  Observations of polariton condensation in semiconductor microcavities suggest
that polaritons can be exploited as a novel type of laser with low input-power
requirements. The low-excitation regime is approximately equivalent to thermal
equilibrium, and a higher excitation results in more dominant nonequilibrium
features. Although standard photon lasing has been experimentally observed in
the high excitation regime, e-h pair binding can still remain even in the
high-excitation regime theoretically. Therefore, the photoluminescence with a
different photon lasing mechanism is predicted to be different from that with a
standard photon lasing. In this paper, we report the temperature dependence of
the change in photoluminescence with the excitation density. The second
threshold behavior transited to the standard photon lasing is not measured at a
low-temperature, high-excitation power regime. Our results suggest that there
may still be an electron--hole pair at this regime to give a different photon
lasing mechanism.
