This work reports the development and spectroscopic studies of blinking-suppressed compact quantum dots. It is shown that a linearly graded alloy shell can be grown on a small CdSe core via a precisely controlled layer-by-layer process, and that this graded shell leads to a dramatic suppression of QD blinking both in organic solvents and in water. A substantial portion (over 25%) of the resulting QDs essentially does not blink (more than 99% of the time in the bright or “on” state). Theoretical modeling studies indicate that this type of linearly graded and relatively thin shells can not only minimize charge carrier access to surface traps, but also reduce accumulated lattice strains and defects at the core/shell interface, both of which are believed to be responsible for carrier trapping and QD blinking. Further, the biological utility of blinking-suppressed QDs by using both polyethylene glycol (PEG)-based and multidentate capping ligands is evaluated, and the results show that their optical properties are maintained regardless of surface coatings or solvating media, and that the blinking-suppressed QDs can provide continuous trajectories in live cell receptor tracking studies.

The unique spectroscopic properties of quantum dots (QDs) are of interest for application in intracellular studies of gene expression. QDs derivatized with single-stranded probe oligonucleotides were used to detect complementary target sequences via hybridization and fluorescence resonance energy transfer (FRET). As nucleic acid targets are not labeled within cells, a displacement assay for nucleic acid detection featuring QDs as FRET donors was developed. QDs conjugated with oligonucleotide probes and then pre-hybridized with labeled target yielded efficient FRET in vitro. Studies in vitro confirmed that displacement kinetics of pre-hybridized target was a function of the stability of the initial hybridized complex. Displacement was observed as reduction in FRET intensity coupled with regeneration of QD fluorescence. By engineering the sequence of the labeled target, faster displacement was possible. The QDprobe+target system was successfully delivered into cells via transfection. Although QDs with their cargo remained sequestered in endosomal vesicles, fluorescent properties were retained.

The unique spectroscopic properties of quantum dots (QDs) are of interest for application in intracellular studies of gene expression. QDs derivatized with single-stranded probe oligonucleotides were used to detect complementary target sequences via hybridization and fluorescence resonance energy transfer (FRET). As nucleic acid targets are not labeled within cells, a displacement assay for nucleic acid detection featuring QDs as FRET donors was developed. QDs conjugated with oligonucleotide probes and then pre-hybridized with labeled target yielded efficient FRET in vitro. Studies in vitro confirmed that displacement kinetics of pre-hybridized target was a function of the stability of the initial hybridized complex. Displacement was observed as reduction in FRET intensity coupled with regeneration of QD fluorescence. By engineering the sequence of the labeled target, faster displacement was possible. The QDprobe+target system was successfully delivered into cells via transfection. Although QDs with their cargo remained sequestered in endosomal vesicles, fluorescent properties were retained.

Inaugural-Dissertation--Philosophische Fakultät--Freiburg i. Br.--Albert-Ludwigs-Universtät, 1989. / Bibliogr. p. 245-273.

Made available in DSpace on 2014-06-12T15:52:33Z (GMT). No. of bitstreams: 2 arquivo415_1.pdf: 1541390 bytes, checksum: 2ab8d4738aeb89e4edfdfacd804ccf72 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2010 / Faculdade de Amparo à Ciência e Tecnologia do Estado de Pernambuco / A lectina ligadora de manose (mannan binding lectin - MBL) é membro das glicoproteínas plasmáticas, um grupo de proteínas que se caracteriza pela interação com um ou mais resíduos de açúcares específicos expressos em vários sistemas biológicos. Concentrações reduzidas de MBL tem sido relacionadas com a diminuição na resposta a varias doenças infecciosas. Inúmeras formas oligoméricas da MBL, com diferentes capacidades funcionais, são encontradas no sangue humano. Estudos controversos lidam com a possível associação entre mutações no gene da MBL e a infecção com o vírus da hepatite C (hepatitis C vírus - HCV). Não existem associações significativas entre pacientes com baixos níveis séricos de MBL e as características de desenvolvimento da doença, incluindo a resposta a terapia antiviral. O presente estudo teve como objetivo propor um ensaio prático para purificação de formas moleculares de MBL em amostras de soro de pacientes infectados com HCV visando investigar a estrutura e o genótipo desta lectina. Os resultados do ensaio de dot-N-man demonstraram uma boa eficiência na separação da MBL utilizando membrana de nitrocelulose revestida de manana. A identificação de MBL foi confirmada em todos os genótipos por método convencional de dot-ELISA. Bandas protéicas em gel de eletroforese revelaram diferentes padrões de migração entre os genótipos AA, A0 e 00, entre 50-90 KDa, e 270 KDa, relacionadas com a variação de baixa e alta massa molecular da subunidade estrutural de MBL, respectivamente. Nas amostras de SDS-PAGE não redutora observou-se uma maior variedade de massas moleculares, como dímeros, trímeros, e as unidades estruturais de MBL, principalmente em indivíduos AA. A analise de western blotting confirmou a presença de alta (128 KDa) e baixa (32KDa) massas moleculares da MBL. Foi também observada a presença de MBL com massa molecular de 128 KDa, a qual não é muito comum. Os resultados mostraram que ambas, baixa e alta formas moleculares, foram identificadas e também houve variação com a genotipagem dos pacientes. Os resultados descrevendo o proteoma das formas moleculares de MBL em pacientes com HCV contribuíram para melhor compreensão da estrutura/genótipo da MBL nesta patologia que pode estar associada à resposta ao tratamento

The area of quantum information promises to deliver a range of new technologies in the fields of quantum computing and quantum communication. Devices based on semiconductor quantum dots hold great potential for the practical realisation of many of the components required in the proposed schemes. This thesis describes the development of several quantum dot devices. By integrating a quantum dot into a p-i-n diode, it was possible to control the dominant emission lines in its photoluminescence spectrum and to maximise the degree of polarisation correlation between the two photons emitted in the biexciton decay. With the same device under a magnetic field, a digital memory was demonstrated. The polarisation information of a single photon was stored as the spin of an electron inside the quantum dot, and was deterministically recovered some time later by the application of an electrical trigger. A fabrication process was developed in order to produce high quality two dimensional slab photonic crystals operating with a photonic band gap at ~ 900 nm. By placing a quantum dot into an appropriately designed H1 photonic crystal cavity, strong coupling was achieved between the dot and the monopole mode of the cavity. The vacuum Rabi splitting was found to be constant for all linear polarisations due to the unpolarised nature of the far-field of the mode. Finally, a new kind of cavity based on photonic crystal waveguides was developed. A Purcell enhancement of the in-plane spontaneous emission from a quantum dot coupled to a unidirectional photonic crystal waveguide was demonstrated.

Semiconductor quantum dots are promising candidates as bright, indistinguishable, single-photon sources---making them desirable for applications in quantum computing and quantum cryptography protocols. By embedding the quantum dots in III-V nanowires, the collection efficiency from the quantum dot is greatly increased. Our goal is to develop a platform that allows for the stable and efficient generation of single-photons on chip. This on-chip design offers an enhanced degree of stability and miniaturization, important in many applications involving the processing of quantum information. In this thesis, we demonstrate the efficient coupling of quantum light generated in a III-V photonic nanowire to a silicon-based photonic integrated circuit. We use high quality SiN waveguide devices fabricated by a foundry (LIGENTEC) to minimize coupling and propagation losses through the waveguide. A hybrid integration of these single-photon sources with a photonic integrated circuit is developed by employing a "pick & place" method which uses a nanomanipulator in a scanning electron microscope setup. By tailoring the nanowire geometry, we are able to maximize the efficient coupling between the optical mode of the photonic nanowire and an accompanying SiN waveguide through evanescent coupling. To determine the effectiveness of our integration method, we compare our hybrid devices with free-standing nanowires on their growth substrate. For each set, we measured the optical properties (brightness, spectral purity, lifetime, and single-photon purity) and efficiencies of the devices. We have shown that using tapered nanowires with embedded quantum dots coupled to on-chip photonic structures is a viable route for the fabrication of stable, high-efficiency, single-photon sources. Although the measured collection efficiencies from device to device were substantially different 9.6%~93%, we have found that the optical properties of the hybrid devices were hardly impacted from the transfer process. In fact, from the same nanowire that achieved 93% coupling efficiency, we were able to measure a single photon purity of 97%. By comparing the amount of emitted light collected from both ends of the nanowire (taper and base), we confirmed that the coupling efficiency of the devices have a strong dependence on the geometry of the nanowire as collection from the taper yielded count rates at least 10x greater than from the base. From our promising results, we can envision integrating the nanowire devices with different types of photonic structures such as ring resonators.

This thesis presents advancements on the modeling of quantum dots using Fourier-space k.p theory and on the use of InGaN quantum dots for ratchet band solar cells. Fourier-space based methods have generally assumed sharp material interfaces for electronic structure, strain and piezoelectric potential calculations in quantum dot systems. Additionally, standard Fourier-space methods have often assumed uniform elastic and dielectric constants for the strain and piezoelectric potential calculations. We present generalized methods to include smoothly varying alloy profiles for the quantum dots, including spatially varying elastic and dielectric constants for the strain and piezoelectric potential calculations. For the case of InGaN/GaN quantum dots, we show that the elastic and dielectric constants corrections are important for accurate strain, piezoelectric potentials, and electronic structure. The smooth alloy profiles are constructed by convolving sharp alloy profiles with a Gaussian, and we show that the electronic structure strongly depends on the smoothing kernel, indicating the need for precise alloy profiles for accurate electronic structures. We also present a new method that facilitates the coupling of strain into the k.p Hamiltonian when considering isolated dots, greatly reducing the computational costs of calculating the Hamiltonian matrix elements. Using the methods, we investigate the use of InGaN/GaN quantum dot superlattices as ratchet band solar cells, where we propose to use the piezoelectric potential to generate a ratchet. The piezoelectric potential can spatially separate confined electron and hole states, creating a spatial ratchet in order to reduce recombination. From our quantum dot k.p model, we calculate optical light absorption cross sections and present an improved method to calculate bound-to-continuum absorption, where electrons are excited out of the dots. In this method, we approximate the continuum states as bulk k.p states for GaN. By coupling our k.p model absorptions into a detailed balance model, we predict power conversion efficiencies. We find that a large number of QD layers is necessary to achieve sufficiently strong absorption as to reach high efficiencies, highlighting one of the key issues of QD-based solar cells. We consider systems which consists of up to 131000 layers of quantum dots and an ideal Lambertian back reflector with two different QD geometries. The first geometry possesses a strong spatial ratchet and can reach a maximum efficiency of 36% under a 1-sun 6000 K black-body spectrum. We verify the existence of the spatial ratchet through the optical properties of the system, showing that it truly has the potential to block recombination. We also present an efficiency optimized system that reaches a 42% detailed balance efficiency but does not have a spatial ratchet.

In this dissertation, a theory of modulation response of a semiconductor quantum dot (QD) laser is developed. The effect of the following factors on the modulation bandwidth of a QD laser is studied and the following results are obtained:<br /><br />1) Carrier capture delay from the optical confinement layer into QDs<br /><br />Closed-form analytical expressions are obtained for the modulation bandwidth omega_{-3 dB} of a QD laser in the limiting cases of fast and slow capture into QDs. omega_{-3 dB} is highest in the case of instantaneous capture into QDs, when the cross-section of carrier capture into a QD sigma_n = infinity. With reducing sigma_n, omega_{-3 dB} decreases and becomes zero at a certain non-vanishing sigma_n^{min}. This sigma_n^{min} presents the minimum tolerable capture cross-section for the lasing to occur at a given dc component j_0 of the injection current density. The higher is j_0, the smaller is sigma_n^{min} and hence the direct modulation of the output power is possible at a slower capture. The use of multiple layers with QDs is shown to considerably improve the modulation response of the laser -- the same omega_{-3 dB} is obtained in a multi-layer structure at a much lower j_0 than in a single-layer structure.<br /><br />2) Internal optical loss in the optical confinement layer<br /><br />The internal optical loss, which increases with free-carrier density in the waveguide region, considerably reduces the modulation bandwidth omega_{-3 dB} of a QD laser. With internal loss cross-section sigma_int increasing and approaching its maximum tolerable value, the modulation bandwidth decreases and becomes zero. There exists the optimum cavity length, at which omega_{-3 dB} is highest; the larger is sigma_int, the longer is the optimum cavity.<br /> <br />3) Excited states in QDs<br /><br />Direct and indirect (excited-state-mediated) mechanisms of capture of carriers from the waveguide region into the lasing ground state in QDs are considered, and the modulation response of a laser is calculated. It is shown that, when only indirect capture is involved, the excited-to-ground-state relaxation delay strongly limits the ground-state modulation bandwidth of the laser -- at the longest tolerable relaxation time, the bandwidth becomes zero. When direct capture is also involved, the effect of excited-to-ground-state relaxation is less significant and the modulation bandwidth is considerably higher.<br /> / Ph. D.

This work develops a comprehensive theoretical model for a semiconductor laser, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). The potential of such a tunneling-injection QD laser for temperature-stable and high-power operation is studied under the realistic conditions of out-tunneling leakage of carriers from QDs (and hence parasitic recombination outside QDs) and the presence of the wetting layer (WL). The following topics are included in the dissertation: 1) Characteristic temperature of a tunneling-injection QD laser The threshold current density jth and the characteristic temperature T0 are mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, the tunneling-injection laser shows the potential for significant improvement of temperature stability of jth — the characteristic temperature T0 remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations. 2) Output power of a tunneling-injection QD laser Closed-form expressions for the light-current characteristic (LCC) and carrier population across the layered structure are derived. Even in the presence of out-tunneling leakage from QDs, the intensity of parasitic recombination outside QDs is shown to remain restricted with increasing injection current. As a consequence, the LCC of a tunneling-injection QD laser exhibits a remarkable feature — it becomes increasingly linear, and the slope efficiency grows closer to unity at high injection currents. The linearity is due to the fact that the current paths connecting the opposite sides of the structure lie entirely within QDs — in view of the three-dimensional confinement in QDs, the out-tunneling fluxes of carriers from dots are limited. 3) Effect of the WL on the output power of a tunneling-injection QD laser In the Stranski-Krastanow self-assembling growth mode, a two-dimensional WL is initially grown followed by the formation of QDs. Due to thermal escape of carriers from QDs, there will be bipolar population and hence electron-hole recombination in the WL, even in a tunneling-injection structure. Since the opposite sides of a tunneling-injection structure are only connected by the current paths through QDs, and the WL is located in the n-side of the structure, the only source of holes for the WL is provided by QDs. It is shown that, due to the zero-dimensional nature of QDs, the rate of the hole supply to the WL remains limited with increasing injection current. For this reason, as in the other parts of the structure outside QDs (QWs and optical confinement layer), the parasitic electron-hole recombination remains restricted in the WL. As a result, even in the presence of the WL, the LCC of a tunneling-injection QD laser becomes increasingly linear at high injection currents, which is a further demonstration of the potential of such a laser for high-power operation. / Ph. D.