  We study the collective effects of an ensemble of organic molecules confined
in an optical cavity based on Holstein-Tavis-Cummings model. By using the
quantum Langevin approach and adiabatically eliminating the degree of freedom
of the vibrational motion, we analytically obtain the expression of the cavity
transmission spectrum to analyze the features of polaritonic states. As an
application, we show that the dependence for the frequency shift of the lower
polaritonic state on the number of molecules can be used in the detection of
the ultra-cold molecules. We also numerically analyze the fluorescence
spectrum. The variation of the spectral profile with various numbers of
molecules gives signatures for the modification of molecular conformation.
