  Using a high-sensitivity cavity perturbation technique (40 to 180 GHz), we
have probed the angle dependent interlayer magneto-electrodynamic response
within the vortex state of the extreme two-dimensional organic superconductor
k-(BEDT-TTF)2Cu(NCS)2. A previously reported Josephson plasma resonance [M.
Mola et al., Phys. Rev. B 62, 5965 (2000)] exhibits a dramatic re-entrant
behavior for fields very close (<1 degree) to alignment with the layers. In
this same narrow angle range, a new resonant mode develops which appears to be
associated with the non-equilibrium critical state. Fits to the angle
dependence of the Josephson plasma resonance provide microscopic information
concerning the superconducting phase correlation function within the vortex
state. We also show that the effect of an in-plane magnetic field on the
temperature dependence of the interlayer phase coherence is quite different
from what we have previously observed for perpendicular magnetic fields.
