  We performed temperature dependent infrared spectroscopy measurements on
BiTeI single crystals, which exhibit large Rashba spin-splitting. Similar to a
previous optical study, we found electronic excitations in good agreement with
spin-split electronic bands. In addition, we report a low energy intraband
transition with an onset energy of about 40 meV and an unexpectedly large
number of vibrational modes in the far-infrared spectral region. At least some
of the modes have asymmetric Fano line-shape. These new observations cannot be
explained considering only the bulk band structure or crystal symmetry of
BiTeI, and we proposed that the optical response is also affected by the
surface topology.
