Table of contents

The EU Research Training Network `Photon-Mediated Phenomena in Semiconductor Nanostructures' (HPRN-CT-2002-00298) comprises seven teams from across Europe: Cambridge, Cardiff, Dortmund, Heraklion, Grenoble, Lund and Paderborn (for details see the Network website http://www.astro.cardiff.ac.uk/research/PMPnetwork/index.html). The first workshop of the Network was held at Gregynog Hall, a conference centre in the beautiful countryside of mid-Wales. There were 44 participants who attended the meeting (7 from France, 2 from Japan, 3 from Germany, 1 from Greece, 2 from Russia, 3 from Sweden, 23 from UK and 3 from USA). Of these, 57\% were students and young postdoctoral research associates.

The talks presented at the meeting were mainly devoted to linear and nonlinear optics of semiconductor nanostructures. Thus the review and research papers included in this special issue of Journal of Physics: Condensed Matter deal with the exciton-mediated optical phenomena in semiconductor quantum wires, quantum wells, planar and spherical microcavities and self-assembled quantum dots. The specific topics covered by the proceedings are

• exciton-mediated optics, including lasing, of semiconductor quantum wires

• Bose–Einstein condensation of excitons and microcavity polaritons

• diffusion, thermalization and photoluminescence of free carriers and excitons in GaAs coupled quantum wells

• polaritons in semiconductor microcavities

• exciton-mediated optics of semiconductor photonic dots

• optical nonlinearities of biexciton waves

• optics of self-assembled quantum dots

• photosensitive metal oxides films

On the first day of the workshop, a special session on presentation skills, lead by Mike Edmunds, was organized for the young researchers. The meeting concluded with a round-table discussion at which key questions on research, organization and management of the Network were identified and discussed.

The second workshop of the Network, organized and chaired by George Kiriakidis, took place at Hersonissos (Crete, Greece) in October 2003. The forthcoming third workshop, organized by Detlef Schikora and Ulrike Woggon, will be held in Paderborn (conference part) and Dortmund (training part) from 4 October 4 through 7 October 2004 (for details visit the Network website).

Finally, I would like to thank my colleagues, Celestino Creatore, Nikolay Nikolaev, Lois Smallwood and Andrew Smith, for their help with preparation of the Proceedings.

High quality T-shaped quantum wire lasers are fabricated by cleaved-edge overgrowth with molecular beam epitaxy on an interface improved by a growth-interrupt high temperature annealing. Microphotoluminescence and photoluminescence (PL) excitation spectroscopy at low temperatures reveals the formation of quantum wires with unprecedentedly high quality, and intrinsic structures of one-dimensional (1D) free excitons, exciton excited states, and 1D continuum states. At high pumping levels, the PL evolves from showing a sharp free exciton peak via exhibiting a biexciton peak to a Coulomb correlated electron–hole plasma PL band. Lasing has been achieved with a low lasing threshold, and its emission patterns are measured in imaging experiments. The lasing energy is in the plasma PL band and is about 5 meV below the free exciton level. The observed shift excludes the possibility of free excitons in the lasing, and suggests a contribution from the electron–hole plasma. Single T-wire samples such as a single-quantum-wire laser and a field-effect-transistor-type doped single quantum wire are fabricated and studied optically.

Quantum states and their optical responses of low-dimensional electron–hole systems in photoexcited semiconductors and/or metals are reviewed from a theoretical viewpoint, stressing the electron–hole Coulomb interaction, the excitonic effects, the Fermi-surface effects and the dimensionality. Recent progress of theoretical studies is stressed and important problems to be solved are introduced. We cover not only single-exciton problems but also few-exciton and many-exciton problems, including electron–hole plasma situations. Dimensionality of the Wannier exciton is clarified in terms of its linear and nonlinear responses. We also discuss a biexciton system, exciton bosonization technique, high-density degenerate electron–hole systems, gas–liquid phase separation in an excited state and the Fermi-edge singularity due to a Mahan exciton in a low-dimensional metal.

That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focusing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.

The dramatic appearance of luminescence rings with radius of several hundred microns in quantum well structures can be understood through a fairly simple nonlinear model of the diffusion and recombination of electrons and holes in a driven nonequilibrium system. The ring corresponds to the boundary between a positive hole gas and a negative electron gas in steady state. While this basic effect is now well understood, we discuss several other experimental results which cannot be explained by this simple model.

The thermalization and photoluminescence (PL) dynamics of indirect excitons in GaAs/AlGaAs coupled quantum wells (QWs) at very low bath temperature, T_b<1 K, are analysed theoretically and modelled numerically in order to clarify the origin of the recently observed sharp increase of the PL signal (PL-jump) right after a high-intensity excitation pulse. We conclude that the PL-jump effect is mainly due to classical cooling of indirect excitons after the pump pulse, which heats the exciton system. Thus the effect cannot unambiguously be attributed to bosonic stimulation of the scattering processes into the ground-state mode . It is argued that the narrowing effect, i.e. an effective screening of in-plane QW disorder by dipole–dipole interacting indirect excitons, naturally explains the observed strongly nonlinear dependence of the PL rise time on the pump intensity. We also discuss and analyse an evaporative optical heating/cooling effect: the change of the effective temperature T of indirect excitons, due to their resonant optical decay.

We present measurements of polariton broadening in energy and momentum space as a function of in-plane momentum in a planar microcavity by directional resonant light scattering in both time-integrated and time-resolved experiments. When optically exciting the lower polariton branch, the strong dispersion versus wavevector results in a directional emission on a ring. For continuous wave excitation, the ring width is shown to be consistent with the polariton spectral line width and dispersion. For pulsed excitation, the ring width decreases with increasing time after excitation, giving evidence for the time–energy uncertainty in the dynamics of the scattering by disorder. The ring width converges for long times to a finite value, a measure of the intrinsic momentum broadening of the polariton states by multiple disorder scattering.

Recently an unusual behaviour of the polariton–polariton scattering in a semiconductor microcavity (MC) under a strong continuous resonant excitation near the lower polariton branch has been observed (Kulakovskii et al 2001 Nanotechnology12 475). The maxima of the scattered photoluminescence signal above the threshold of parametric scattering does not shift along the microcavity lower polariton branch with change of the pump detuning or angle of incidence but is always directed approximately perpendicular to the MC plane. We discuss theoretically a possible explanation of such behaviour via a competition between two instabilities in the polariton–polariton scattering, stimulated polariton–polariton scattering and bistability of the pumped polariton mode response.

We study theoretically the second-order coherence g⁽²⁾(0) of light emitted by polariton lasers, i.e., devices based on stimulated relaxation and condensation of exciton–polaritons in microcavities. We solve kinetic equations for the polaritons in different approximations and show that (i) the coherence introduced into the polariton condensate by an external source can be conserved by the system over a macroscopically long time, and (ii) if the total number of polaritons is fixed by the excitation conditions, the correlations between the populations of the ground and excited polariton states can also result in the spontaneous buildup of second-order coherence in the polariton condensate. Both results are obtained neglecting polariton–polariton interactions in the condensate.

We present some experimental study of CdTe II–VI microcavity polaritons generated by strong non-resonant optical excitation. We first compare spectra of luminescence under high excitation and white light reflectivity showing that the polariton stimulation occurs in the strong coupling regime. In a second part we present a measurement of the integrated polariton emission intensity versus the excitation power: the stimulated emission exhibits an exponential behaviour which excludes a polariton/polariton quadratic collision process. In a third part we present an angle-resolved experiment showing that the stimulation peak is dispersion free while the residual spontaneous emission keeps its usual dispersion curve; we exclude a simple scattering phenomenon due to surface roughness and discuss the possibility of real space shrinkage or polariton collisions within a coherent population inspired by the theoretical work of Ciuti and co-workers.

Due to their large oscillator strengths, ZnSe microcavities with (Zn, Cd)Se quantum wells are particularly suited for investigation of the photon–exciton coupling behaviour in semiconductors. We have observed a strong coupling between the excitonic and photonic modes in a ZnSe microcavity with four (Zn, Cd)Se quantum wells and distributed Bragg mirrors of ZnS and YF₃. A very large Rabi splitting  meV was observed in temperature dependent photoluminescence investigations.

A previously proposed Green function of cavity polariton associated with quantum well excitons in a distributed Bragg reflector cavity has been generalized to an arbitrary three-dimensional cavity containing multilevel excitons. This Green function describes the electromagnetic field, both inside and outside the cavity, caused by the cavity dielectrics and the linear polarization of excitons. The electromagnetic field induced by any other polarizations can be calculated as a convolution with the Green function. In this way, we can include in the Green function approach all the problems which deal with boundary conditions and the radiative shift/width of excitons.

We develop coherent optics of dipole-active, dispersionless excitons in spherical semiconductor photonic dots (PDs). In the absence of any incoherent scattering, both the strong and weak coupling regimes can intrinsically be realized simply by changing the parameters of the dot and surrounding medium. A criterion, which attributes the transition between these two regimes to a discrete topological change of the relevant dispersion curves, is found and approximated by an analytic expression. The transition depends upon the intrinsic radiative lifetime of the PD photon eigenstates, i.e. it is determined by the parameters of the structure (the oscillator strength of the exciton–photon interaction, PD radius and the ratio of the background dielectric constants inside and outside of the dot). We propose the use of high-precision modulation spectroscopy in order to visualize the above 'phase' transition between a well-developed polariton picture (the strong coupling regime) and weakly-interacting exciton and PD photon states (the weak coupling regime). It is shown that the radiative decay of optically dressed PD excitons, coherently distributed among the relevant PD eigenstates, is non-monotonous against the dot radius a: a size-dependent increase of the effective oscillator strength at small a saturates at , and with a increasing further towards the optical lifetime of excitons starts to increase proportionally to a, reflecting the ballistic escape of nearly bulk polaritons from the PD. The numerical simulations are scaled to dispersionless excitons in PDs fabricated from cyanine dye J aggregates.

We propose an experimental scheme to evaluate the intrinsic anharmonicity of the biexciton ensemble by extending the conventional four-wave mixing technique to coherent biexciton waves, which can be prepared by two-photon absorption of ultrashort optical pulses. The anharmonicity originates from the two-body interaction of the bosonic quasi-particles at low densities and leads to Kerr-type nonlinearity for the coherent biexcitonic waves generated in the crystal. We discuss the feasibility of determining the two-body biexciton interaction energy in CuCl by measuring the anharmonicity of the biexcitonic ensemble.

A variable semiconductor optical buffer based on the electromagnetically induced transparency in a quantum dot waveguide is theoretically investigated with feasible parameters for applications to a 40 Gbps optical network. We show the refractive index and absorption spectra of the quantum dot waveguide at various pump levels, which exhibit an optimal pump power for maximum slow-down factor, in agreement with the previous experimental observation using a Pr-doped solid. The group velocity slow-down factor is theoretically analysed as a function of the pump intensity at different broadened linewidths. Inhomogeneous broadening in self-assembled quantum dots degrades the slow-down factor. In order to reduce the inhomogeneous broadening effects, we propose to use a resonant microcavity structure with quantum dots embedded in the active layer to enhance the slow-down factor.

InP quantum dots grown on GaInP by the Stranski–Krastanow technique are less well studied than InAs quantum dots grown on GaAs. We here give a review of the main experimental evidence for the InP dots being charged when grown in between n-type barriers.

We have measured the modal optical absorption spectrum of a three-layer system of InAs quantum dots in a slab waveguide geometry, observing distinct absorption peaks for the ground and excited states. The spectrally integrated absorption cross section for the ground and first excited states are determined to be σ₀ = (0.43 ± 0.1) × 10⁻¹⁵ and (0.92 ± 0.2) × 10⁻¹⁵ cm² eV, respectively. Assuming that the spectral shapes are determined primarily by the inhomogeneous size distribution of dots the Gaussian linewidths are 16 and 19 meV for the ground and first excited state transitions, respectively. The peak ground state absorption cross section is 1.1 × 10⁻¹⁴ cm². The ground state spectrally integrated cross section estimated by a theory with the envelope function overlap integral taken to be unity is 0.40 × 10⁻¹⁵ cm² eV, in agreement with the measured value. We conclude that on the basis of the spectrally integrated cross section there is no evidence for a substantial reduction in the strength of the fundamental light–matter interaction in dots compared with systems of higher dimensionality.

In this work, we review the electrical and optical properties of InO_x and ZnO films, and their use as dynamic optical materials. The transport properties of these films are analysed and correlated with the growth parameters. We have studied the permanent light-induced refractive index changes in our dc-sputtered InO_x thin films using pulsed ultraviolet laser radiation at 193 nm. Non-permanent holography recording of information has been achieved in InO_x films upon illumination with UV radiation (325 nm). The recording, which appears to be independent of the electrical state of the material, disappears in the absence of UV light with decay times that depend on the material's specific properties, as these are influenced by the film fabrication conditions. We also review our work on waveguide reflection submicron relief gratings on multilayer waveguides operating near 1550 nm, fabricated by excimer laser ablation at 248 nm.