Observation of the Transition from Lasing Driven by a Bosonic to a Fermionic Reservoir in a GaAs Quantum Well Microcavity

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.

Figure 1

(a) Scanning electron microscope images of a mesa with electrical contacts (yellow overlay) on the sidewalls. The left image shows the whole device and the right image an enlargement of the contact to the quantum wells. (b) Momentum-resolved PL spectra at three different excitation powers representing the linear polariton (left), polariton laser (middle), and photon laser regimes (right). White dashed lines are theoretical dispersions of the lower polariton fitted to the low-power spectrum. Green (red) dashed lines are bare photon (exciton) dispersions used in the fit.

Figure 2

(a) Average number of photons emitted per pulse as a function of excitation power without external bias. At 20–30 mW, emission from both the polariton (black squares) and the photon laser (red circles) is visible due to the pulsed excitation scheme. (b) The same as (a) for an external bias of 5 V corresponding to an in-plane electric field of $1\mathrm{kV}/\mathrm{cm}$. (c) Lateral photocurrent as a function of excitation power for an external bias of 5 V showing a clear kink at the second threshold. The inset shows the photocurrent for $P<50\mathrm{mW}$ in the linear scale.

Figure 3

(a) Current-voltage characteristics at different excitation powers. The current is normalized to the total number of excited carriers at each power. (b) Differential conductance from a linear fit to the current-voltage characteristics for $U>10\mathrm{V}$. Dashed lines are guides to the eye. A kink similar to Fig. 2 is observed. (c) Line spectra at $k_{\parallel }=0$ for fixed excitation power and increasing bias. Spectra are not normalized and have a constant vertical offset. Because of the pulsed excitation scheme, emission from both polariton and photon lasers can be seen.