  It was recently shown that BiTeI, a semiconductor with polar crystal
structure, possesses a giant spin-splitting of electrons, which has been
interpreted in terms of Rashba-type spin-orbit coupling. Here, we use high
field magneto-optical spectroscopy to quantify the deviations of the
conduction-band profile from this appealing, but at the same time, strongly
simplifying model. We find that the optical response -- comprising a series of
inter-Landau level excitations -- can be described by the Rashba model only at
low magnetic fields. In contrast, the high-field response appears to be more
consistent with a simple picture of massless electrons in a conical band. This
points towards more linear rather than parabolic at energies well above the
bottom of the conduction band.
