R.M. Stevenson

Heavy photon dispersions in photonic crystal waveguides

Heavy photon dispersion curves exhibiting group velocities suppressed by two orders of magnitude are measured directly for deeply etched AlGaAs waveguide structures by means of surface coupling techniques. It is shown that due to the wave vector-selective nature of surface coupling, such techniques permit the excitation of modes of specific, known dispersion in photonic crystal waveguides. Coupling to regions of very strong anomalous dispersion is demonstrated, with potential to be developed into a method for excitation of gap solitons.

Exciton polaritons in single and coupled microcavities

Recent work on strong coupling exciton–polariton phenomena in single and coupled microcavities is presented. We describe experiments for single cavities where the strong coupling nature of the excitations manifests itself. It is also shown that coupled cavities enable optically induced coupling between macroscopically separated exciton states to be achieved, and polaritons with strongly anisotropic properties to be realised. Results for both inorganic and organic microcavities are presented.

Stimulated Polariton Scattering in Semiconductor Microcavities: New Physics and Potential Applications

Stimulated polariton scattering in semiconductor microcavities is reported. The phenomena arise from the bosonic character of the coupled exciton-photon modes of the system. The ability to manipulate and control the polariton dispersions in microcavities is shown to be a key factor underlying the new observations. The potential of such phenomena to form the basis of a new type of coherent light source (polariton lasers) and highly efficient optical parametric oscillators is discussed.

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

High-occupancy effects and stimulation phenomena in semiconductor microcavities

This paper describes recent work on high-occupancy effects in semiconductor microcavities, with emphasis on the variety of new physics and the potential for applications that has been demonstrated recently. It is shown that the ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton-photon coupled modes, exciton polaritons, leads to a number of very interesting phenomena, which are either difficult or impossible to achieve in bulk semiconductors or quantum wells. The very low polariton density of states enables state occupancies greater than one to be easily achieved, and hence stimulation phenomena to be realized under conditions of resonant excitation. The particular form of the lower polariton dispersion curve in microcavities allows energy and momentum conserving polariton-polariton scattering under resonant excitation. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the center of the Brillouin zone, and to a companion state at high wave vector. The stimulation phenomena lead to the formation of highly occupied states with macroscopic coherence in two specific regions of k space. The results are contrasted with phenomena that occur under conditions of nonresonant excitation. Prospects to achieve polariton lasing under nonresonant excitation, and high-gain, room-temperature ultrafast amplifiers and low-threshold optical parametric oscillator under resonant excitation conditions are discussed.

Determination of the band structure of photonic crystal waveguides

Abstract We report an experimental and theoretical investigation of the near infra-red reflectivity properties of two-dimensional (2D) periodically patterned semiconductor waveguides. The coupling of incident electromagnetic radiation to leaky modes of the photonic crystal waveguide is shown to clearly manifest itself by the appearance of sharp features in the reflectivity spectra. By determining the energy dependence of the resonant features on the angle of incidence along different symmetry directions we are able to map out the photonic band structure. Theoretical spectra obtained using an advanced scattering matrix theory are found to agree well with experiment. The vertical confinement provided by the waveguide is found to have a profound effect on the in-plane band structure of the photonic crystal.

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.