  Ultrafast carrier dynamics in the topological insulator Bi2Se3 have 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, the 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
exclusively influenced by whether the Dirac point is presented between the
Dirac cones of the higher energy (~1.5 eV) surface states (known as SS2). We
recognized that SS2 act as a valve substantially slowing down the relaxation of
carriers when the gap between the Dirac cones exceeds the phonon and resonant
defects energies. The progressive accumulation of carriers in the gapped SS2
becomes detectable through the inverse bremsstrahlung type free carrier
absorption.
