Submillimeter spectroscopy of tilted Nd_{1.85}Ce_{0.15}CuO_{4-\delta}   films: observation of a mixed a-c excitation
  The anisotropic conductivity of a series of tilted Nd_{1.85}Ce_{0.15}CuO_{4-\delta} thin films was measured by quasioptical spectroscopy in the frequency range 3cm^{-1}<\nu <40cm^{-1}. Two characteristic features have been observed in the low-temperature transmission spectra. The first one at \nu = 12cm^{-1} was shown to reflect the c-axis plasma frequency of Nd_{1.85}Ce_{0.15}CuO_{4-\delta}. The second feature represents a new mixed ab-plane/c-axis excitation. The frequency of this resonance may be changed in a controllable way by rotating the polarization of the incident radiation. 
A. Pimenov, A. V. Pronin, A. Loidl, A. Kampf, S. I. Krasnosvobodtsev,   and V. S. Nozdrin