  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.
