Optical response of the Josephson vortex lattice in a stack of intrinsic Josephson junctions

Optical response of a Josephson vortex lattice in a stack of intrinsic Josephson junctions is investigated in two experimental configurations for measuring a microwave absorption spectrum and reflectivity on the bc-plane in high-temperature superconductors. The effective dielectric functions for an oscillatory electric field parallel to the c-direction are derived in these two configurations, which are expressed in terms of the variables containing information about the spatial dependence of the gauge-invariant phase differences in a static Josephson vortex lattice. The optically active small phase oscillation modes of a triangular Josephson vortex lattice are clarified. The lowest resonant frequency under an in-plane magnetic field, H, is located at nearly half the zero-field Josephson plasma frequency and is a weakly decreasing function of H. This result is consistent with the experimental results in Bi-2212 and YBCO.