V.N. Astratov

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.

Cavity-polariton dispersion and polarization splitting in single and coupled semiconductor microcavities

Recent theoretical and experimental work on linear exciton-light coupling in single and coupled semiconductor microcavities is reviewed: emphasis is given to angular dispersion and polarization effects in the strong-coupling regime, where cavity-polariton states are formed. The theoretical formulation is based on semiclassical theory. The energy of single-cavity modes is determined by the Fabry-Pérot frequency ωc as well as by the center of the stop band ωs of the dielectric mirrors; the phase delay in the dielectric mirrors carries a nontrivial angle-and polarization dependence. The polarization splitting of cavity modes depends on the mismatch between ωc and ωs, and increases with internal angle as sin2θeff. Interaction between the cavity mode and quantum-well (QW) excitons is described at each angle by a two-oscillator model, whose parameters are expressed in terms of microscopic quantities. Weak and strong coupling regimes and the formation of cavity polaritons are described. Comparison with experimental results on a GaAs-based cavity with In0.13Ga0.87As QWs shows that a quantitative understanding of polariton dispersion and polarization splitting has been achieved. Coupling of two identical cavities through a central dielectric mirror induces an optical splitting between symmetric and antisymmetric modes. When QW excitons are embedded in both cavities at antinode positions, the system behaves as four coupled oscillators, leading to a splitting of otherwise degenerate exciton states and to separate anticrossing of symmetric and antisymmetric modes. These features are confirmed by experimental results on coupled GaAs cavities with In0.06Ga0.94As QWs. An analysis of reflectivity lineshapes requires the inclusion of the effect of resonance narrowing of cavity polaritons. Finally, the polarization splitting in a coupled cavity depends both on the single-cavity factors and on the angle-and polarization dependence of the optical coupling between the cavities. Inclusion of all these effects provides a good description of the experimental findings.

Opal photonic crystals infiltrated with chalcogenide glasses

Composite opal structures for nonlinear applications are obtained by infiltration with chalcogenide glasses As2S3 and AsSe by precipitation from solution. Analysis of spatially resolved optical spectra reveals that the glass aggregates into submillimeter areas inside the opal. These areas exhibit large shifts in the optical stop bands by up to 80 nm, and by comparison with modelling are shown to have uniform glass filling factors of opal pores up to 40%. Characterization of the domain structure of the opals prior to infiltration by large area angle-resolved spectroscopy is an important step in the analysis of the properties of the infiltrated regions.

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.

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

Excitons and Polaritons in Semiconductor Microcavities

We review various aspects of the behaviour of semiconductor microcavities, focusing on the important role that polaritons play in determining their optical properties. The review covers angular dependent measurements of cavity polariton dispersions, the effects of motional narrowing on the inhomogeneous line widths, and the possibilities for observing consequences of Bose-Einstein statistics in microcavities.

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.