  This work focuses on the low frequency Drude response of bulk-insulating
topological insulator Bi$_2$Se$_3$ films. The frequency and field dependence of
the mobility and carrier density are measured simultaneously via time-domain
terahertz spectroscopy. These films are grown on buffer layers, capped by Se,
and have been exposed in air for months. Under a magnetic field up to 7 Tesla,
we observe prominent cyclotron resonances (CRs). We attribute the sharp CR to
two different topological surface states (TSSs) from both surfaces of the
films. The CR sharpens at high fields due to an electron-impurity scattering.
By using magneto-terahertz spectroscopy, we confirm that these films are
bulk-insulating, which paves the way to use intrinsic topological insulators
without bulk carriers for applications including topological spintronics and
quantum computing.
