Nonlinear semiconductor microcavity reflectance and photoluminescence from normal-mode coupling to lasing

The transition from the nonperturbative reversible emission regime of normal-mode coupling all the way to the perturbative regime of irreversible lasing emission is studied. The microcavity samples contain one or two InGaAs/GaAs quantum wells with very narrow absorption linewidths (1 meV) resulting in record normal-mode-coupling splitting-to-linewidth ratios. For zero exciton-cavity detuning, the transmission peaks and reflectance dips vanish with increased carrier density with little change in splitting. This new nonlinear behavior is observed because exciton broadening with little reduction in oscillator strength is the dominant nonlinearity at low densities for such narrow-linewidth excitons. A microscopic theory, where the effects of carrier and polarization scattering are included at the microscopic level, explains these experiments. The photoluminescence emitted perpendicular to the layers also shows curious density-dependent behavior. When the cavity mode is tuned energetically above the exciton resonance, the upper-polariton peak is weaker (stronger) than the lower-polariton peak at low (high) carrier densities. This crossover occurs at a carrier density less than a factor of two below that for lasing threshold. This behavior is in good agreement with a full quantum mechanic description for both light and carrier, where the mutual Coulomb interaction of carriers is included.