Dynamical Corrections to Spin Wave Excitations in Quantum Wells due to Coulomb Interactions and Magnetic Ions

We have measured dispersions of spin-flip waves and spin-flip single-particle excitations of a spin polarized two-dimensional electron gas in a CdMnTe quantum well using resonant Raman scattering. We find the energy of the spin-flip wave to be below the spin-flip single particle excitation continuum, a contradiction to the theory of spin waves in diluted magnetic semiconductors put forth in [Phys. Rev. B 70, 045205 (2004)]. We show that the inclusion of terms accounting for the Coulomb interaction between carriers in the spin wave propagator leads to an agreement with our experimental results. The dominant Coulomb contribution leads to an overall red shift of the mixed electron-Mn spin modes while the dynamical coupling between Mn ions results in a small blue shift. We provide a simulated model system which shows the reverse situation but at an extremely large magnetic field.Comment: 6 pages, 7 figure

Fabry-Pérot-multichannel spectrometer tandem for ultra-high resolution Raman spectroscopy

We present a novel ultra-high resolution Raman spectroscopy technique based in a Fabry-Pérot/triple spectrometer tandem with multichannel acquisition. We describe the system, detail the calibration process, and experimentally test the technique, showing that effective finesses in excess of 1000 are possible. The technique is specifically tailored for low intensity, complex and spectrally extended Raman spectra, providing shorter acquisition times with respect to similar tandem systems with monochannel detectors.Fil: Rozas, Guillermo. Comisión Nacional de Energía Atómica; Argentina. Comision Nacional de Energia Atomica. Gerencia del _rea de Investigaciones y Aplicaciones No Nucleares. Gerencia de Física (CAB). Laboratorio de Propiedades Opticas; ArgentinaFil: Jusserand, Bernard. Universite Pierre et Marie Curie; FranciaFil: Fainstein, Alejandro. Comisión Nacional de Energía Atómica; Argentina. Comision Nacional de Energía Atómica. Gerencia de Área Investigaciones y Aplicaciones no Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Física de Metales; Argentin

Raman-Brillouin electronic density in short-period superlattices

International audienceWe analyze and interpret resonant Raman-Brillouin scattering by folded acoustic vibrations in short-period GaAs/AlAs superlattices. Analysis of the spectra and their resonance behavior is performed using a Raman-Brillouin electronic density constructed by combining thousands transitions between electronic eigenstates of the system according to their weight in the light-scattering process. We show that plots of this effective electronic density allow for capturing the essential physics of the electron-phonon interaction and of the resonant light-scattering process in a situation where complex effects are simultaneously present: electronic confinement in the quantum wells and wave-function delocalization due to interlayer coupling, folding of acoustic dispersion and symmetry changes in the deformation fields, resonant selection of optical transitions. Comparison between the measured spectra and those simulated using the Raman-Brillouin quantum model and the photoelastic model are presented. Activation and/or deactivation of the scattering by acoustic vibration doublets and changes in their intensity ratio with excitation energy are directly related to the Raman-Brillouin electronic density distribution along the superlattices axis. Limitations of the photoelastic model are discussed by comparing the steplike variation in the photoelastic coefficient to the Raman-Brillouin electronic density profiles

Superlattice and disorder effects on vibrations in III-V compounds

We analyse the perturbation induced either by a random atomic distribution (mixed crystal) or a one-dimensional periodic one (superlattice) onto the vibrations in III-V compounds and in particular onto the Raman backscattering active modes. As a consequence of the induced relaxation of the wavevector selection rule; some new modes become Raman active. We analyse their frequency and activity as a function of respectively the alloy concentration and the thicknesses of layers constituting the superlattice. We illustrate these results on structures based on the GaAs and AlAs compounds

Raman scattering in resonant cavities

International audienceThe modification of the optical properties of matter due to photon confinement in optical microcavities has been an active field of research in the last ten years. This review addresses the problem of Raman scattering in these optically confining structures. Two completely different regimes exist and will be discussed here. Firstly, a situation in which the action of the microcavity is basically to enhance, and to spatially and spectrally confine, the photon field, but otherwise the light-matter interaction process remains unaltered. This is the case when the laser and scattered photon energies are well below those of the excitoilic transitions in the structure, or when the coupling between the latter and the cavity mode can be treated in a ``weak coupling'' approximation. This regime will be labeled here as ``optical resonant Raman scattering''. And secondly, a ``strong-coupling'' regime in which exciton and cavity-photon modes cannot be treated separately, leading to coupled excitations, so-called cavity polaritons. In this second case, when the laser or scattered photons are tuned to the excitonic energies, and thus an electronic resonant Raman-scattering process is achieved, the Raman-scattering process has to be described in a fundamentally different way. This regime will be labeled as ``cavity-polariton-mediated Raman scattering''. The Chapter will begin, after a brief historical introduction, with a review of the fundamental properties of optical microcavities, and of the different strategies implemented to achieve double optical resonant Raman scattering in planar microcavities. A section will be then devoted to experimental results that highlight the different characteristics and potentialities of Raman scattering under optical confinement, including, in some detail, an analysis of the performance of these structures for Raman amplification. A subsequent section will present a series of research efforts devoted to the study of nanostructure phonon physics that rely on microcavities for Raman enhancement. In particular, emphasis will be given to recent investigations on acoustic cavities that parallel their optical counterparts but that, instead, confine hypersound in the GHz-THz range. The Chapter will then turn to a quite different topic, that of cavity-polariton-mediated scattering. The theory developed in the 1970s for bulk materials will be briefly introduced, and its modifications to account for photon confinement in planar structures will be addressed. Finally, a series of experiments that demonstrate the involvement of polaritons in the inelastic scattering of light in strongly coupled cavities will be reviewed. The Chapter will end with some conclusions and prospects for future developments on this subject. also a stimulated emission of coherent phonons. It is an interesting issue whether acoustic and optical cavities can contribute essential ingredients for the development of phonon ``lasers''. Acoustic cavities based on distributed Bragg reflectors as described here could provide the required feedback mechanism, while optical microcavities could be exploited to efficiently seed the stimulated-emission process. All the experiments reviewed here, and to the best of our knowledge all the reported investigations of Raman amplification in condensed-matter cavities, rely on planar one-dimensional confining structures. However, and in the same way as micrometer-size liquid droplets have led to enormous Raman gain, it is possible to conceive higher-dimensional optical confining structures that could be exploited for Raman amplification. These include pillars that, based on 1D cavities, achieve 3D optical confinement by lateral nanostructuring. Also whispering gallery modes in high-Q disk-like cavities can be a possible choice in the search of Raman signals from single, or a few, semiconductor dots. It would also be interesting to search for Raman amplification using high-Q modes related to defects and high density of states extended states in 2- and 3-dimensional photonic bandgap, devices. Last but not least, the reported experiments dealing with cavity-polariton-mediated scattering are extremely few, and the available theories quite crude. We believe that this remains an interesting and quite unexplored territory for research on a very basic and fundamental light-matter interaction process

Microspectroscopie Raman des nanofils de GaAs

This thesis presents a study by micro-Raman spectroscopy of a new type of nanostructures: semiconductor nanowires. It is focused onto GaAs nanowires with diameter between 50 and 150nm, fabricated by Molecular beam epitaxy (MBE). This is the first systematic study of vibrations and related electronic resonances in the two crystalline forms available for GaAs nanowires depending of the growth conditions: zinc-blende (ZB) as in bulk GaAs and wurtzite (WZ), a form not stable in bulk form. Raman spectroscopy is performed on single nanowires thanks to a dedicated micro-Raman set-up developed during this work. Based on an overall characterization of optical vibrations, this work has been mainly focused on two research lines. 1) the study of Raman polarizations of the recorded lines. It has been evidenced that the microscopic symmetry of nanowires has a limited influence onto the observed polarizations which appear to be mostly controlled by the shape anisotropy of nanowires. A model of dielectric confinement has been developed and well reproduces most of the experimental observations. It predicts a significant dependence onto the diameter which has been confirmed by preliminary experiments on wires with different diameters. 2) the study of Raman resonances. It has been first observed that Raman micro spectroscopy in standard conditions, based on an Argon ion laser with wavelengths around 500nm, is very difficult and generally leads to the destruction of the wires because of strong light absorption of GaAs nanowires in this range of wavelengths. It has been demonstrated that on the contrary the use of wavelengths in the range 700-900nm is very well adapted and allows a full detailed, non destructive study of vibrations in GaAs nanowires. A novel original observation has been obtained in this energy range, the Fröhlich resonance of the LO phonon reflecting an excitonic transition in WZ nanowires shifted by around 100meV from the well established resonance in ZB GaAs. This novel observation well agrees with recent ab initio calculations of WZ GaAs electronic band structure and allows to establish the ordering and the symmetry of the lowest energy conduction states which remains controversial among different versions of published ab initio calculationsCette thèse présente une étude de micro spectroscopie Raman d un type nouveau de nanostructures : les nanofils de semiconducteurs. Elle porte sur des nanofils de GaAs, d un diamètre compris entre 50 et 150 nm, fabriqués par épitaxie par jets moléculaires. Il s agit de la première étude systématique des modes de vibration et des résonances électroniques associées dans les deux formes cristallines qui peuvent être obtenues suivant les conditions de croissance : la blende de zinc comme pour GaAs massif et la WZ qui n existe pas à l état massif. La spectroscopie est réalisée sur un nanofil unique grâce à un dispositif de micro-Raman original développé pendant la thèse. Après une caractérisation d ensemble des vibrations optiques, le travail s est concentré sur deux volets particuliers : 1) l étude des polarisations Raman des raies observées. Il a été mis en évidence que la symétrie microscopique des matériaux a une influence faible sur les polarisations observées qui sont entièrement dominées par l anisotropie de forme des nanofils. Un modèle de confinement diélectrique a été développé qui reproduit bien l essentiel des observations expérimentales et prédit une influence très forte du diamètre des fils qui a fait l objet de vérifications préliminaires. 2) l étude des résonances Raman. Il a d abord été observé que la micro spectroscopie Raman standard avec une illumination par un laser à Argon ionisé à des longueurs d onde vers 500nm est très difficile et conduit généralement à la destruction des fils étudiés à cause de la forte absorption optique dans cette gamme de longueurs d onde. Il a été démontré qu au contraire l utilisation de longueurs d onde dans la gamme 700-900nm est très bien adaptée et permet une étude complète et non destructive des nanofils de GaAs. Une observation très originale a été réalisée, une résonance de Fröhlich du phonon LO qui traduit une transition excitonique très fortement décalée en énergie dans les fils de forme WZ par rapport aux fils de forme blende de zinc où les observations sont conformes à ce qui est connu pour GaAs massif. Cette observation originale est en bon accord avec les prédictions de calculs ab initio de la structure de bande électronique de GaAs WZ et permet de préciser l ordre et la symétrie des états de conduction qui fait l objet de débats entre plusieurs variantes des calculs ab initio publiésPARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF