We consider a system of few electrons trapped in a two-dimensional circularquantum dot with harmonic confinement and in the presence of ahomogeneous magnetic field, with focus on the role of e-e interaction. Byperforming the exact diagonalization of the Hamiltonian in second quantization,the low-lying energy levels for spin polarized system are obtained. The singlet-triplet oscillation in the ground state of the two-electron system showing up inthe result is explained due to the role of Coulomb interaction. The splitting ofthe lowest Landau level is another effect of the e-e interaction, which is alsoobserved in the results.

Στην εργασία αυτή μελετούμε φαινόμενα μεταβατικής (χρονικά εξαρτημένης) απορρόφησης σε δύο συστήματα κβαντικών τελειών, κάτω από συνθήκες που οδηγούν τις κβαντικές τελείες να εμφανίσουν σε στάσιμη κατάσταση φαινόμενα οπτικής διαφάνειας, και πιο συγκεκριμένα ηλεκτρομαγνητικά επαγόμενη διαφάνεια και οπτική διαφάνεια λόγω εξωτερικού δυναμικού. Μετά από μια σχετικά σύντομη εισαγωγή στις κβαντικές τελείες και στο φαινόμενο και την ιστορία της ηλεκτρομαγνητικά επαγόμενης διαφάνειας, τα συστήματα των κβαντικών τελειών μελετώνται με την μεθοδολογία του πίνακα πυκνότητας, η οποία αναπτύσσεται στην παρούσα εργασία. Τα αποτελέσματα που εξάγουμε προκύπτουν από αριθμητικές λύσεις των αντίστοιχων εξισώσεων του πίνακα πυκνότητας για διάφορες τιμές παραμέτρων. / In this thesis we study transient absorption in two quantum dot systems under the conditions of optical transparency. We will study two types of transparency; the first is voltage-controlled transparency and the second electromagnetically induced transparency. The thesis starts with a short introduction in quantum dot nanostructures that is followed by an introduction to the phenomenon and the history of electromagnetically induced transparency. We then study the coherent interaction of the electromagnetic fields with the quantum dots with the methodology of the density matrix that is analysed in the thesis. We solve the appropriate density matrix equations numerically for the structures under study and present results for several system parameters.

No description available.

No description available.

Within the realm of biosensing, DNA analysis has become an indispensable research tool in medicine, enabling the investigation of relationships among genes, proteins, and drugs. Conventional DNA microarray technology uses multiple lasers and complex optics, resulting in expensive and bulky systems which are not suitable for point-of-care medical diagnostics. The immobilization of DNA probes across the microarray substrate also results in substantial spatial variation. To mitigate the above shortcomings, this thesis presents a set of techniques developed for the CMOS image sensor for point-of-care spectrally-multiplexed fluorescent DNA sensing and other fluorescence biosensing applications. First, a CMOS tunable-wavelength multi-color photogate (CPG) sensor is presented. The CPG exploits the absorption property of a polysilicon gate to form an optical filter, thus the sensor does not require an external color filter. A prototype has been fabricated in a standard 0.35μm digital CMOS technology and demonstrates intensity measurements of blue (450nm), green (520nm), and red (620nm) illumination. Second, a wide dynamic range CMOS multi-color image sensor is presented. An analysis is performed for the wide dynamic-range, asynchronous self-reset with residue readout architecture where photon shot noise is taken into consideration. A prototype was fabricated in a standard 0.35μm CMOS process and is validated in color light sensing. The readout circuit achieves a measured dynamic range of 82dB with a peak SNR of 46.2dB. Third, a low-power CMOS image sensor VLSI architecture for use with comparator based ADCs is presented. By eliminating the in-pixel source follower, power consumption is reduced, compared to the conventional active pixel sensor. A 64×64 prototype with a 10μm pixel pitch has been fabricated in a 0.35μm standard CMOS technology and validated experimentally. Fourth, a spectrally-multiplexed fluorescence contact imaging microsystem for DNA analysis is presented. The microsystem has been quantitatively modeled and validated in the detection of marker gene sequences for spinal muscular atropy disease and the E. coli bacteria. Spectral multiplexing enables the two DNA targets to be simultaneously detected with a measured detection limit of 240nM and 210nM of target concentration at a sample volume of 10μL for the green and red transduction channels, respectively.

Within the realm of biosensing, DNA analysis has become an indispensable research tool in medicine, enabling the investigation of relationships among genes, proteins, and drugs. Conventional DNA microarray technology uses multiple lasers and complex optics, resulting in expensive and bulky systems which are not suitable for point-of-care medical diagnostics. The immobilization of DNA probes across the microarray substrate also results in substantial spatial variation. To mitigate the above shortcomings, this thesis presents a set of techniques developed for the CMOS image sensor for point-of-care spectrally-multiplexed fluorescent DNA sensing and other fluorescence biosensing applications. First, a CMOS tunable-wavelength multi-color photogate (CPG) sensor is presented. The CPG exploits the absorption property of a polysilicon gate to form an optical filter, thus the sensor does not require an external color filter. A prototype has been fabricated in a standard 0.35μm digital CMOS technology and demonstrates intensity measurements of blue (450nm), green (520nm), and red (620nm) illumination. Second, a wide dynamic range CMOS multi-color image sensor is presented. An analysis is performed for the wide dynamic-range, asynchronous self-reset with residue readout architecture where photon shot noise is taken into consideration. A prototype was fabricated in a standard 0.35μm CMOS process and is validated in color light sensing. The readout circuit achieves a measured dynamic range of 82dB with a peak SNR of 46.2dB. Third, a low-power CMOS image sensor VLSI architecture for use with comparator based ADCs is presented. By eliminating the in-pixel source follower, power consumption is reduced, compared to the conventional active pixel sensor. A 64×64 prototype with a 10μm pixel pitch has been fabricated in a 0.35μm standard CMOS technology and validated experimentally. Fourth, a spectrally-multiplexed fluorescence contact imaging microsystem for DNA analysis is presented. The microsystem has been quantitatively modeled and validated in the detection of marker gene sequences for spinal muscular atropy disease and the E. coli bacteria. Spectral multiplexing enables the two DNA targets to be simultaneously detected with a measured detection limit of 240nM and 210nM of target concentration at a sample volume of 10μL for the green and red transduction channels, respectively.

Spatial confinement of electrons and their interactions as well as confinement of the spin dimensionality often yield drastic changes of the electronic and magnetic properties of solids. Novel quantum transport and optical phenomena, involving electronic spin degrees of freedom in semiconductor heterostructures, as well as a rich variety of exotic quantum ground states and magnetic excitations in complex transition metal oxides that arise upon such confinements, belong therefore to topical problems of contemporary condensed matter physics. In this work electron spin systems in reduced dimensions are studied with Electron Spin Resonance (ESR) spectroscopy, a method which can provide important information on the energy spectrum of the spin states, spin dynamics, and magnetic correlations. The studied systems include quasi onedimensional spin chain materials based on transition metals Cu and Ni. Another class of materials are semiconductor heterostructures made of Si and Ge. Part I deals with the theoretical background of ESR and the description of the experimental ESR setups used which have been optimized for the purposes of the present work. In particular, the development and implementation of axial and transverse cylindrical resonant cavities for high-field highfrequency ESR experiments is discussed. The high quality factors of these cavities allow for sensitive measurements on μm-sized samples. They are used for the investigations on the spin-chain materials. The implementation and characterization of a setup for electrical detected magnetic resonance is presented. In Part II ESR studies and complementary results of other experimental techniques on two spin chain materials are presented. The Cu-based material Linarite is investigated in the paramagnetic regime above T > 2.8 K. This natural crystal constitutes a highly frustrated spin 1/2 Heisenberg chain with ferromagnetic nearest-neighbor and antiferromagnetic next-nearestneighbor interactions. The ESR data reveals that the significant magnetic anisotropy is due to anisotropy of the g-factor. Quantitative analysis of the critical broadening of the linewidth suggest appreciable interchain and interlayer spin correlations well above the ordering temperature. The Ni-based system is an organic-anorganic hybrid material where the Ni2+ ions possessing the integer spin S = 1 are magnetically coupled along one spatial direction. Indeed, the ESR study reveals an isotropic spin-1 Heisenberg chain in this system which unlike the Cu half integer spin-1/2 chain is expected to possess a qualitatively different non-magnetic singlet ground state separated from an excited magnetic state by a so-called Haldane gap. Surprisingly, in contrast to the expected Haldane behavior a competition between a magnetically ordered ground state and a potentially gapped state is revealed. In Part III investigations on SiGe/Si quantum dot structures are presented. The ESR investigations reveal narrowlines close to the free electron g-factor associated with electrons on the quantum dots. Their dephasing and relaxation times are determined. Manipulations with sub-bandgap light allow to change the relative population between the observed states. On the basis of extensive characterizations, strain, electronic structure and confined states on the Si-based structures are modeled with the program nextnano3. A qualitative model, explaining the energy spectrum of the spin states is proposed.

Colloidal semiconductor nanocrystals (NCs) have attracted a great deal of interest as bright and stable chromophores for a variety of applications. Their superior physicochemical properties depend on characteristics of the inorganic core, as well as on the chemical nature and structure of the stabilizing organic ligand shell. To evaluate the promising material, a thorough knowledge of structure-property relationships is still demanded. The present work addresses this challenge to three water-soluble NC systems, namely thiol-capped CdTe, thiol-capped CdHgTe, and DNA-functionalized CdTe NCs with special emphasis on the investigation of structure, modification, and influence of the ligand shell. Remarkably, CdTe NCs show bright emission in the visible spectral region and can be synthesized in high quality directly in water. It was shown that the aqueous synthesis also facilitates the preparation of strongly near-infrared (NIR) emitting CdHgTe NCs. The current work presents a detailed study on parameters, by which the emission can be tuned, such as the growth time, the initial Cd : Hg ratio, and the choice of ligand. These insights contribute to the knowledge, which is essential for the design of highly emissive and long-term stable NIR emitting NCs. Further variations of the NC/ligand system include the modification of the ligand shell of CdTe NCs with oligonucleotides based on the strong attachment of DNA molecules to the NC. The successful functionalization of NCs with single-stranded DNA molecules is very promising for the precise and programmable assembly of NCs using DNA origami structures as templates. For both, functionality and optical properties, the surface chemistry of the NCs plays a substantial role and was subject to an extensive investigation. As there is no generally applicable technique to determine the amount of stabilizers and the structure of the ligand shell, the presented study is based on a combination of various methods particularly tailored to the analysis of water-soluble CdTe NCs capped by short-chain thiols. CdTe NCs served as a model system for the described analysis of the ligand shell, since they are thoroughly studied regarding synthesis and features of the core. Aiming for the quantification of thiols, a straightforward colorimetric assay, the Ellman\'s test, is for the first time introduced for the analysis of NCs. Accompanied by elemental analysis an approximate number of thiols per NC becomes accessible. Moreover, theoretical calculations were performed to estimate the amount of ligand that would cover the NC in a monolayer of covalently bound molecules. In contrast to these results, the experimental values point to a larger amount of thiols immobilized on the NC. Attempts to remove the ligand indicate the presence of Cd in the ligand shell and thermogravimetric studies show that the ligands are not loosely assembled in the ligand shell. The outstanding conclusion of these findings involves the presence of Cd-thiol complexes in the ligand shell. Further results unambiguously show that the amount of Cd-thiol complexes present in the NC solution strongly influences the concentration-dependent emission yield of the NCs. Additional studies dedicated to the considerable influence of the ligand shell highlight a strong effect of pH, NC concentration, type and purity of the solvent, and the number of precipitation steps on the emission of water-soluble semiconductor NCs. These substantial investigations emphasize the need to carefully control the conditions applied for handling, optical measurements, and application of NCs. In order to gain a deeper insight into the complex structure of the native ligand shell, techniques deliberately chosen for the in situ analysis were applied for thioglycolic acid-capped CdTe NCs. Information from dynamic light scattering (DLS) regarding the stability and the shell thickness are consistent with previous results showing a large ligand network on the NC surface and a decreasing stability of the NCs upon dilution. Importantly, nuclear magnetic resonance (NMR) spectroscopy allows for the distinction of bound and free ligands directly in solution and proves the presence of these species for the NCs studied. In particular, the results indicate that the ligands are not strongly bound to the NC core and that both, free and bound ligand species, consist of modified thiol molecules, such as Cd-thiol complexes. These findings support previous assumptions and allow to establish a distinct picture of the ligand shell of water-soluble semiconductor NCs. Further insights were obtained from small-angle X-ray scattering (SAXS), which facilitates the identification and the determination of the composition of NC core as well as ligand shell. Element-specific SAXS yields the final proof of the presence of Cd in the ligand shell. The model developed for the optimal fitting of the experimental scattering curves additionally confirms the findings from the other methods. In conclusion, the present work contributes to the challenging goal of a comprehensive knowledge of interactions between the NC core and the ligands. The fundamental development of a structural model of water-soluble CdTe NCs including information on stoichiometries is accomplished by the combination of the techniques presented and emphasizes the challenge to assign a clear border between the ligand shell and the Cd-thiol complexes in solution. Altogether, CdTe NCs capped by thioglycolic acid are best described by a crystalline core surrounded by a water-swollen Cd-thiolate shell that considerably affects the optical properties of the system. Notably, the results of the versatile study provide the opportunity to control the overall properties and to evaluate water-soluble semiconductor NCs for particular applications in photonics and optoelectronics.

Submitted by Murilo Oliveira (murilo.oliveira@df.ufscar.br) on 2018-05-28T14:03:10Z No. of bitstreams: 1 MuriloOliveira_dissertacao.pdf: 3213764 bytes, checksum: eaf61f839f12dceed4b235ef7970e674 (MD5) / Rejected by Ronildo Prado (ri.bco@ufscar.br), reason: Oi Murilo, Faltou enviar a Carta comprovante assinada pelo orientador. Solicite o modelo em sua Secretaria de Pós-graduação, preencha e colete a assinatura com o orientador e acesse novamente o sistema para fazer o Upload. Fico no aguardo para finalizarmos o processo. Abraços Ronildo on 2018-06-04T17:58:14Z (GMT) / Submitted by Murilo Oliveira (murilo.oliveira@df.ufscar.br) on 2018-06-04T18:39:43Z No. of bitstreams: 2 MuriloOliveira_dissertacao.pdf: 3213764 bytes, checksum: eaf61f839f12dceed4b235ef7970e674 (MD5) IMG_20180604_0001.pdf: 522878 bytes, checksum: b7fa478fbcdf1b45955f8a02f8403af0 (MD5) / Approved for entry into archive by Ronildo Prado (ri.bco@ufscar.br) on 2018-06-06T17:29:19Z (GMT) No. of bitstreams: 2 MuriloOliveira_dissertacao.pdf: 3213764 bytes, checksum: eaf61f839f12dceed4b235ef7970e674 (MD5) IMG_20180604_0001.pdf: 522878 bytes, checksum: b7fa478fbcdf1b45955f8a02f8403af0 (MD5) / Approved for entry into archive by Ronildo Prado (ri.bco@ufscar.br) on 2018-06-06T17:32:09Z (GMT) No. of bitstreams: 2 MuriloOliveira_dissertacao.pdf: 3213764 bytes, checksum: eaf61f839f12dceed4b235ef7970e674 (MD5) IMG_20180604_0001.pdf: 522878 bytes, checksum: b7fa478fbcdf1b45955f8a02f8403af0 (MD5) / Made available in DSpace on 2018-06-06T17:32:18Z (GMT). No. of bitstreams: 2 MuriloOliveira_dissertacao.pdf: 3213764 bytes, checksum: eaf61f839f12dceed4b235ef7970e674 (MD5) IMG_20180604_0001.pdf: 522878 bytes, checksum: b7fa478fbcdf1b45955f8a02f8403af0 (MD5) Previous issue date: 2018-02-28 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / The aim of this work is to study the electromagnetically induced transparency (EIT) and the coherent population trapping (CPT) phenomena in three-level systems, such as atoms and quantum dot molecules (QDM). The present work can be basically divided into two parts. The first one focus on comparing and explaining the differences between the optical reponses of systems composed by atoms and QDM and on proposing a possible application in cavity linewidth narrowing. In the second part, we turn our attention to the study of controlable optical bistability in three-level systems in the regime of CPT, bringing new perspectives to the study of these bistable systems and some practical applications as well. / O presente trabalho de mestrado tem como objetivo estudar o fenômeno da transparência eletromagneticamente induzida (EIT) e do aprisionamento coerente de populações (CPT) em sistemas de três níveis, como átomos e moléculas de ponto quântico (QDM). Essa dissertação pode ser dividida basicamente em duas partes. A primeira se encarrega de comparar e explicar a origem das diferenças observadas na resposta óptica de sistemas compostos por átomos e sistemas compostos por QDM, bem como propor uma possível aplicação para o sistema de QDM no estreitamento da largura de linha de cavidades. Já a segunda parte consiste de um estudo da biestabilidade óptica controlável em sistemas de 3 de níveis no regime de CPT, trazendo novas perspectivas de estudo desses sistemas biestáveis e aplicações como sensores de pequenas flutuações de frequência ou de amplitudes de campos eletromagnéticos. / CNPq: 131392/2016-6

Ce travail de thèse porte sur l’étude fondamentale de systèmes nano-électroniques,mesurés à très basse température. Nous avons réalisé des interféromètres électroniques àdeux chemins à partir d’électrons balistiques obtenus dans un gaz 2D d’électrons d’unehétéro-structure GaAs/AlGaAs. Nous montrons que la phase des électrons, et ainsileur état quantique,peut être contrôlée par des grilles électrostatiques. Ces dispositifsse révèlent être des candidats prometteurs pour la réalisation d’un qubit volant. Nousavons développé une simulation numérique évoluée d’un modèle de liaisons fortes à partirde transport quantique ballistique qui décrit toutes les découvertes expérimentales etnous apporte une connaissance approfondie sur les signatures expérimentales de cesdispositifs particuliers. Nous proposons des mesures complémentaires de ce système dequbit volants. Pour atteindre le but ultime, à savoir un qubit volant à un électron unique,nous avons assemblé la source à électron unique précédemment développée dans notreéquipe à un beam splitter électronique. Les électrons sont alors injectés depuis une boîtequantique à un train de boîte quantiques en mouvement. Ce potentiel électrostatique enmouvement est généré par des ondes acoustiques de surface créées par des transducteursinter-digités sur le substrat GaAs piézo-électrique. Nous avons étudié et optimisé chacunde ces composants fondamentaux nécessaires à la réalisation d’un beam splitter à électronunique et développé un procédé local et fiable de fabrication. Ce dispositif nous permet d’étudier les interactions électroniques pour des électrons isolés et pourra servir de basede mesure pour des expériences d’optique quantiques sur un système électronique del’état condensé. Enfin, nous avons développé un outil puissant de simulation du potentielélectrostatique à partir de la géométrie des grilles. Ceci permet d’optimiser la conceptiondes échantillons avant même leur réalisation. Nous proposons ainsi un prototype optimiséde beam splitter à électron unique. / This thesis contains the fundamental study of nano-electronic systems at cryogenictemperatures. We made use of ballistic electrons in a two-dimensional electron gasin a GaAs/AlGaAs heterostructure to form a real two-path electronic interferometerand showed how the phase of the electrons and hence their quantum state can becontrolled by means of electrostatic gates. The device represents a promising candidateof a flying qubit. We developed a sophisticated numerical tight-binding model based onballistic quantum transport, which reproduces all experimental findings and allows togain profound knowledge about the subtle experimental features of this particular device.We proposed further measurements with this flying qubit system. With the ultimate goalof building a single electron flying qubit, we combined the single electron source that hasbeen developed in our lab prior to this manuscript with an electronic beam splitter. Theelectrons are injected from static quantum dots into a train of moving quantum dots.This moving potential landscape is induced in the piezoelectric substrate of GaAs bysurface acoustic waves from interdigial transducers. We studied and optimized all keycomponents, which are necessary to build a single electron beam splitter and built up areliable local fabrication process. The device is capable of studying electron interactionson the single electron level and can serve as a measurement platform for quantum opticsexperiments in electronic solid state systems. Finally, we developed a powerful toolcapable of calculating the potential landscapes of any surface gate geometry, which canbe used as a fast feedback optimization tool for device design and proposed an optimizedprototype for the single electron beam splitter.