We have trapped rubidium atoms in the magnetic field produced by a superconducting atom chip operated at liquid helium temperatures. Up to 8.2x10(5) atoms are held in a Ioffe-Pritchard trap at a distance of 440 microm from the chip surface, with a temperature of 40 microK. The trap lifetime reaches 115 s at low atomic densities. These results open the way to the exploration of atom-surface interactions and coherent atomic transport in a superconducting environment, whose properties are radically different from normal metals at room temperature.

We study the effects of electron-polariton scattering in a semiconductor microcavity that allows control of the electron density. We show that this process can efficiently drive polaritons to the ground polariton state and that it is more effective whenever the relaxation bottleneck is present on the LP branch. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.

Photoluminescence (PL) image size and angular resolved spectra were studied in a GaAs/AlAs planar microcavity (MC), with an embedded GaAs quantum well (QW) that contains a photogencrated, variable density, two-dimensional electron gas (2DEG). The PL was measured at T = 7 K, under nonresonant, cw excitation. The in-plane polariton migration from the photoexcitation spot (whose radius is similar to30 mum) is deduced from the PL image size. It is observed to be anisotropic in the MC plane, and for negative detuning energy values (delta = E-C - E-X) it extends over in-plane distances of similar to300 mum. Introducing a 2DEG into the QW while maintaining the strong coupling between the excitons and the MC Photon mode, reduces the migration range. On the other hand, the angular resolved PL spectra show that the emission from the k(\\) similar to 0 polariton states is enhanced as the 2DEG density is increased. From these observations we propose that the polaritons migrate while they occupy the long-lived states above the relaxation bottleneck in the lower polariton (LP) branch. The efficient electron-polariton scattering processes transfer the polaritons through the bottleneck into the radiative trap (k(\\) similar to 0) in the LP branch. These scattering processes occur at the point of photoexcitation in the MC plane and therefore they decrease the migration range. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We review the spectroscopic and dynamic properties of cavity polaritons in a λ-wide GaAs/GaAlAs microcavity (MC) structure having a single GaAs/AlAs quantum well in which a variable density two-dimensional electron gas (2DEG) is photoexcited, with a density in the range 0 < ne < 5 × 1011 cm−2. The main observed effects that will be discussed here are: (a) photoexcitation of negatively charged polaritons. These polaritons result of the coupling of the MC photon and the negatively charged excitons. They appear at low temperatures (T <25 K), in the range of low 2DEG densities (ne < 5 × 1010 cm−2), and their coupling strength increases as √ne. (b) The effect of electron–polariton scattering on the polariton spectral linewidth, its complex dependence on the detuning energy, and the electron-induced transition from strong to weak coupling regimes, as ne increases. (c) The efficient polariton transfer across the relaxation bottleneck in the lower branch induced by electron–polariton scattering, and the resulting non-linear dependence of the polariton photoluminescence intensity on ne.

We present a study of a microcavity polariton system in the presence of an optically induced electron gas. Angle resolved photoluminescence measurements with nonresonant excitation show enhanced emission from the k /spl ap/ 0 states as the electron density is increased. This indicates that electron-polariton scattering can assist in relaxing the polariton bottleneck.

In semiconductor microcavities, electron-polariton scattering has been proposed as an efficient process that can drive polaritons from the bottleneck region to the ground state, achieving Bose amplification of the optical emission. We present clear experimental observation of this process in a structure that allows control of the electron density and we report substantial enhancement of photoluminescence. We show that this enhancement is more effective at higher temperatures due to the different way that electron scattering processes either broaden or relax polaritons.

Electron-polariton scattering is studied by photoluminescence spectroscopy in a GaAs/AlGaAs microcavity with an embedded quantum well containing a variable density electron gas. The photoluminescence spectra are measured as a function of two parameters: (a) The detuning energy between the cavity-confined photon and the heavy exciton and (b) the intensity of the laser that generates the electron gas. The integrated photoluminescence intensity varies nonlinearly with increasing electron density, showing a large enhancement that reaches a factor of 35 over the intensity observed without an electron gas. The spectra are analyzed in the strong coupling regime, by calculating the energy and electron gas density dependence of the electron-polariton scattering rates. The most effective scattering process is the dissociation of the charged exciton component in the polariton states.

The effect of a classical electron gas on the cavity polariton linewidths is studied experimentally at T = 80 K in a GaAs/AlAs microcavity with an embedded quantum well. The linewidth dependence on the electron density and on the cavity mode energy is explained by a theoretical model based on the asymmetric bare exciton line shape, due to electron-exciton scattering.

We report on a detailed study of the electron-exciton scattering in a GaAs quantum well (QW) with a variable density two-dimensional electron gas. Both a bare QW and a QW embedded in a GaAs/AlGaAs microcavity are studied by measuring the reflection linewidths of the bare excitons and the cavity polaritons as a function of photoexcitation intensity, temperature, and a perpendicularly applied magnetic field. This field induces the formation of charged polaritons at temperatures higher than the range of the charged exciton binding energy. The bare exciton linewidth dependence on electron density shows a transition when the electron gas transforms from classical to degenerate state. The lowest polariton linewidth dependence on electron gas and detuning energy is explained by the overlap of the cavity mode with the asymmetric exciton lineshape.

The effect of a classical electron gas on the cavity polariton linewidths is studied experimentally in a GaAs/AlAs microcavity with an embedded quantum well that contains photogenerated electrons (at T=80 K). The line-width dependence on the electron density and on the cavity mode energy is explained by a theoretical model based on calculating the asymmetric bare exciton line shape, due to electron-exciton scattering. For a given electron density, the exciton line shape is inserted into the linear dispersion theory of the coupled excitons and cavity mode, yielding the reflection spectra that are formed of the three polariton branches. This model reproduces the reduced lower-polariton linewidth and the complex dependence of the middle- and upper-polariton linewidths on the cavity mode energy.

We report on a detailed study of the free electron scattering effects on the (e1: hh1)1S and (e1: 1h1)1S excitons in a GaAs quantum well with a variable density two-dimensional electron gas, that is either bare or embedded in a GaAs/AlGaAs microcavity. These effects are studied by measuring the reflection line width of the bare excitons or cavity polaritons as a function of photoexcitation intensity. temperature (2 < T < 80 K) and a perpendicularly applied magnetic field (0 < B < 7 T). This field induces the formation of charged polaritons at temperatures higher than the range of charged exciton existence without a magnetic field. In order to interpret the line-width data observed at high temperatures, when no charged polaritons exist, we developed a theoretical model that is based on calculating the exciton-electron direct and exchange interaction matrix elements, from which we derive the scattering rates of bare excitons. The model accounts well for the polariton line-widths dependence on microcavity-photon detuning energy and the electron density, (C) 2002 Elsevier Science B.V. All rights reserved.

We report on an experimental study of the free electron scattering effects on the cavity polaritons (CP's) energy and linewidth in a GaAs/AlGaAs microcavity (MC), with an embedded quantum well (QW) that contains a variable density two dimensional electron gas (2DEG). Reflection spectra were measured (at T = 80K) as a function of the MC mode energy, in the spectral range of the (e1:hh1)1S and the (e1:1h1)1S excitons, and with a 2DEG density n(e) < 5 x 10(11) cm(-2). The main observations are: a. Over a large detuning energy range the linewidth of the lowest CP branch does not vary while those of the higher CP's increase strongly with increasing n(e). It indicates that the lowest CP branch bottleneck is not suppressed even by the efficient electron CP scattering. b. When the MC mode energy is close to resonance with each of the excitons, a strong increase in the modes linewidths is seen, indicating a resonant enhancement in the free electrons - CP's scattering c. In the n(e) similar to 10(11) cm(-2) range, the linewidths of all CP branches increase and the exciton - MC confined photon system transforms from the strong to the weak coupling regime. This is attributed to an increased CP dephasing (resulting from electron - CP scattering) and not from screening and phase space filling effects.

When a quantum well containing a low density two-dimensional electron gas is embedded in a microcavity, the photoexcited charged excitons that interact with the confined photons form charged polaritons, The objective of this paper is to describe the experimental spectroscopic evidence for the appearance of charged microcavity polaritons, and the analysis of their energy and oscillator strength dependence on the 2DEG density. Then, the model calculations are extended to predict that the charged polaritonic signal can be electrically drifted in the QW plane over measurable distances.

The strong resonant interaction between microcavity (MC) confined photons and quantum well (QW) interband excitations near the onset of the exciton continuum is investigated in lambda - wide, planar GaAs/AlGaAs MC's, having a single GaAs/AlAs QW (200A wide). The spectra show sharp lines of the cavity polaritons that are formed of the (1e:hh1)1S, (1e:1h1)1S, (e1:hh1)2S and (e1:1h1)2S excitons, the interband transition between the N = 1 electron and hole Landau states, as well as of unidentified magnetoexcitons with energies around that of the (e1:hh1)2S. All the near bandgap cavity polaritons. are observed only when the magneto excitons, of which they are formed, are tuned into resonance with the MC confined photons, and their intensity is observed to increase with increasing B. The measured (1e:hh1)1S and (e1:hh1)2S diamagnetic shift and Rabi splitting are used in order to estimate the magnetoexcitons wavefunction in-plane radius decrease with increasing B (in the low field range).

We report on an experimental study of the microcavity polariton linewidth in an AlAs/AlGaAs microcavity containing GaAs quantum wells (QW) in which a variable density electron gas is photogenerated. The linewidth of the lowest polariton branch shows a clear suppression while that of the other branches show enhancement at narrow detuning energy ranges. A transformation is observed from a strong to weak exciton-cavity mode coupling at a very low photoexcitation intensity.