abstract: Nanowires are one-dimensional (1D) structures with diameter on the nanometer scales with a high length-to-diameter aspect ratio. Nanowires of various materials including semiconductors, dielectrics and metals have been intensively researched in the past two decades for applications to electrical and optical devices. Typically, nanowires are synthesized using the vapor-liquid-solid (VLS) approach, which allows defect-free 1D growth despite the lattice mismatch between nanowires and substrates. Lattice mismatch issue is a serious problem in high-quality thin film growth of many semiconductors and non-semiconductors. Therefore, nanowires provide promising platforms for the applications requiring high crystal quality materials. With the 1D geometry, nanowires are natural optical waveguides for light guiding and propagation. By introducing feedback mechanisms to nanowire waveguides, such as the cleaved end facets, the nanowires can work as ultra-small size lasers. Since the first demonstration of the room-temperature ultraviolet nanowire lasers in 2001, the nanowire lasers covering from ultraviolet to mid infrared wavelength ranges have been intensively studied. This dissertation focuses on the optical characterization and laser fabrication of two nanowire materials: erbium chloride silicate nanowires and composition-graded CdSSe semiconductor alloy nanowires. Chapter 1 – 5 of this dissertation presents a comprehensive characterization of a newly developed erbium compound material, erbium chloride silicate (ECS) in a nanowire form. Extensive experiments demonstrated the high crystal quality and excellent optical properties of ECS nanowires. Optical gain higher than 30 dB/cm at 1.53 μm wavelength is demonstrated on single ECS nanowires, which is higher than the gain of any reported erbium materials. An ultra-high Q photonic crystal micro-cavity is designed on a single ECS nanowire towards the ultra-compact lasers at communication wavelengths. Such ECS nanowire lasers show the potential applications of on-chip photonics integration. Chapter 6 – 7 presents the design and demonstration of dynamical color-controllable lasers on a single CdSSe alloy nanowire. Through the defect-free VLS growth, engineering of the alloy composition in a single nanowire is achieved. The alloy composition of CdSxSe1-x uniformly varies along the nanowire axis from x=1 to x=0, giving the opportunity of multi-color lasing in a monolithic structure. By looping the wide-bandgap end of the alloy nanowire through nanoscale manipulation, the simultaneous two-color lasing at green and red colors are demonstrated. The 107 nm wavelength separation of the two lasing colors is much larger than the gain bandwidth of typical semiconductors. Since the two-color lasing shares the output port, the color of the total lasing output can be controlled dynamically between the two fundamental colors by changing the relative output power of two lasing colors. Such multi-color lasing and continuous color tuning in a wide spectral range would eventually enable color-by-design lasers to be used for lighting, display and many other applications. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2015

FundaÃÃo de Amparo à Pesquisa do Estado do Cearà / Nesta dissertaÃÃo à realizado um estudo da polarizaÃÃo da luz e suas aplicaÃÃes em sistemas de comunicaÃÃes quÃnticas. Inicialmente, sÃo apresentadas as ferramentas matemÃticas necessÃrias ao tratamento da polarizaÃÃo da luz de fÃtons isolados e estados coerentes: matriz coerÃncia, parÃmetro de Stokes e grau de polarizaÃÃo. Em seguida à apresentada, atravÃs de simulaÃÃo numÃrica, a dinÃmica do grau de polarizaÃÃo da luz de um fÃton durante a propagaÃÃo em um canal despolarizador. Por fim, o resultado de um experimento usando estados coerentes, objetivando medir o grau de polarizaÃÃo da luz apÃs propagaÃÃo em um trecho de 200 m de fibra, à apresentado. O experimento à Ãtil para a determinaÃÃo do parÃmetro do modelo de canal despolarizador de qubits. Sendo a polarizaÃÃo da luz uma propriedade facilmente alterada por condiÃÃes ambientais, sÃo estudados os esquemas Ãpticos que, dentro de certas restriÃÃes, podem corrigir variaÃÃes aleatÃrias da polarizaÃÃo da luz durante a propagaÃÃo na fibra. Tais esquemas sÃo empregados como corretores de erros em sistemas de comunicaÃÃes quÃnticas. Baseados em esquemas de correÃÃo encontrados na literatura, foram propostos dois novos esquemas, um para a utilizaÃÃo em sistemas que empregam polarizaÃÃo de luz de fÃtons isolados e outro para sistemas que empregam polarizaÃÃo de estados coerentes bimodais, sendo o primeiro ativo, isto Ã, requer um protocolo de controle e sincronismo, e o segundo passivo. Por fim, à feita a anÃlise da seguranÃa de um sistema de distribuiÃÃo quÃntica de chaves, empregando o esquema proposto de correÃÃo de erros, quando o mesmo sofre o ataque de Fuchs-Peres-Brandt. à mostrado que a utilizaÃÃo do esquema de correÃÃo de erros proposto favorece a visibilidade da presenÃa de um espiÃo monitorando o canal quÃntico. / In this dissertation it is realized a study about quantum key distribution (QKD) in optical networks. Initially, a review of the theory of quantum key distribution and some of its implementations with strongly attenuated coherent states, as well a review of an optical system for physical encryptation using mesoscopic coherent states are realized. Following, it is analyzed the use of an error correction scheme in the physical encryptation system, and two new schemes for quantum key distribution are proposed. The first is a possible implementation of a hybrid system, that is, using weak and mesoscopic coherent states, for QKD and quantum authentication of classical messages. The second is an implementation of a QKD system based on relative phase modulation of sidebands frequency, using two RF carriers and an amplitude modulator in Alice and a phase modulator in Bob. After, an experimental realization of a simple QKD setup using polarization states of strongly attenuated coherent states for B92 protocol is presented. At last, it is realized an experimental characterization of an optical receiver for optical communication applications.