  Ultrafast carrier dynamics in the topological insulator Bi2Se3 have recently
been intensively studied using a variety of techniques. However, we are not
aware of any successful experiments exploiting transient absorption (TA)
spectroscopy for these purposes. Here we demonstrate that if the ~730 nm
wavelength pumping (~1.7 eV photon energy) is applied to ultrathin Bi2Se3
films, TA spectra cover the entire visible region, thus unambiguously pointing
to two-photon excitation (~3.4 eV). The carrier relaxation dynamics is found to
be governed by the polar optical phonon cascade emission occurring in both the
bulk states and the Dirac surface states (SS), including SS-bulk-SS vertical
electron transport and being also exclusively influenced by whether the Dirac
point is presented between the Dirac cones of the higher energy (~1.5 eV) Dirac
SS (known as SS2). We have recognized that SS2 act as a valve substantially
slowing down the relaxation of electrons when the gap between Dirac cones
exceeds the polar optical phonon and resonant defects energies. The resulting
progressive accumulation of electrons in the gapped SS2 becomes detectable
through the inverse bremsstrahlung type free carrier absorption.
