  The recent discovery of topology-protected charge transport of ultimate
thinness on surfaces of three-dimensional topological insulators (TIs) are
breaking new ground in fundamental quantum science and transformative
technology. Yet a challenge remains on how to isolate and disentangle helical
spin transport on the surface from bulk conduction. Here we show that selective
midinfrared femtosecond photoexcitation of exclusive intraband electronic
transitions at low temperature underpins topological enhancement of terahertz
(THz) surface transport in doped Bi2Se3, with no complication from interband
excitations or need for controlled doping. The unique, hot electron state is
characterized by conserved populations of surface/bulk bands and by
frequency-dependent hot carrier cooling times that directly distinguish the
faster surface channel than the bulk. We determine the topological enhancement
ratio between bulk and surface scattering rates, i.e.,
$\gamma_\text{BS}/\gamma_\text{SS}\sim$3.80 in equilibrium. These behaviors are
absent at elevated lattice temperatures and for high pumpphoton frequencies and
uences. The selective, mid-infrared-induced THz conductivity provides a new
paradigm to characterize TIs and may apply to emerging topological semimetals
in order to separate the transport connected with the Weyl nodes from other
bulk bands.
