  We report measurements of the optical response of polycrystalline DyN thin
films. The frequency-dependent complex refractive index in the near
IR-visible-near UV was determined by fitting reflection/transmission spectra.
In conjunction with resistivity measurements these identify DyN as a
semiconductor with 1.2 eV optical gap. When doped by nitrogen vacancies it
shows free carrier absorption and a blue-shifted gap associated with the
Moss-Burstein effect. The refractive index of 2.0+/-0.1 depends only weakly on
energy. Far infrared reflectivity data show a polar phonon of frequency 280
cm-1 and dielectric strength delta epsilon= 20.
