  A cavity perturbation technique is used to study the microwave response of
the organic superconductor k-(BEDT-TTF)2Cu(NCS)2. Observation of a Josephson
plasma resonance, below Tc (approx. 10 K), enables investigation of the vortex
structure within the mixed state of this highly anisotropic, type-II,
superconductor. Contrary to previous assumptions, frequency dependent studies
(28 - 153 GHz) indicate that the squared plasma frequency depends exponentially
on the magnetic field strength. Such behavior has been predicted for a weakly
pinned quasi-two-dimensional vortex lattice [Bulaevskii et al. Phys. Rev. Lett.
74, 801 (1995)], but has not so far been observed experimentally. Our data also
suggests a transition in the vortex structure near the irreversibility line not
previously reported for an organic superconductor using this technique.
