Made available in DSpace on 2016-06-02T20:15:30Z (GMT). No. of bitstreams: 1 5738.pdf: 3104025 bytes, checksum: 27f8126e91dc4b23ddd37a2e733a23fa (MD5) Previous issue date: 2014-01-29 / Universidade Federal de Sao Carlos / In the present thesis, we realize a computational modeling of semiconductor structures based on multiple quantum wells with filter barriers and on quantum wells with semiconductor diluted magnetic layers. We numerically solve the time-dependent Schrödinger s equation within the effective mass approximation, using the Split Operator method. Through the time evolved wave functions we access the dynamics quantities as the light assisted couplings of the states, in which the light is described by the inclusion of an oscillating electric field in the Hamiltonian. Then we determine the probabilities of absorption, oscillator strengths of the intersubband transitions induced by the light. Moreover we analyze the transmission probabilities and, in special, the system s photocurrent. The eigenstates and the eigenfunctions of the stationary states are also obtained within the method by simply making an imaginary time evolution. In the first work, the photocurrent of a multiple quantum well structure with filter barriers modulating the continuum above the wells was analyzed as a function of the applied bias. We find out an interesting dependence of the photocurrent with the applied field, as a differential negative photoconductance controlled by the field. We attribute this negative conductance to the interaction between the localized and extended states in the continuum, expressed by anticrossings between these states and the enhancement of the photocurrent at the crossings by the Landau-Zener-Stückelberg-Majorama like transitions. In the second work, it was evaluated the spin polarized photocurrent arising from quantum well s structures of GaMnAs, under light, electric and magnetic fields of few teslas. The study shows the existence of spectral domains in the THz ranges for which the proposed structure is strongly spin selective. For such photon frequencies, the photocurrent is spin polarized and the application of the external electric field reverts the polarization s signal. This behavior suggests the possibility of conveniently simple switching mechanisms. The physics underlying these results is studied and understood in terms of the spin-dependent coupling strengths emerging from the particular potential profiles of the heterostructures. We present two additional works related to the main ones. In the first additional one, we evaluated the dark current of the multiple quantum well structure with and without filter barriers. For doing this, we add totally the transmission probability through the structure in the Levine s model for the dark current. We observe that dark current is considerably reduced for the structure with the filter barriers when compared to the structure without these barriers. In the second additional work, we calculate the photocurrent in a ZnMnSe structure. We observe the generation of a spin polarized photocurrent controlled by the external electric field, as in the case of the GaMnAs structures. / Na presente tese, realizamos a modelagem computacional de estruturas semicondutoras baseadas em poços quânticos múltiplos com barreiras de filtro e em poços quânticos com camadas de material semicondutor magnético diluído. Para tanto, resolvemos numericamente a equação de Schrödinger dependente do tempo na aproximação de massa efetiva, por meio da evolução temporal das funções de onda do sistema, utilizando o chamado método do Split- Operator. Com as funções de onda evoluídas no tempo temos acesso às variáveis dinâmicas do sistema, como os acoplamentos entre os estados pela presença de luz, descrita na forma de um campo elétrico oscilante. Determinamos assim as probabilidades de absorção, forças de oscilador das transições intersubbandas geradas por essa excitação com luz, as probabilidades de transmissão através da estrutura e, em especial, o espectro de fotocorrente proveniente desses sistemas semicondutores. As autofunções e as autoenergias dos estados estacionários dos sistemas são obtidas pelo mesmo método realizando a evolução em tempo imaginário. No primeiro trabalho, a fotocorrente da estrutura de poços quânticos múltiplos com barreiras de filtro foi analisada em função do campo elétrico aplicado à estrutura. Foi encontrada uma dependência da fotocorrente com o campo elétrico bastante interessante, na forma de uma fotocondutância negativa controlada pelo campo elétrico aplicado à heteroestrutura. Atribuímos essa condutância negativa à interação entre estados localizados e estendidos no continuo se manifestando na forma de anticrossings e o aumento da fotocorrente para os valores de campo elétrico nos quais ocorrem esses crossings foi associado a transições de dois níveis do tipo Landau-Zener-Stückelberg-Majorama. No segundo trabalho, foi calculada a fotocorrente polarizada em spin de estruturas de poços quânticos de GaMnAs, na presença de um campo elétrico varável e um campo magnético de poucos teslas. O estudo mostrou a existência de domínios espectrais na região de THz do espectro eletromagnético, para os quais as estruturas propostas são altamente seletivas em spin. Para tais frequências, encontramos que a fotocorrente é polarizada em spin e a aplicação do campo elétrico é capaz de reverter forma muito eficiente o sinal da polarização. O comportamento observado sugere a possibilidade de mecanismos simples de controle sobre a fotocorrente e a Física por trás de tais efeitos foi entendida em termos dos acoplamentos dependentes de spin dos estados da estrutura, emergentes do perfil de potencial particular das heteroestruturas. Apresentamos dois trabalhos adicionais diretamente relacionados aos trabalhos principais. No primeiro trabalho, calculamos a corrente de escuro proveniente da estrutura de poços quânticos múltiplos com e sem barreiras de filtro, adicionando de forma integral a probabilidade de transmissão através da estrutura no modelo de Levine que determina essa corrente. Observamos que a presença das barreiras de filtro diminui significativamente a corrente de escuro dessa estrutura no regime de altos valores de campo elétrico. No segundo trabalho adicional, foi calculada a fotocorrente de uma estrutura de PQ com camada DMS, composta por ZnMnSe. Observamos a possibilidade de controle da polarização de spin com o campo elétrico, assim como no caso da estrutura composta de GaMnAs.

The study of quadrupolar nuclei using NMR spectroscopy in the solid state significantly increased in popularity from the end of the 20th century, with the introduction of specific methods to acquire spectra free from the effects of the quadrupolar interaction, that results in broadened lineshapes that cannot be completely removed by spinning the sample at the magic angle (MAS), unlike most of the other interactions present in the solid state. The first technique which allows, without any specific hardware, the removal of this broadening has been the Multiple-Quantum MQMAS experiment. The method quickly gained a popularity within the NMR community, with numerous successful applications published. However, the multiple-quantum filtration step in this experiment relies on severely limits sensitivity, restricting application to the most sensitive nuclei. Extending the applicability of MQMAS to less receptive nuclei requires the use of signal improvement techniques. There are multiple examples of such approaches in the literature, but most of these require additional optimisation that may be time-consuming, or simply impossible, on less receptive nuclei. This work introduces a novel signal improvement technique for MQMAS, called FAM-N. Its optimisation is solely based on density matrix simulations using SIMPSON, implying no additional experimental optimisation is required, while improving the signal in MQMAS spectra by equivalent or higher amounts than other common methods. In order to prove the applicability of this method on virtually any system, FAM-N has been investigated by simulation, and tested experimentally using a number of model samples, as well as samples known to be challenging to study by NMR. This work also explores other aspects of NMR spectroscopy on quadrupolar nuclei. Adiabatic inversion of the satellite populations can be performed to improve the central transition signal in static or MAS spectra. A range of methods has been tested and compared, with particular attention given to hyperbolic secant-shaped pulses, for which its performance have been described. Finally, cross-polarisation from a spin I = 1/2 nucleus to a quadrupolar nucleus has been investigated. After reviewing the theory for the static case, simulations have been performed under MAS in order to identify the conditions for efficient magnetisation transfer, with applications in spectral editing or for the combination with MQMAS.

NMR spectroscopy is a very powerful technique for the characterization of structure and dynamics of a variety of systems starting from small organic molecules to large biological macromolecules. In solids, the study of protons becomes more interesting because they are very sensitive to inter-molecular packing and are directly involved in hydrogen-bonding and aromatic π-π interactions, etc. The present thesis is devoted essentially to utilizing information from proton resonances obtained using multinuclear and multiple-quantum approaches. The thesis has two parts. The first part deals with methodological developments in the area of solid-state NMR, relevant to the study of rigid powder samples as well as partially ordered liquid crystalline materials. Methods have been proposed to investigate the structure of small molecules at moderate spinning frequencies and thermotropic liquid crystals at static conditions. Proton detected heteronuclear experimental methods based on both first and second-order cross polarization at moderate and ultra-fast magic angle spinning rates are also proposed. The second part of the thesis deals with the application of both newly proposed and existing solid state NMR methods to the study of several biologically relevant systems. These include the study of several designed as well as naturally occurring peptides. The use of first-principles calculations based on GIPAW method for supporting the experimentally obtained results has also been made. The thesis is divided into five chapters. In the second chapter, a new pulse sequence to correlate Double Quantum (DQ) proton frequencies to carbon Single Quantum (SQ) chemical shifts in the solid state has been proposed. In this sequence, named as MAS-J-1H (DQ)-13C-HMQC, the correlation between 1H and 13C is achieved through scalar coupling, while the double-quantum coherence among protons is generated through dipolar couplings. This experiment is particularly suited for the study of 13C in natural abundance. The advantages of the technique with applications to alanine, histidine and a model liquid crystalline material have been demonstrated. The assignment of 13C spectra of partially ordered systems has also been considered. In this case the assignment of the spectrum is a major challenge due to the interplay of anisotropic order and chemical shift parameters. The DQ-SQ correlation experiment described in the thesis has been applied to a well known liquid crystal and also to a novel thiophene based liquid crystal and the local order parameters of the liquid-crystal have been obtained. The thesis also presents results on the azelaic acid -isonicotinamide co-crystal as well as the drug ibuprofen obtained by using novel methodologies. In the case of the former, the problem of overlap of resonances was overcome with the use of the REVERSE-CP approach to separate out the carbon attached protons from the rest of the protons. Subsequently, by the use of several combined approaches, the structural features were identified. A new heteronuclear correlation pulse sequence for solids under fast MAS conditions has also been tested. With low r.f powers, a second-order dipolar term mediated transfer of magnetization between I and S spin known as second order cross-polarization (SOCP) was exploited to obtain the entire spin system connectivity. Both carbon detected and proton detected experiments have been carried out and their utility evaluated. Similar approaches to shed light on the structure and conformation of a set of proline and pseudoproline based designed β-turn peptides that are used as templates for understanding protein folding have been made. Results of studies on two biologically important forms of the short-chain peptides namely glutathione reduced (GSH) and oxidized (GSSG) tripeptides are also presented.

No description available.

Cette thèse est consacrée à l’étude des propriétés optiques et optoélectroniques autour de la bande interdite directe des structures à puits quantiques Ge/SiGe pour la modulation, la photodétection et l’émission de lumière sur la plateforme silicium. Les principaux composants réalisés sont : un modulateur optique en guide d’onde, rapide et à faible puissance électrique, basé sur l’Effet Stark Confiné Quantiquement, les premières photodiodes Ge/SiGe dont le comportement fréquentiel est compatible avec les transmissions de données à 40 Gbit/s, et la première diode à électroluminescence à puits quantiques Ge/SiGe, base sur la transition directe de ces structures et fonctionnant à température ambiante. Les caractérisations statiques et fréquentielles ont été réalisées sur l’ensemble des composants, qui ont tous été fabriqués avec la même structure épitaxiée et les mêmes procédés de fabrication. Des modèles théoriques simples ont ensuite été utilisés pour décrire analyser les comportements observés. Finalement les études menées permettent de conclure que les structures à puits quantiques Ge/SiGe sont un candidat de choix pour la réalisation d’une nouvelle plateforme photonique à haut débit, totalement compatible avec les technologies silicium. / This PhD thesis is devoted to study electro-optic properties of Gemanium/Silicon-Germanium (Ge/SiGe) multiple quantum wells (MQWs) for light modulation, detection, and emission on Si platform. It reports the first development of high speed, low energy Ge/SiGe electro-absorption modulator in a waveguide configuration based on the quantum-confined Stark effect (QCSE), demonstrates the first Ge/SiGe photodiode with high speed performance compatible with 40 Gb/s data transmission, and realizes the first Ge/SiGe light emitting diode based on Ge direct gap transition at room temperature. Extensive DC and RF measurements were performed on each tested prototype, which was realized using the same epitaxial growth and fabrication process. Simple theoretical models were employed to describe experimental properties of the Ge/SiGe MQWs. The studies show that Ge/SiGe MQWs could potentially be employed as a new photonics platform for the development of a high speed optical link fully compatible with silicon technology.

L’amplification optique large bande à base de SOA est devenue indispensable pour la montée en débit des systèmes de transmissions optiques et pour pouvoir exploiter au mieux la bande optique des fibres optiques. Ce travail présente une étude théorique et expérimentale d’un SOA large bande passante développé par Alcatel Thales III-V Lab dans le cadre des projets ANR AROME et UltraWIDE. Dans cette thèse, nous avons d’abord effectué une modélisation semi-phénoménologique du gain matériau et du coefficient de gain d’une structure à base de multi-puits quantiques avec un nombre réduit de paramètres. L’intégration de notre modèle dans un modèle de SOA déjà développé au laboratoire a montré son efficacité pour restituer quantitativement le comportement statiques (gain, facteur de bruit) des nouvelles structures SOA large bande sur une large plage de longueurs d’onde (> 110 nm), de courants d’alimentation et de puissances optiques. A l’aide de ce modèle, nous avons étudié l’influence de la structure du SOA sur la bande passante pour un gain cible en jouant sur la longueur, le nombre d’électrode et le courant d’alimentation du SOA. Nous avons mis en évidence qu’une structure bi-électrodes n’apportait pas d’amélioration de la bande passante optimisée par rapport au cas mono-électrode. En revanche, la structure bi-électrode permet d’optimiser la puissance de saturation et le facteur de bruit du SOA, sans sacrifier ni le gain maximal ni la bande passante optique. Nous avons aussi montré que, pour ce type de composants, une augmentation de la puissance optique injectée pouvait être compensée par une augmentation du courant d’alimentation pour maintenir une large bande passante optique. Nous avons également mis en place deux techniques d’élargissement de la bande passante optique de SOA à large bande. La première technique est fondée sur le filtrage en réflexion spectralement sélectif (ESOA). Le dispositif expérimental a permis d’amplifier simultanément 8 canaux CWDM dans une bande passante (définie à −1 dB) de 140 nm. La deuxième technique, basée sur un amplificateur hybride Raman-SOA, a fourni une bande passante optique (définie à −1 dB) de 89 nm avec un gain de 17 dB. Nous avons ainsi pu réaliser une transmission simultanée de 5 canaux CWDM allant jusqu’à 10 Gb/s sur 100 km. / SOA-based optical amplification became crucial for increasing optical system capacity and to benefit from the broad bandwidth of optical fibers. In this work we present both theoretical and experimental studies for a new broadband SOA developed by Alcatel Thales III-V lab in the framework of AROME and UltraWIDE ANR projects.We developed firstly a semi-phenomenological model for both the material gain and the gain coefficient of a multi-quantum well -based SOA structure with a reduced set of parameters. This material gain model has been integrated in an existing SOA model and proved its performance in reproducing steady state behavior of this new broadband SOA (gain and noise figure) for a wide range of wavelengths, input powers and bias currents. Thanks to this model, we studied the influence of the SOA geometrical structure on the optical bandwidth for a given target gain, by varying length, number of electrodes and bias current. We showed that two-electrode SOA structures do not provide any improvement of the bandwidth compared to the one-electrode case. However, the two-electrode structure allows the optimization of both the SOA saturation power and the noise figure, without sacrificing neither the maximum gain nor the optical bandwidth. We have also shown that for this kind of component, an increase in the injected optical power could be compensated by an increase in the supply current to maintain a wide optical bandwidth.We have also investigated two techniques to widen the optical bandwidth of our broadband SOA. The first one is based on a modification of the SOA structure by introducing a selective reflection filter (ESOA). Its experimental implementation allowed the amplification of an 8-CWDM-channel comb in a bandwidth (defined at -1 dB) of 140 nm. The second one, based on a hybrid Raman-SOA amplifier, provided an optical bandwidth (defined at -1 dB) of 89 nm with a gain of 17 dB. With this last technique, we were able to achieve a 5-CWDM-channel comb transmission up to 10 Gb/s over 100 km.

This thesis describes the incorporation within a semiconductor laser of a multiple quantum well InGaAsP/InP Electron Bragg Reflector (EBR). The EBR is intended to improve laser performance by inhibiting the escape of hot electrons from the laser active region by quantum mechanical Bragg reflection. To the author's knowledge, this investigation represents the first attempt to realize an EBR in the InGaAsP/InP material system. Computer models based on a transfer matrix method for the solution of Schrodinger's equation were written to obtain the EBR design. The transfer matrix method is described. Extensions to the transfer matrix method for optics are presented and are demonstrated to provide more than an order of magnitude improvement in computational efficiency for the calculation of the complex TE-mode propagation constant for planar graded-index waveguides with absorption or gain. The EBR designed for this work incorporates several new features. Deleterious band bending in the vicinity of the EBR is minimized by exploiting material strain to reduce the density of hole states in the EBR quantum wells. To maximize reflection bandwidth and relax fabrication tolerances, the EBR design used well widths that decreased with increasing depth into the p-type InP cladding. By the placement of the EBR adjacent to the separate confinement region, a return path was provided for electrons that scattered inelastically within the EBR. Moreover, the EBR structure was designed to support no bound electron states, so that the recombination of electrons with holes in the EBR would be minimal. To the author's knowledge, the EBR-equipped laser fabricated for this work represents the first attempt to exploit electron state exclusion. To explore the effectiveness of EBRs in the InGaAsP/InP material system, two nearly identical ridge waveguide lasers (one with an EBR, and one without) were designed, fabricated, and tested. The EBR-equipped lasers exhibited an anomalous threshold current temperature dependence which featured a "negative-To" regime (in which the threshold current decreases with increasing temperature), attaining a minimum in threshold current between T=150 K and T=200 K. These lasers had a threshold current temperature stability superior to that of standard lasers within a ~70 K window around the minimum threshold temperature. Experimental evidence suggests that the improved stability is not due to quantum mechanical Bragg reflection provided by the EBR, but is attributable to the temperature-dependent rate of hole escape from the EBR quantum wells into the separate confinement region. The proposed mechanism is described in detail and is supported by theoretical and experimental evidence. The results have implications for device design, because the mechanism by which the superior temperature stability is achieved does not rely on the electron coherence effects; the mathematical model suggests that the mechanism can be exploited to provide superior temperature stability in semiconductor lasers at 300 K or above. / Thesis / Master of Engineering (ME)