  We show that, by monitoring the free carrier reservoir in a GaAs-based
quantum well microcavity under nonresonant pulsed optical pumping, lasing
supported by a fermionic reservoir (photon lasing) can be distinguished from
lasing supported by a reservoir of bosons (polariton lasing). Carrier densities
are probed by measuring the photocurrent between lateral contacts deposited
directly on the quantum wells of a microcavity that are partially exposed by
wet chemical etching. We identify two clear thresholds in the input-output
characteristic of the photoluminescence signal which can be attributed to
polariton and photon lasing, respectively. The power dependence of the probed
photocurrent shows a distinct kink at the threshold power for photon lasing due
to an increased radiative recombination of free carriers as stimulated emission
into the cavity mode sets in. At the polariton lasing threshold, on the other
hand, the nonlinear increase of the luminescence is caused by stimulated
scattering of exciton polaritons to the ground state which do not contribute
directly to the photocurrent.
