antimony-doped bismuth selenide MOSFETs grown by
chemical vapor deposition have displayed millimeter-long diffusion lengths at cryogenic tem-
peratures with a tunable chemical potential. Motivated by the hypothesis that these highly
extended and efficient photocurrents are reliant on the topological surface states, I will dis-
cuss recent experiments to probe this observation through several avenues: first, by creating
MOSFET architecture that allows us to locally gate the chemical potential; second, through
ultrafast photocurrent studies achieved with a Ti:sapphire laser to dramatically modulate the
carrier concentration; third, by studying illuminated magnetotransport studies to shed light
on contributions to weak antilocalization; fourth, through a tandem approach of experimental
and theoretical investigations into the excitations generated through helical radiation. Our
results provide clues about the nature of the carriers responsible for these uniquely nonlocal
photocurrents which may be due to the carrier states forming an exciton condensate.
ii