Interplay of the exciton and electron-hole plasma recombination on the photoluminescence dynamics in bulk GaAs

We present a systematic study of the exciton/electron-hole plasma photoluminescence dynamics in bulk GaAs for various lattice temperatures and excitation densities. The competition between the exciton and electron-hole pair recombination dominates the onset of the luminescence. We show that the metal-to-insulator transition, induced by temperature and/or excitation density, can be directly monitored by the carrier dynamics and the time-resolved spectral characteristics of the light emission. The dependence on carrier density of the photoluminescence rise time is strongly modified around a lattice temperature of 49 K, corresponding to the exciton binding energy (4.2 meV). In a similar way, the rise-time dependence on lattice temperature undergoes a relatively abrupt change at an excitation density of 120-180x10^15 cm^-3, which is about five times greater than the calculated Mott density in GaAs taking into account many body corrections.Comment: 15 pages, 7 figures, submitted to Phys. Rev.

Observation of long-lived polariton states in semiconductor microcavities across the parametric threshold

The excitation spectrum around the pump-only stationary state of a polariton optical parametric oscillator (OPO) in semiconductor microcavities is investigated by time-resolved photoluminescence. The response to a weak pulsed perturbation in the vicinity of the idler mode is directly related to the lifetime of the elementary excitations. A dramatic increase of the lifetime is observed for a pump intensity approaching and exceeding the OPO threshold. The observations can be explained in terms of a critical slowing down of the dynamics upon approaching the threshold and the following onset of the soft Goldstone mode

Interrelaciones entre GSH y ascorbato en células de mamífero: Implicaciones fisiológicas y clínicas

Las vitaminas poseen diferentes papeles fisiológicos y clínicos además del de la prevención de enfennedades carenciales. Este es el caso de la vitamina C, que más allá de prevenir el escorbuto, posee actividad antioxidante bien descrita y la capacidad de ahorrar GSH. El GSH es un tripéptido ampliamente distribuido en las células de marniferos, el cual no es requerido en la dieta. El ciclo del gamma glutamilo es el responsable de la síntesis y de la degradación del GSH. Este tripéptido provee a la célula de un medio reductor a través de la acción de la glutation difulfuro reductasa. La administración de ácido ascórbico puede contribuir también al sistema reductor de las células. Existen numerosos datos científicos que apoyan el hecho de que algunas condiciones asociadas con estrés oxidativo podrían ser mejoradas por una terapia que mantuviera los niveles nonnales de GSH. Esto se puede conseguir por la administración de ésteres de GSH, aumentando la capacidad de síntesis de GSH al proporcionar los substratos, como el N-acety-L-cysteina y/o aumentando la disponibilidad de compuestos como el ascorbato que puede ahorrar GSH. Todos estos efectos podrían ser de interés clínico para el diseño de un "cocktail" adecuado que mantuviera el GSH intracelular dentro de valores nonnales para tejidos de mamíferos, en condiciones en las que el GSH estuviera disminuido.Vitamins have different physiological and clinical roles besides preventing deficiency diseases. This is the case of vitamin C that beyond preventing scurvy, it has a well known antioxidant activity and the capacity to spare GSH. GSH is a tripeptide widely distributed in marnmalian cells, which is not required in the diet. The gamma-glutamyl cycle is responsible for the synthesis and degradation of GSH. This tripeptide provides the cell with a reducing milieu that is achieved through the action of glutathione disulfide reductase. Administration of ascorbic acid may also contribute to the reducing properties of cells. There is enough scientific background to support the fact that several conditions associated with oxidative stress might be improved by therapy that maintain GSH within normal leves. This can be achieved by the administration of GSH-esters, increasing the capacity for GSH synthesis by providing substrates such as N-acetyl-L-cysteine aml/or by increasing the availibility of compounds such as ascorbate that can spare GSH. AII these facts could be of clinical interest in the design of the right "cocktail" in order to keep intracellular GSH within normal values in marnmalian tissues under those situations were GSH is depleted

Optical control of spin textures in quasi-one-dimensional polariton condensates

We investigate, through polarization-resolved spectroscopy, the spin transport by propagating polariton condensates in a quasi one-dimensional microcavity ridge along macroscopic distances. Under circularly polarized, continuous-wave, non-resonant excitation, a sinusoidal precession of the spin in real space is observed, whose phase depends on the emission energy. The experiments are compared with simulations of the spinor-polariton condensate dynamics based on a generalized Gross-Pitaevskii equation, modified to account for incoherent pumping, decay and energy relaxation within the condensate.Comment: 10 pages, 9 figure

Pauli blockade of the electron spin flip in bulk GaAs

By means of time-resolved optical orientation under strong optical pumping, the k-dependence of the electron spin-flip time (t_sf) in undoped GaAs is experimentally determined. t_sf monotonically decreases by more than one order of magnitude when the electron kinetic energy varies from 2 to 30 meV. At the high excitation densities and low temperatures of the reported experiments the main spin-flip mechanism of the conduction band electrons is the Bir-Aronov-Pikus. By means of Monte-Carlo simulations we evidence that phase-space filling effects result in the blocking of the spin flip, yielding an increase of t_sf with excitation density. These effects obtain values of t_sf up to 30 ns at k=0, the longest reported spin-relaxation time in undoped GaAs in the absence of a magnetic field.Comment: new author added, major changes in section IV (phenomenological model), minor changes throughout the entire manuscrip

Spin relaxation in low-dimensional systems

We review some of the newest findings on the spin dynamics of carriers and excitons in GaAs/GaAlAs quantum wells. In intrinsic wells, where the optical properties are dominated by excitonic effects, we show that exciton-exciton interaction produces a breaking of the spin degeneracy in two-dimensional semiconductors. In doped wells, the two spin components of an optically created two-dimensional electron gas are well described by Fermi-Dirac distributions with a common temperature but different chemical potentials. The rate of the spin depolarization of the electron gas is found to be independent of the mean electron kinetic energy but accelerated by thermal spreading of the carriers.Comment: 1 PDF file, 13 eps figures, Proceedings of the 1998 International Workshop on Nanophysics and Electronics (NPE-98)- Lecce (Italy

Remarks on the naturality of quantization

Hamiltonian quantization of an integral compact symplectic manifold M depends on a choice of compatible almost complex structure J. For open sets U in the set of compatible almost complex structures and small enough values of Planck's constant, the Hilbert spaces of the quantization form a bundle over U with a natural connection. In this paper we examine the dependence of the Hilbert spaces on the choice of J, by computing the semi-classical limit of the curvature of this connection. We also show that parallel transport provides a link between the action of the group Symp(M) of symplectomorphisms of M and the Schrodinger equation.Comment: 20 page

Polarization Control of the Non-linear Emission on Semiconductor Microcavities

The degree of circular polarization ($\wp$) of the non-linear emission in semiconductor microcavities is controlled by changing the exciton-cavity detuning. The polariton relaxation towards \textbf{K} $\sim 0$ cavity-like states is governed by final-state stimulated scattering. The helicity of the emission is selected due to the lifting of the degeneracy of the $\pm 1$ spin levels at \textbf{K} $\sim 0$. At short times after a pulsed excitation $\wp$ reaches very large values, either positive or negative, as a result of stimulated scattering to the spin level of lowest energy ($+1/-1$ spin for positive/negative detuning).Comment: 8 pages, 3 eps figures, RevTeX, Physical Review Letters (accepted