We have investigated the band structure at the Γ point of the
three-dimensional (3D) topological insulator Bi2Se3 using
magneto-spectroscopy over a wide range of energies (0.55−2.2\,eV) and in
ultrahigh magnetic fields up to 150\,T. At such high energies (E>0.6\,eV) the
parabolic approximation for the massive Dirac fermions breaks down and the
Landau level dispersion becomes nonlinear. At even higher energies around 0.99
and 1.6 eV, new additional strong absorptions are observed with a temperature
and magnetic-field dependence which suggest that they originate from higher
band gaps. Spin orbit splittings for the further lying conduction and valence
bands are found to be 0.196 and 0.264 eV