  Diagonal and off-diagonal optical conductivity spectra have been determined
form the measured reflectivity and magneto-optical Kerr effect (MOKE) over a
broad range of photon energy in the itinerant ferromagnetic phase of
CuCr$_2$Se$_4$ at various temperatures down to T=10 K. Besides the low-energy
metallic contribution and the lower-lying charge transfer transition at
$E$$\approx$2 eV, a sharp and distinct optical transition was observed in the
mid-infrared region around $E$$=$0.5 eV with huge magneto-optical activity.
This excitation is attributed to a parity allowed transition through the Se-Cr
hybridization-induced gap in the majority spin channel. The large off-diagonal
conductivity is explained by the high spin polarization in the vicinity of the
Fermi level and the strong spin-orbit interaction for the related charge
carriers. The results are discussed in connection with band structure
calculations.
