  Ultrafast dynamics in three-dimensional topological insulators (TIs) opens
new routes for increasing the speed of information transport up to frequencies
thousand times faster than in modern electronics. However, up to date,
disentangling the exact contributions from bulk and surface transport to the
subpicosecond dynamics of these materials remains a difficult challenge. Here,
using time- and angle-resolved photoemission, we demonstrate that driving a TI
from the bulk-conducting into the bulk-insulating transport regime allows to
selectively switch on and off the emergent channels of ultrafast transport
between the surface and the bulk. We thus establish that ultrafast transport is
one of the main driving mechanisms responsible for the decay of excited
electrons in prototypical TIs following laser excitation. We further show how
ultrafast transport strongly affects the thermalization and scattering dynamics
of the excited states up to high energies above the Fermi level. In particular,
we observe how inhibiting the transport channels leads to a thermalization
bottleneck that substantially slows down electron-hole recombination via
electron-electron scatterings. Our results pave the way for exploiting
ultrafast transport to control thermalization time scales in TI-based
optoelectronic applications, and expand the capabilities of TIs as intrinsic
solar cells.
