No description available.

Cette thèse porte sur l'étude des ondes de spin dans des puits quantiques CdMnTe dopés n, par rotation Kerr résolue en temps (TRKR) et par mélange à quatre ondes (FWM). Nous avons étudié trois échantillons de haute mobilité et de caractéristiques différentes.La technique TRKR donne accès uniquement aux excitations de vecteur d'onde nul, dans notre cas l'onde spin-flip en q=0. Nous avons étudié l'anticroisement qui apparait entre l'onde spin-flip et l'excitation spin-flip des ions manganèse. Nous avons étudié la variation du gap, et donc de l'énergie de couplage, entre ces modes en fonction de la puissance d'excitation et du champ magnétique. En particulier nous avons étendu les mesures des modes mixtes à plus basse concentration en Mn (jusqu'à 0.07%) et contrairement à ce qui était attendu, nous avons trouvé que le régime de couplage fort persiste à cette concentration.Nous nous sommes ensuite intéressés à la détermination de la polarisation en spin ζ du gaz d'électrons bidimensionnel, qui peut être déduite de l'énergie de couplage entre les modes mixtes. Nous avons trouvé que la polarisation mesurée par cette méthode excède la polarisation théorique calculée en prenant en compte le renforcement de la susceptibilité par les effets à N corps. Nous avons également mesuré les temps de relaxation des électrons confinés dans le puits quantique, et nous avons montré l'influence de l'échauffement de l'échantillon par le laser sur le temps de relaxation de spin des électrons.Dans la deuxième partie de cette thèse, nous avons étudié par FWM l'amortissement et la dispersion des ondes de spin de vecteur d'onde non nul pour l'un de nos échantillons. Nous avons démontré qu'on peut effectivement générer les ondes de spin en excitation femtoseconde, et les détecter en FWM. Nous avons trouvé que leur dispersion est plus faible que celle observée dans les expériences de Raman. Cette faible dispersion pourrait être imputable à la forte densité d'excitation utilisée dans les expériences de FWM (typiquement trois à quatre ordres de grandeur supérieurs à celle du Raman), et/ou au fait que deux ondes de vecteur d'ondes q et –q, ayant des dispersions différentes, sont sondées simultanément en FWM. / This thesis focuses on the study of spin waves in n-doped CdMnTe quantum wells using respectively time-resolved Kerr rotation (TRKR) and four-wave mixing (FWM) techniques. We studied three high mobility samples with different characteristics.The TRKR technique gives access only to zero wave vector excitations, in our case the spin- flip wave q = 0 . We studied the anticrossing that appears between the spin -flip wave and the manganese spin -flip excitation. We studied the gap variation energy between these modes as function on the power excitation and the magnetic field. In particular, we have extended the measurements of mixed modes at lower Mn concentration (up 0.07 %) and contrary to what were expected; we found that the strong coupling regime persists at this concentration.We are then interested in determining the two dimensional electron gas spin polarization ζ, which can be deduced from the energy coupling between the mixed modes. We found that the measured polarization exceeds the theoretical polarization calculated taking into account the increased susceptibility by many-body effects. We also measured the electron spin relaxation time and we have shown that it is influenced by thermal effects inherent to optical pump-probe experiments on this time.In the second part of this thesis, we studied by FWM the damping and the dispersion of the non-zero wave vector spin waves for one of our samples. We have demonstrated that we can actually generate spin waves in femtosecond excitation and deted them by FWM. We found that the dispersion is lower than that observed in the Raman experiments. This low dispersion may be due to the strong excitation density used in the FWM experiments (typically three to four orders of magnitude higher than the Raman ones) and / or the fact that two waves of wave vector q and - q, having different dispersions are simultaneously probed in FWM .

We correlated experimental results with theoretical estimations of the dielectric function ε(ω) in two contexts: the effect of an electric field in quantum wells and that of the spin polarization of an interacting electron-hole plasma in bulk semiconductors. In the first part, we recorded photoreflectance spectra from Ge/GeSi quantum wells of different widths but having comparable builtin electric fields caused by doping. The reason why the spectra differed in overall shape was difficult to understand by conventional methods, for example, by calculating the allowed transition energies or by fitting the data with lineshape functions at each transition energy. Instead, we computed the photoreflectance spectra from first-principles by using the confined electron and hole wavefunctions. This method showed that the spectra differ in overall shape because of the experimentally hitherto unobserved trend in quantum well electro-optical properties, from the quantum confined Franz-Keldysh effect to the bulk Franz-Keldysh effect, as the well width is increased. The second part develops a threeband microscopic theory for the optical properties due to spin-polarized carriers in quasiequilibrium. We show that calculations based on this theory reproduce all the trends observed in a recent circularly polarized pump-probe experiment reported in the literature. To make the computation less intensive, we proposed a simplified, two-band version of this theory which captured the main experimental features. Besides, we constructed a cw diode laser-based pump-probe setup for our own optical studies of spin-polarized carriers by Kerr rotation. We achieved a sensitivity of detection of Kerr rotation of 3 x 10¯ 8 rad, corresponding to an order of magnitude improvement over the best reports in the literature. The efficacy of our setup allowed for the demonstration of a pumpinduced spin polarization in bulk GaAs, under the unfavorable conditions of steady-state and room temperature.

No description available.

No description available.

No description available.

Nesta tese, analisamos dois sistemas de semicondutores magnéticos: um semicondutor magnético cristalino de EuTe e uma heteroestrutura formada por um poço quântico de GaAs/AlGaAs ao lado uma barreira tipo delta de Mn, que, ao difundir-se, forma o semicondutor magnético diluído de (Ga,Mn)As. Nossos estudos foram focados na possibilidade de manipularmos oticamente a orientação magnética de ambos os sistemas. No semicondutor magnético de EuTe, a indução de magnetização se dá pela formação de polarons magnéticos ao redor de elétrons fotoexcitados. Para o estudo dos polarons, um modelo teórico elaborado foi adaptado para a construção de um sistema computacional baseado no método de Monte Carlo. Essa sistema permitiu o cálculo do momento magnético e do raio do polaron em temperaturas finitas, muito acima da temperatura de Néel. O modelo foi elaborado para reproduzir tanto as propriedades do EuTe sem o polaron (temperatura de Néel e campo crítico), quanto o deslocamento da linha de fotoluminescência devido a formação do polaron. Além do desenvolvimento do próprio método computacional, que pode ser utilizado para estudar outros sistemas, o conhecimento adquirido com o estudo do EuTe serviu como base para o estudo de um sistema mais complexo, que é a heteroestrutura de GaAs/AlGaAs + dMn. O estudo da heteroestrutura de GaAs/AlGaAs + dMn foi feito em cima de medidas experimentais de rotação de Kerr com resolução temporal. O sistema de medição construído permite, também, medidas de rotação de Kerr com resolução espacial, que servem para o estudo de transporte e hélice de spin em semicondutores, e está detalhadamente descrito em um dos capítulo desta tese. Na amostra estudada, o controle da magnetização dos íons de Mn é feito através da interação de troca com o elétron fotoexcitado no poço quântico. Os resultados obtidos das medidas de rotação de Kerr mostram uma frequência de precessão dependente do tempo, que revela a existência de dois processos com dinâmicas diferentes: uma primeira orientação do spin dos íons de Mn devido à polarização do par elétron-buraco no poço quântico, seguida por um realinhamento desses spins com o campo magnético externo, a partir do momento em que a coerência dos spins dos buracos desaparece. Esse resultado sugere que a interação entre os elétrons fotoexcitados e os íons de Mn ocorre por intermédio dos buracos fotoexcitados, ao contrário do que havia sido proposto em estudos anteriores de estruturas similares, mas de acordo com o modelo de interação sp-d, utilizado para explicar o ferromagnetismo do (Ga,Mn)As. / In this thesis we analyzed two magnetic semiconductor systems: one intrinsic magnetic semiconductor crystal of EuTe and one GaAs-based heterostructure with a GaAs/AlGaAs quantum well close to delta-type Mn barrier, that forms a diluted magnetic semiconductor of (Ga,Mn)As after diffusion. Our studies on both systems were focused on the possibility of optical manipulation of magnetic order. In EuTe pure semiconductor, the magnetization control occurs due to de formation od magnetic polarons around photo-excited electrons. To study magnetic polarons we adapted a theoretical model to build a computer simulation system based on Monte Carlo\'s method. This system allowed us to calculate the magnetic moment and radius of the polaron at finite temperatures fair above Néel Temperature. The computational model was tested to reproduce EuTe properties without polarons (Néel Temperature and critical magnetic field) and with polarons (photoluminescence line shift). Beside the development of this computational model, that can be used to study other systems, the knowledge acquired during the studies on EuTe helped us to better understand the more complex system of the GaAs/AlGaAs +dMn heterostructure. The studies about the GaAs/AlGaAs + dMn heterostructure were based on experimental measurements of time-resolved Kerr rotation. The measurement system we built also allows us to perform spatial-resolved Kerr rotation measurements to study spin transport and spin helix on semiconductors and it is described in details in one chapter of this thesis. The optical manipulation of Mn ions magnetization on the studied sample is consequence of the exchange interaction with the photoexcited electron inside the quantum well. The results of Kerr rotation measurements show a time-dependent precession frequency that reveals the existence of two process with distinct dynamics: the initial orientation of Mn ions spins with the photoexcited electron-hole pair, followed by the realignment of these spins with the external magnetic field, as soon as the photoexcited hole spins loose its coherence. These results indicate that the exchange interaction between the photoexcited electron inside the quantum well and the Mn ions is mediated by the photoexcited holes, in opposition of what was being proposed in previous studies of similar structures, but in agreement with the sp-d model, used to explain the (Ga,Mn)As ferromagnetism.

Nesta tese, analisamos dois sistemas de semicondutores magnéticos: um semicondutor magnético cristalino de EuTe e uma heteroestrutura formada por um poço quântico de GaAs/AlGaAs ao lado uma barreira tipo delta de Mn, que, ao difundir-se, forma o semicondutor magnético diluído de (Ga,Mn)As. Nossos estudos foram focados na possibilidade de manipularmos oticamente a orientação magnética de ambos os sistemas. No semicondutor magnético de EuTe, a indução de magnetização se dá pela formação de polarons magnéticos ao redor de elétrons fotoexcitados. Para o estudo dos polarons, um modelo teórico elaborado foi adaptado para a construção de um sistema computacional baseado no método de Monte Carlo. Essa sistema permitiu o cálculo do momento magnético e do raio do polaron em temperaturas finitas, muito acima da temperatura de Néel. O modelo foi elaborado para reproduzir tanto as propriedades do EuTe sem o polaron (temperatura de Néel e campo crítico), quanto o deslocamento da linha de fotoluminescência devido a formação do polaron. Além do desenvolvimento do próprio método computacional, que pode ser utilizado para estudar outros sistemas, o conhecimento adquirido com o estudo do EuTe serviu como base para o estudo de um sistema mais complexo, que é a heteroestrutura de GaAs/AlGaAs + dMn. O estudo da heteroestrutura de GaAs/AlGaAs + dMn foi feito em cima de medidas experimentais de rotação de Kerr com resolução temporal. O sistema de medição construído permite, também, medidas de rotação de Kerr com resolução espacial, que servem para o estudo de transporte e hélice de spin em semicondutores, e está detalhadamente descrito em um dos capítulo desta tese. Na amostra estudada, o controle da magnetização dos íons de Mn é feito através da interação de troca com o elétron fotoexcitado no poço quântico. Os resultados obtidos das medidas de rotação de Kerr mostram uma frequência de precessão dependente do tempo, que revela a existência de dois processos com dinâmicas diferentes: uma primeira orientação do spin dos íons de Mn devido à polarização do par elétron-buraco no poço quântico, seguida por um realinhamento desses spins com o campo magnético externo, a partir do momento em que a coerência dos spins dos buracos desaparece. Esse resultado sugere que a interação entre os elétrons fotoexcitados e os íons de Mn ocorre por intermédio dos buracos fotoexcitados, ao contrário do que havia sido proposto em estudos anteriores de estruturas similares, mas de acordo com o modelo de interação sp-d, utilizado para explicar o ferromagnetismo do (Ga,Mn)As. / In this thesis we analyzed two magnetic semiconductor systems: one intrinsic magnetic semiconductor crystal of EuTe and one GaAs-based heterostructure with a GaAs/AlGaAs quantum well close to delta-type Mn barrier, that forms a diluted magnetic semiconductor of (Ga,Mn)As after diffusion. Our studies on both systems were focused on the possibility of optical manipulation of magnetic order. In EuTe pure semiconductor, the magnetization control occurs due to de formation od magnetic polarons around photo-excited electrons. To study magnetic polarons we adapted a theoretical model to build a computer simulation system based on Monte Carlo\'s method. This system allowed us to calculate the magnetic moment and radius of the polaron at finite temperatures fair above Néel Temperature. The computational model was tested to reproduce EuTe properties without polarons (Néel Temperature and critical magnetic field) and with polarons (photoluminescence line shift). Beside the development of this computational model, that can be used to study other systems, the knowledge acquired during the studies on EuTe helped us to better understand the more complex system of the GaAs/AlGaAs +dMn heterostructure. The studies about the GaAs/AlGaAs + dMn heterostructure were based on experimental measurements of time-resolved Kerr rotation. The measurement system we built also allows us to perform spatial-resolved Kerr rotation measurements to study spin transport and spin helix on semiconductors and it is described in details in one chapter of this thesis. The optical manipulation of Mn ions magnetization on the studied sample is consequence of the exchange interaction with the photoexcited electron inside the quantum well. The results of Kerr rotation measurements show a time-dependent precession frequency that reveals the existence of two process with distinct dynamics: the initial orientation of Mn ions spins with the photoexcited electron-hole pair, followed by the realignment of these spins with the external magnetic field, as soon as the photoexcited hole spins loose its coherence. These results indicate that the exchange interaction between the photoexcited electron inside the quantum well and the Mn ions is mediated by the photoexcited holes, in opposition of what was being proposed in previous studies of similar structures, but in agreement with the sp-d model, used to explain the (Ga,Mn)As ferromagnetism.