  Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological
insulator whose Dirac surface states have been extensively studied
theoretically and experimentally. Surprisingly little, however, is known about
the energetics and dynamics of electrons and holes within the bulk band
structure of the semiconductor. We use mid-infrared femtosecond transient
reflectance measurements in a thick exfoliated nanoflake to study the ultrafast
thermalization and recombination dynamics of photoexcited electrons and holes
within the extended bulk band structure over a wide energy range (0.3-1.2 eV).
Theoretical modeling of the reflectivity spectral lineshapes at 10 K
demonstrates that the electrons and holes are photoexcited within a dense and
cold electron gas with a Fermi level positioned well above the bottom of the
lowest conduction band. Direct optical transitions from the first and the
second spin-orbit split valence bands to the Fermi level above the lowest
conduction band minimum are identified. The photoexcited carriers thermalize
rapidly to the lattice temperature within a couple of picoseconds due to
optical phonon emission and scattering with the cold electron gas. The minority
carrier holes recombine with the dense electron gas within 150 ps at 10 K and
50 ps at 300 K. Such knowledge of interaction of electrons and holes within the
bulk band structure provides a foundation for understanding of how such states
interact dynamically with the topologically protected Dirac surface states.
