  We study the spectra of collective low excitations of two atomic ensembles
coupled indirectly through a single-mode cavity field. When the left ensemble
is driven with an external optical field, its corresponding response spectrum
to the incident optical light shows an electromagnetically induced
transparency- (EIT-) like phenomenon when the layers are arranged in the
sequence of node-antinode but not in the sequence of antinode-node. In the case
of antinode-antinode sequence, the response spectrum shows an EIT-like
phenomenon with two transparent windows. We also investigate the fluctuation
spectra of the atomic collective excitation modes, which show similar EIT-like
phenomena.
