Submitted by Renata Lopes (renatasil82@gmail.com) on 2017-06-27T11:13:15Z No. of bitstreams: 1 leandrodecastroguarnieri.pdf: 14716199 bytes, checksum: d864ab0333d3140dd1b45ea0361a21d8 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2017-08-07T20:52:00Z (GMT) No. of bitstreams: 1 leandrodecastroguarnieri.pdf: 14716199 bytes, checksum: d864ab0333d3140dd1b45ea0361a21d8 (MD5) / Made available in DSpace on 2017-08-07T20:52:00Z (GMT). No. of bitstreams: 1 leandrodecastroguarnieri.pdf: 14716199 bytes, checksum: d864ab0333d3140dd1b45ea0361a21d8 (MD5) Previous issue date: 2011-08-19 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta tese, é feito um estudo teórico sobre as propriedades de transporte de elétrons e de emissão de um feixe coerente de fônons com frequências na faixa de terahetz, conhecido como saser, a partir de um dispositivo semicondutor de dimensões nano-métricas, formado por uma heteroestrutura de dupla barreira (HDB) e controlado por um potencial externo. O sistema é descrito por um hamiltoniano na aproximação de tight-binding no qual são incluídos as contribuições dos elétrons, dos fônons e das interações elétron-fônon e elétron-elétron. Para o estudo das propriedades de transporte eletrônico, utilizou-se do formalismo de Keldysh que considera, de forma direta, as interações de muitos corpos com a finalidade de reduzir uma corrente de elétrons fora do equilíbrio a uma que depende das funções Green retardadas que estão em equilíbrio termodinâmico a uma dada temperatura. Um procedimento conhecido como dizimação é empregado repetidamente ao longo deste trabalho para a obtenção das funções de Green retardadas. Para o estudo das propriedades de emissão dos fônons TA, são utilizadas as soluções das equações cinéticas que governam as populações dos elétrons nos dois primeiros níveis ressonantes e dos fônons dentro do poço. O perfil de energia potencial e a distribuição de cargas são calculados auto-consistentemente através de um sistema de equações no qual se inclui a equação de Poisson. Verificaram-se nos resultados as conhecidas regiões de instabilidade no início da emissão dos fônons LO e de biestabilidade no final do segundo pico da corrente. Uma análise sobre o comportamento destas regiões é feita através de mudanças nos parâmetros da HDB. A influência da temperatura sobre o saser também é estudada. A baixa temperatura, os resultados encontrados aqui coincidem com aqueles existentes na literatura. / In this thesis, we study theoretically the transport properties of electrons and emission of a coherent beam of TA phonons with terahetz frequencies, known as saser, from a semiconductor device of nanometric dimensions formed by a double barrier heterostructure and controlled by an external potential. The system is described by a Hamiltonian in the 'tight-binding' approximation in which the electrons, the phonons, the electron-phonon and the electron-electron interactions contributions are included. To study the electronic transport properties we use the Keldysh formalism, which directly considers the many body's interactions, to reduce the flow of electrons out of equilibrium to one that depends on the retarded Green functions that are in the thermodynamic equilibrium at a given temperature. A procedure known as decimation is used repeatedly throughout this work to find such retarded Green functions. To study the emission properties of the TA phonons, we solve the kinetic equations that govern the populations of electrons in the first two resonant levels and the phonons in the well. The potential profile and the charge distribution energy are calculated self-consistently, through a system of equations which includes the Poisson equation. We verify in our results the well known regions of instability at the beginning LO phonon emission and bistability at the end of the second peak current. We analyze the behavior of these regions through the change in the barriers thickness of the HDB. A study of the influence of temperature on saser is carried out in this work. At low temperatures, our results coincide with those in the literature.

Les propriétés acoustiques des bulles sont reconnues pour leur potentiel dans des applications tant biologiques que médicales. Capables de provoquer la lyse des cellules en générant des écoulements intenses, elles peuvent aussi servir d'agent de contraste en échographie.Ce manuscrit traite de la dynamique de vibration de bulles confinées entre les deux murs d'un canal microfluidique. Ces bulles exhibent une pulsation en volume aux faibles amplitudes d'excitation, à laquelle se superpose un mode de surface paramétrique aux plus fortes amplitudes. Le matériau constituant le canal étant élastique, la pulsation de la bulle confinée a pour effet de générer des ondes de Rayleigh sur les parois du canal. Grâce à ces ondes de surface, les bulles vont pouvoir se coupler les unes aux autres. Ce couplage a un effet sur les écoulements hydrodynamiques autour de ces bulles. En effet, la présence d'une bulle voisine engendre l'apparition d'un mode de translation de la bulle qui, couplé à sa pulsation en volume, conduira à la génération d'écoulements à longue portée. Ce même couplage permet aux bulles de s'auto-organiser en réseau. Afin d'étudier de manière contrôlée les effets collectifs des bulles, leur position a été fixée à l'aide de puits capillaires. Les conditions d'amplification et de synchronisation de la vibration des bulles sont recherchées en vue de créer de nouveaux méta-matériaux. / The pulsation properties of air bubbles under ultrasound have received much attention since the development of sonoporation and contrast agents. Spherical bubbles are well known to induce streaming when excited by ultrasound.We report in this manuscript the acoustic vibration of microbubbles confined between the two walls of a microfluidic channel. These bubbles exhibit a volumetric pulsation at low intensities of ultrasound, superimposed with a parametric surface mode for higher intensities of the pressure field. Because the channel walls are elastic, the bubble pulsation leads to the generation of Rayleigh waves at the channel wall interface. The bubble coupling induced by these surface waves has hydrodynamic consequences. In fact, a neighbouring bubble will create a translation mode of the bubble, in addition to its volumetric pulsation. It gives rise to a long-range mixed-mode streaming. The Rayleigh waves lead also to a self-organization of the bubbles in a network. In order to study the collective effects of these bubble networks in a controlled manner, bubble positions were fixed by capillarity on micropits. Conditions for an amplification or a synchronization of the bubble pulsations are sought in order to develop new bubble metamaterials.