D. M. Whittaker

Polariton-polariton interactions and stimulated scattering in semiconductor microcavities

Recent work on polariton-polariton scattering in semiconductor microcavities under continuous wave excitation conditions is reviewed. For weak non-resonant laser excitation, a marked bottleneck in the polariton distribution is observed, but which is suppressed by polariton-polariton scattering as the laser intensity is increased. However, the high excitation conditions necessary to observe stimulated emission lead to loss of strong coupling and conventional lasing in the weak coupling regime, By contrast for resonant excitation, polaritons are created directly in the polariton trap formed by the microcavity dispersion curve. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the centre of the Brillouin zone, and to a companion state at high wavevector. The stimulation phenomena lead to condensation of the bosonic quasi-particles to two specific regions of k-space, and to the formation of a new state with macroscopic coherence. The prospects to achieve a polariton laser under conditions of non-resonant excitation are discussed

Steady states of a chi(3) parametric oscillator with coupled polarizations

Polarization effects in the microcavity parametric oscillator are studied using a simple model in which two Χ 3 optical parametric oscillators are coupled together. It is found that there are, in general, a number of steady states of the model under continuous pumping. There are both continuous and discontinuous thresholds, at which new steady states appear as the driving intensity is increased: at the continuous thresholds, the new state has zero output intensity, whereas at the discontinuous threshold it has a finite output intensity. The discontinuous thresholds have no analog in the uncoupled device. The coupling also generates rotations of the linear polarization of the output compared with the pump, and shifts in the output frequencies as the driving polarization or intensity is varied. For large ratios of the interaction between polarizations to the interaction within polarizations, of the order of 5, one of the thresholds has its lowest value when the pump is elliptically polarized. This is consistent with recent experiments in which the maximum output was achieved with an elliptically polarized pump.

Angular-asymmetric nonlinear polariton dynamics in semiconductor microcavities

Parametric interactions in semiconductor heterostructures in which two exciton quasiparticles mutually scatter are typically weak because they are restricted by energy-momentum conservation. Here, semiconductor microcavities in the strong coupling regime are optically pumped at specific resonant energy and angle of incidence to enormously enhance the parametric interactions. A femtosecond goniometer is devised to allow time-, wavelength- and polarisation-resolved pump-probe spectroscopy. Time-resolved images of the microcavity emission at different angles reveal a highly asymmetric luminescence distribution which switches on as the pump power is increased. Both signal and idler beams can be identified when a probe purse initiates stimulated scattering of polaritons from a reservoir. The images show the rich variety of interactions produced by the coupling of light-matter modes, and the future promise of dispersion engineering.