  We present a comparative analysis of a 'conventional' phonoriton (coherent
superposition of exciton-photon-phonon) and a polariton (coherent
exciton-photon superposition) 'weakly' coupled to the LA-phonons bath.
Depending on duration of the pumping laser field the phonon-induced decoherence
results in two distinct types of excitation. Long ($ms$) laser pumping pulses
form an 'equilibrium' polariton. The generic feature here is a pronounced
photo-thermal bi-stability. i.e. formation of four distinct branches.
Transitions between branches can be achieved by excitation energy fluctuations
as small as $200 neV$ which may impede BEC of the para-excitons. Short ($\mu
s$) laser pulses create a 'quasi-equilibrium' polariton. In the latter case,
for some critical intensity of the laser field we demonstrate possibility of
strong luminicence from a highly unstable localized state on the lower
polariton branch.
