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.

8 pages, 3 figures Proc. 6th NEC Symposium on Fundamental Approaches to New Material Phases (Karuizawa, Japan 1996)