Exciton condensates in semiconductor quantum wells emit coherent light

We show that a quasi-two dimensional condensate of optically active excitons emits coherent light even in the absence of population inversion. This allows an unambiguous and clear experimental detection of the condensed phase. We prove that, due to the exciton-photon coupling, quantum and thermal fluctuations do not destroy condensation at finite temperature. Suitable conditions to achieve condensation are temperatures of a few K for typical exciton densities, and the use of a pulsed, and preferably circularly polarized, laser.Comment: 5 pages, no figure

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

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

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

Spin degree of freedom in two dimensional exciton condensates

We present a theoretical analysis of a spin-dependent multicomponent condensate in two dimensions. The case of a condensate of resonantly photoexcited excitons having two different spin orientations is studied in detail. The energy and the chemical potentials of this system depend strongly on the spin polarization . When electrons and holes are located in two different planes, the condensate can be either totally spin polarized or spin unpolarized, a property that is measurable. The phase diagram in terms of the total density and electron-hole separation is discussed.Comment: 4 pages, 3 figures, Accepted for publication in Physical Review Letter

Dark-bright magneto-exciton mixing induced by Coulomb interaction in strained quantum wells

Coupled magneto-exciton states between allowed (`bright') and forbidden (`dark') transitions are found in absorption spectra of strained In$_{0.2}$Ga$_{0.8}$As/GaAs quantum wells with increasing magnetic field up to 30 T. We found large (~ 10 meV) energy splittings in the mixed states. The observed anticrossing behavior is independent of polarization, and sensitive only to the parity of the quantum confined states. Detailed experimental and theoretical investigations indicate that the excitonic Coulomb interaction rather than valence band complexity is responsible for the splittings. In addition, we determine the spin composition of the mixed states.Comment: 4 pages, 4 figure

Control of non-Markovian effects in the dynamics of polaritons in semiconductor microcavities

We report on time-resolved photoluminescence from semiconductor microcavities showing that an optically controllable mechanism exists to turn on and off memory effects in a polariton system. By increasing the laser pumping pulse intensity we observe revivals of the decaying time-resolved photoluminescence signal, a manifestly non-Markovian behavior of the optically active polaritons. Based on an open quantum system approach we perform a comprehensive analytical and numerical study of the coupling of optically active polaritons to a structured reservoir to confirm the origin of the observed features. Our findings show that negative detunings and strong excitation should occur simultaneously for memory effects to take place.Comment: 8 pages, 5 figure