  It is often taken for granted that in pump-probe experiments on the
topological insulator (TI) Bi2Se3 using IR pumping with a commercial
Ti:Sapphire laser [~800 nm (1.55 eV photon energy)], the electrons are excited
in the one-photon absorption regime, even when pumped with absorbed fluences in
the mJ cm-2 range. Here, using UV-Vis transient absorption (TA) spectroscopy,
we show that even at low-power IR pumping with absorbed fluences in the mkJ
cm-2 range, the TA spectra of the TI Bi2Se3 extend across a part of the UV and
the entire visible region. This observation suggests unambiguously that the
two-photon pumping regime accompanies the usual one-photon pumping regime even
at low laser powers applied. We attribute the high efficiency of two-photon
pumping to the giant nonlinearity of Dirac fermions in the Dirac surface states
(SS). On the contrary, one-photon pumping is associated with the excitation of
bound valence electrons in the bulk into the conduction band. Two mechanisms of
absorption bleaching were also revealed since they manifest themselves in
different spectral regions of probing. These two mechanisms were assigned to
the filling of the phase-space in the Dirac SS and bulk states and the
corresponding Pauli blocking.
