Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field

The effect of an external electric field F on the excitonic photoluminescence (PL) spectra of a symmetric coupled double quantum well (DQW) is investigated both theoretically and experimentally. We show that the variational method in a two-particle electron-hole wave function approximation gives a good agreement with measurements of PL on a narrow DQW in a wide interval of F including flat-band regime. The experimental data are presented for an MBE-grown DQW consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers. The bias voltage is applied along the growth direction. Spatially direct and indirect excitonic transitions are identified, and the radius of the exciton and squeezing of the exciton in the growth direction are evaluated variationally. The excitonic binding energies, recombination energies, oscillator strengths, and relative intensities of the transitions as functions of the applied field are calculated. Our analysis demonstrates that this simple model is applicable in case of narrow DQWs not just for a qualitative description of the PL peak positions but also for the estimation of their individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B

Hydrogen intercalation of epitaxial graphene and buffer layer probed by mid-infrared absorption and Raman spectroscopy

We have measured optical absorption in mid-infrared spectral range on hydrogen intercalated single layer epitaxial graphene and buffer layer grown on silicon face of SiC. We have used attenuated total reflection geometry to enhance absorption related to the surface and SiC/graphene interface. The Raman spectroscopy is used to show presence of buffer layer and single layer graphene prior to intercalation. We also present Raman spectra of quasi free standing monolayer and bilayer graphene after hydrogen intercalation at temperatures between 790 and 1510°C. We have found that although the Si-H bonds form at as low temperatures as 790°C, the well developed bond order has been reached only for samples intercalated at temperatures exceeding 1000°C. We also study temporal stability of hydrogen intercalated samples stored in ambient air. The optical spectroscopy shows on a formation of silyl and silylene groups on the SiC/graphene interface due to the residual atomic hydrogen left from the intercalation process

MAGNETO - OPTICAL EFFECTS IN THE IMPURITY SPECTRAL REGION OF CdCr2Se4

Nous avons mesuré l'absorption optique et le dichroïsme circulaire magnétique sur les monocristaux de CdCr2Se4. On compare l'effet des traitements thermiques soit dans le vide soit dans la vapeur de sélénium avec celui qu'on observe dans les spectres des cristaux dopés à l'argent ou irradiés par les deuterons.Optical absorption and magnetic circular dichroïsm measurements in CdCr2Se4 single crystals are reported. We compare the samples annealed in vacuum or in selenium vapour with the silver doped ones and with the samples irradiated by deuterons