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In this thesis the author describes the use of transient grating techniques to study the transport of electrons and holes perpendicular to the layers of a GaAs/AlGaAs multiple quantum well (MQW).

Neste trabalho, investigamos a influência da estrutura de energia das minibandas dos estados eletrônicos ocupados no tempo de recombinação em poços quânticos múltiplos (MQW) fracamente acoplados de GaAs / AlGaAs. Um dos melhores métodos para estudar o efeito da estrutura energética consiste em medir o tempo de recombinação eletrônica em função de parâmetros expostas à influência externa que afeta a estrutura energética, por isso, aplicamos um campo magnético externo. O espectro da emissão de fotoluminescência foi composta pelas contribuições das minibandas da banda de condução, Γ – Γ e Γ – XZ. Observou-se um aumento notável do tempo de recombinação quando o campo magnético causou a despopulação da minibanda de maior energia, Γ – XZ. O efeito observado é atribuído à variação induzida pelo campo magnético na densidade dos estados eletrônicos. / In the present work, we investigate the influence of the miniband energy structure of the populated electron states on the recombination time in GaAs/AlGaAs weakly coupled multiple quantum wells (MQW). The best method to study the effect of the energy structure is to measure the recombination time in the same sample subject to external influence which affects the energy structure, therefore, we apply an external magnetic field. The photoluminescence emission was composed of the contributions from the Γ – Γ and Γ – XZ conduction band minibands. Remarkable enhancement of the recombination time was observed when the magnetic field caused depopulation of the higher energy Γ – XZ miniband. The observed effect is attributed to the magnetic field induced variation of the electron density of states.

Neste trabalho, investigamos a influência da estrutura de energia das minibandas dos estados eletrônicos ocupados no tempo de recombinação em poços quânticos múltiplos (MQW) fracamente acoplados de GaAs / AlGaAs. Um dos melhores métodos para estudar o efeito da estrutura energética consiste em medir o tempo de recombinação eletrônica em função de parâmetros expostas à influência externa que afeta a estrutura energética, por isso, aplicamos um campo magnético externo. O espectro da emissão de fotoluminescência foi composta pelas contribuições das minibandas da banda de condução, Γ – Γ e Γ – XZ. Observou-se um aumento notável do tempo de recombinação quando o campo magnético causou a despopulação da minibanda de maior energia, Γ – XZ. O efeito observado é atribuído à variação induzida pelo campo magnético na densidade dos estados eletrônicos. / In the present work, we investigate the influence of the miniband energy structure of the populated electron states on the recombination time in GaAs/AlGaAs weakly coupled multiple quantum wells (MQW). The best method to study the effect of the energy structure is to measure the recombination time in the same sample subject to external influence which affects the energy structure, therefore, we apply an external magnetic field. The photoluminescence emission was composed of the contributions from the Γ – Γ and Γ – XZ conduction band minibands. Remarkable enhancement of the recombination time was observed when the magnetic field caused depopulation of the higher energy Γ – XZ miniband. The observed effect is attributed to the magnetic field induced variation of the electron density of states.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18540号 / 理博第4016号 / 新制||理||1579(附属図書館) / 31440 / 京都大学大学院理学研究科化学専攻 / (主査)教授 竹腰 清乃理, 教授 吉村 一良, 教授 北川 宏 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

<p>Asymmetric multiple quantum wells (AMQWs) can provide broad and flat gain spectra. Broadly tunable diode lasers can be realized with AMQW active regions and without the need for antireflection coatings on cleaved facets.</p> <p> This thesis reports the application of AMQW broadly tunable lasers with uncoated facets for Fourier domain and synthesized optical coherence tomography (OCT). A depth resolution of 13 μm in air was obtained with a test bed OCT system that used diffractive optical elements, short external cavities, and AMQW InGaAsP/InP broadly tunable lasers as the light sources for the Fourier domain and the synthesized OCT measurements. The centre wavelengths of the broadly tunable sources were 1550 nm and the tunable ranges were ≤ 117 nm.</p> <p>The features of broad and flat gain spectra of AMQWs also make AMQWs ideal candidates for broad spectral width superluminescent diodes (SLDs). 1300 nm AMQW InGaAsP/InP SLDs were designed and fabricated for application to time domain OCT. For the design of the active region, it was found by simulation of gain and the comparison of two growths that the transition carrier density (TCD) has to be reasonably high to achieve high power SLDs. A transfer matrix method was used to solve for the modes of planar optical waveguides with arbitrary layers and the thicknesses of these layers were optimized with a Marquardt nonlinear fitting method. With the optimization of the optical waveguide and with AMQWs with high TCDs, the output power of SLDs could reach 2 mW with > 90 nm spectral width. It is shown by time domain OCT measurements that the depth resolution of the OCT measurements could reach 7.85 μmin air with double section SLDs.</p> <p>Two dimensional OCT images of a glass cover slip were built with the imageSC function in Matlab™. Image enhancement with blind/not-blind deconvolution was performed based on the measured point spread function (PSF) of the OCT setup. A Richardson-Lucy algorithm was used as the blind deconvolution method and a not-blind version of a Jansson-Van Cittert method was used.</p> / Thesis / Doctor of Philosophy (PhD)

<p> Compositionally asymmetric multiple quantum well (AMQW) lasers are used for the demonstration of the gain flattening coating functionality. The gain spectra of the lasers are extracted using a non-linear least square fitting method. An optimum facet reflectance spectrum is calculated for a chosen current. For manufacturability, a modified reflectance spectrum of the gain flattening coating is proposed, in order to achieve operation over a wider spectral range without the 'difficult' gap which was a region where lasing was difficult or impossible to achieve due to insufficient gains at these wavelengths. </p> <p> Silicon oxides films with high, medium, and low refractive indices fabricated in an inductively coupled plasma (ICP) enhanced chemical vapor deposition (CVD) system are chosen as the building blocks of the gain flattening coating. An 18-layer coating is designed by the insertion of needle-like refractive index variation with a few optimization methods applied to minimize the merit function. A laser bar holder is custom designed and fabricated. Experiments and modification on the laser bar holder are carried out for better performance. The 18-layer gain flattening coating is then fabricated in the ICPCVD system with an in-situ spectroscopic ellipsometric measurement. It is observed that the non-lasing gap has disappeared after the coating is applied. Without external feedback, the coated laser shows tuning over 85 nm with the central wavelength of 1593 nm, while the uncoated laser has a non-lasing gap of about 25 nm in the central region of the tuning range of 70 nm. </p> <p> Finally, the coherence length of a low coherent source synthesized from the gain flattening coated AMQW laser is measured by using Michelson interferometer. The highest depth resolution that can be achieved is measured as 40 μm. The power intensity of the synthesized low coherence light source from the gain flattening coated AMQW laser is rendered from the interferogram using fast Fourier transform (FFT). </p> / Thesis / Doctor of Philosophy (PhD)

Distant dipolar field (DDF)-based nuclear magnetic resonance is an active research area with many fundamental properties still not well understood. Already several intriguing applications have developed, like HOMOGENIZED and IDEAL spectroscopy, that allow high resolution spectra to be obtained in inhomogeneous fields, such as in-vivo. The theoretical and experimental research in this thesis concentrates on the fundamental signal properties of DDF-based sequences in the presence of relaxation (T1 and T2) and diffusion. A general introduction to magnetic resonance phenomenon is followed by a more in depth introduction to the DDF and its effects. A novel analytical signal equation has been developed to describe the effects of T2 relaxation and diffusing spatially modulated longitudinal spins during the signal build period of an HOMOGENIZED cross peak. Diffusion of the longitudinal spins results in a lengthening of the effective dipolar demagnetization time, delaying the re-phasing of coupled anti-phase states in the quantum picture. In the classical picture the unwinding rate of spatially twisted magnetization is no longer constant, but decays exponentially with time. The expression is experimentally verified for the HOMOGENIZED spectrum of 100mM TSP in H2O at 4.7T. Equations have also been developed for the case of multiple repetition steady state 1d and 2d spectroscopic sequences with incomplete magnetization recovery, leading to spatially varying longitudinal magnetization. Experimental verification has been accomplished by imaging the profile. The equations should be found generally applicable for those interested in DDF-based spectroscopy and imaging.

L'innovation de la technologie de pointe et l'exigence du marché électronique se concentrent toujours sur l'électronique bon marché, qui présente une fabrication facile, avec des performances sans cesse améliorées. Les pérovskites hybrides organiques-inorganiques, qui combinent les propriétés des semi-conducteurs organiques et inorganiques, sont des candidats prometteurs pour de futurs dispositifs opto-électroniques. L’énergie de liaison des excitons et la force d'oscillateur sont très élevées dans ces systèmes, ce qui rend possible leurs applications à température ambiante. Dans cette thèse, nous avons étudié des couches minces auto-assemblées de molécules de pérovskite (R-NH3)2PbX4. En modifiant la structure R, des pérovskites avec des propriétés optimisées (propriétés optiques d’émission, rugosité de surface et photostabilité) ont été découvertes. Nous avons aussi développé des méthodes pour fabriquer des cristaux massifs et des nanoparticules de pérovskites, et nous avons créé de nouveaux cristaux de pérovskite mixtes: (RNH3)2PbYxX4-x et AB-(NH3)2PbX4. Des cavités verticales en régime de couplage fort ont été réalisées avec ces matériaux, l’émission du polariton de basse énergie a été observée à température ambiante. / The innovation of advanced technology and the requirement of electronic market are always focusing on low cost electronics, presenting an easy processing and having enhanced performance. Organic-inorganic hybrid perovskites, which combine the properties of organic and inorganic semiconductors, are hopeful candidates for future opto-electronic devices. The exciton binding energies and oscillator strengths are very large in these systems making the applications at room temperature possible. In this thesis, we study the flexibility and photostability of self-assembled two-dimensional layered perovskites (R-NH3)2PbX4. By modifying the R structure, perovskites with optimized photoluminescence efficiency, surface roughness and photostability are discovered. We develop also some methodologies to fabricate crystal bulks and nanoparticles of perovskites, and we create new mixed perovskite crystals: (RNH3)2PbYxX4-x and AB-(NH3)2PbX4. Vertical microcavities containing these new materials and working in the strong coupling regime at room temperature have been realized, the emission of the lower energy polariton is observed.