  Gated terahertz cyclotron resonance measurements on epitaxial Bi2Se3 thin
films capped with In2Se3 enable the first spectroscopic characterization of a
single topological interface state from the vicinity of the Dirac point to
above the conduction band edge. A precipitous drop in the scattering rate with
Fermi energy is observed that is interpreted as the surface state decoupling
from bulk states and evidence of a shift of the Dirac point towards mid-gap.
Near the Dirac point, potential fluctuations of 50 meV are deduced from an
observed loss of differential optical spectral weight near the Dirac point.
Potential fluctuations are reduced by a factor of two at higher surface Fermi
levels in the vicinity of the conduction band edge inferred from the width of
the scattering rate step. The passivated topological interface state attains a
high mobility of 3500 cm2/Vsec near the Dirac point.
