  Using microscopic models in which both photons and excitons are treated as
microscopic degrees of freedom, we discuss polaritons of two cases: One is the
case when excitonic parameters are time dependent. The time dependence causes
creation of polaritons from a "false vacuum." % We present the creation spectra
of both % the lower- and upper-branch polaritons. It is shown that both the
creation sepctra and the creation efficiency are much different from the
results of the previous studies. The other is polaritons in absorptive and
inhomogeneous cavities. A polariton in such a system cannot be viewed as a back
and forth oscillation between a photon state and an exciton state.
