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In het spintronica-onderzoeksveld wordt de elektronspin naar voren geschoven als informatiedrager. Na eerdere successen bij metaal-gebaseerde componenten, is het gebruik van de spin in halfgeleidercomponenten een volgende integratiestap. Een belangrijke uitdaging is een elektrische creatie van een spinpolarisatie in halfgeleiders. Een directe injectie van spins vanuit ferromagnetische materialen lijkt hier de meest aangewezen aanpak. In deze thesis hebben we elektrische spininjectie in GaAs bestudeerd gebruik makende van verschillende tunnelcontacten. Een meting van de circulaire polarisatie van het licht dat uitgezonden werd in een licht-emitterende diode (LED) werd gebruikt om de geinjecteerde spinpolarisatie te meten. Het oblique Hanle effect, een techniek die gebaseerd is op spinprecessie in een schuin magnetisch veld, laat ons toe zowel de spinpolarisatie als informatie over de spindynamica in de halfgeleider te bekomen. Om spininjectie vanuit verschillende tunnelcontacten te meten, werden verschillende spin-LEDs ontworpen en gefabriceerd. Spin-LEDs gebaseerd op tunnelinjectie van een gesputterd ferromagnetisch metaal (CoFe) door een oxidebarriere, vanuit een epitaxiaal ferromagnetisch metaal (MnSb) door de Schottky barriere en vanuit een ferromagnetische halfgeleider ((Ga,Mn)As) resulteerden in een hoge geinjecteerde spinpolarisatie. Kamertemperatuur spininjectie werd gemeten voor de CoFe-oxide-injector en een heel hoge polarisatie werd behaald voor de (Ga,Mn)As-injector bij lage temperatuur. Bovendien hebben we de spanningsafhankelijkheid bestudeerd en verklaard voor de verschillende spin-LEDs. De experimentele geometrie liet toe de invloed te meten van de elektronspins op de spins van de kernen in het GaAs rooster. We hebben deze spininteracties onderzocht door transiente effecten te bestuderen na een verandering van het teken van de elektronspins en een resonante depolarisatie van de nucleaire spins in een alternerend magnetisch veld. Tenslotte hebben we het ontwerp bekeken van volledig-elektrische spin-injectie-detectie componenten. Gebruik makende van zelf-consistente simulaties, hebben we het dopingsprofiel ontworpen voor zowel de injectiecontacten als voor het spin-transportgebied. In the field of spintronics the electron spin is used as information carrier in addition to the charge. After earlier successes in metal-based spin-devices, using the spin in semiconductor devices would mean a next integration step. An important challenge for such devices is the creation of a non-equilibrium spin polarization in semiconductors. The most straightforward approach is direct injection of spins from ferromagnetic materials. In this work we have studied electrical spin injection in GaAs using a variety of tunneling contacts. A measurement of the circular polarization of the light emitted in a light-emitting diode (LED) has been used to determine the injected spin polarization. Using the oblique Hanle effect, a technique that relies on spin precession in an oblique magnetic field, both the injected spin polarization and information about the spin dynamics can be obtained. In order to study spin injection from different tunnel contacts, different spin-LEDs have been designed and fabricated. Spin-LEDs based on tunnel injection from a sputtered CoFe ferromagnetic metal through an oxide barrier, from an epitaxial MnSb ferromagnetic metal through the Schottky barrier and from a ferromagnetic semiconductor ((Ga,Mn)As) have been shown to result in a high injected electron spin polarization. The polarization persisted up to room temperature for the injector based on the oxide tunnel barrier and a very high polarization has been obtained for the injector based on (Ga,Mn)As at low temperature. Furthermore we have examined and explained the bias dependence of the injected spin polarization for the different devices. The experimental geometry also allowed to measure the influence of the injected electron spins on the spins of the nuclei in the GaAs lattice. We have investigated the electronic-nuclear spin interactions by examining transient effects upon changing the sign of the injected electron spins and resonant depolarization of the nuclear spins by an alternating magnetic field. Finally, we have looked into the design of all-electrical spin injection-detection devices. Using self-consistent simulations, we have designed the doping profile for both the injection contacts and for the spin-transport region. Spintronica, of spinafhankelijke elektronica is een onderzoeksgebied waarin onderzocht wordt hoe het intrinsiek magnetisch moment van elektronen - hun spin - kan gebruikt worden om nieuwe elektronische of spintronische componenten te ontwerpen. Na eerdere successen bij metaal-gebaseerde componenten, is het gebruik van de spin in halfgeleidercomponenten een volgende integratiestap. Een belangrijke uitdaging is een elektrische creatie van een spinpolarisatie in halfgeleiders. Een directe injectie van spins vanuit ferromagnetische materialen lijkt hier de meest aangewezen aanpak. In deze thesis hebben we elektrische spininjectie in GaAs bestudeerd gebruik makende van verschillende tunnelcontacten. Een meting van de circulaire polarisatie van het licht dat uitgezonden werd in een licht-emitterende diode (LED) werd gebruikt om de geinjecteerde spinpolarisatie te meten. De meetmethode was gebaseerd op de precessie van spins in een schuin magnetisch veld. De effecten van die precessie vertaalden zich in veranderingen in de circulaire polarisatie van het licht. Op die manier konden we zowel de polarisatie van de spins als informatie over hun dynamica te weten komen. Om spininjectie vanuit verschillende tunnelcontacten te meten, werden verschillende spin-LEDs ontworpen en gefabriceerd. Spin-LEDs gebaseerd op tunnelinjectie van een ferromagnetisch metaal door een tunnelbarriere en vanuit een ferromagnetische halfgeleider via een interband tunnelproces resulteerden in een hoge geinjecteerde spinpolarisatie. Kamertemperatuur spininjectie werd gemeten voor de injector gebaseerd op het ferromagnetische metaal en een heel hoge polarisatie werd behaald voor de injector gebaseerd op de ferromagnetische halfgeleider bij cryogene omstandigheden. Bovendien hebben we de spanningsafhankelijkheid bestudeerd en verklaard voor de verschillende spin-LEDs. De experimentele geometrie liet toe de invloed te meten van de elektronspins op de spins van de kernen in het GaAs rooster. We hebben deze spininteracties onderzocht door tijdsafhankelijke effecten te bestuderen na een verandering van het teken van de elektronspins en een resonante depolarisatie van de nucleaire spins in een alternerend magnetisch veld. Tenslotte hebben we het ontwerp bekeken van volledig-elektrische spin-injectie-detectie componenten. Gebruik makende van simulaties, hebben we een component ontworpen, waarbij zowel de dopering van de injectiecontacten als die van die van het spin-transportgebied geoptimaliseerd werd. Spintronics, or spin-dependent electronics contains research that is focussed on the use of the intrinsic magnetic moment of electrons - their spin - in new electronic, or rather spintronic devices. After earlier successes in metal-based components is the use of the spin in semiconductor devices a next integration step. An important challenge for such devices is the creation of a non-equilibrium spin polarization in semiconductors. The most straightforward approach is direct injection of spins from ferromagnetic materials.In this work we have studied electrical spin injection in GaAs using a variety of tunneling contacts. A measurement of the circular polarization of the light emitted in a light-emitting diode (LED) has been used to determine the injected spin polarization. The measurement technique was based on spin precession in an oblique magnetic field. This allowed both a determination of the injected spin polarization and insightful information about the spin dynamics. In order to study spin injection from different tunnel contacts, different spin-LEDs have been designed and fabricated. Spin-LEDs based on tunnel injection from ferromagnetic metals and from a ferromagnetic semiconductor have been shown to result in a high injected electron spin polarization. The polarization persisted up to room temperature for the injector based on the ferromagnetic metal and a very high polarization has been obtained for the injector based on the ferromagnetic semiconductor at low temperature. Furthermore we have examined and explained the bias dependence of the injected spin polarization for the different devices. The experimental geometry also allowed to measure the influence of the injected electron spins on the spins of the nuclei in the GaAs lattice. We have investigated the electronic-nuclear spin interactions by examining transient effects upon changing the sign of the injected electron spins and resonant depolarization of the nuclear spins by an alternating magnetic field. Finally, we have looked into the design of all-electrical spin injection-detection devices. Using self-consistent simulations, we have designed the doping profile for both the injection contacts and for the spin-transport region.

Academic collection --- Theses

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Fluorescence microscopy has become an indispensable tool in biology and medicine. It is routinely used in drug development, to diagnose health disorders and in DNA sequencing. Despite enormous progress in instrumentation and deployment, high-resolution microscopes remain expensive, bulky devices that require skilled operators. As a consequence, they are found only in specialized labs, limiting their accessibility. The next big push in microscopy with large societal impact will come from extremely compact and robust optical systems that will make high-resolution microscopy highly accessible. This push to miniaturization can be facilitated by photonic integrated circuits. These are extremely compact chip-scale devices that are mass-produced using CMOS process technology. All lens-based and state-of-the-art lens-free microscopy solutions use light propagation in three dimensional space to illuminate a sample. For a fluorescence microscope that is miniaturized to be fully on a chip, the illumination is confined to a two dimensional slab-waveguide. By placing the sample on top of the slab region, the fluorescent signal is then activated by the evanescent light in the vicinity of the contact region. When the entire slab region is to be illuminated simultaneously, due to the nature of light, the emitted fluorescence signal will be blurred when detected, making it impossible to reconstruct the image. The challenge is to selectively illuminate the sample. Through interference of multiple laser beams a light pattern can be generated in the slab waveguide. Such patterns can be in general classified as so-called moiré patterns, a curious geometrical phenomenon of periodic patterns that are made to overlap. The theory of generating structured illumination with full control over its geometrical features using the theory of moiré patterns is not sufficiently developed in the literature. In this thesis, a robust mathematical framework is developed and an algorithm to design various illumination patterns that are suitable for fluorescence microscopy is proposed.

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Fluorescence microscopy is an important tool for every biologist, and in recent years huge progress was made in increasing the resolution and image quality. This progress comes at the cost of bulky equipment, expensive optical setups and the requirement of highly skilled operators. In the IROCSIM project at imec, a world leading research center for nanotechnology, a new type of microscopy is proposed. By using photonic integrated circuits, combined with a monolithic integrated CMOS imager all free-space optics are eliminated and a fluorescence microscope can be fully integrated on a chip, making it affordable, portable and easy to use. The resolution is maintained by using dedicated structured illumination patterns based on interference of light from different waveguides. The excitation of the sample is based on total internal reflection (TIRF), providing high axial resolution and leading to high signal-to-background ratios. In this thesis a system model for integrated photonics-based microscopy is developed to guide the design optimization process of the microscopy chips. The system model helps in understanding the combination of on-chip waveguide-based excitation of the sample and lens-free imaging with an on-chip integrated imager. In the model, all parts of the on-chip microscope are included, starting with the photonic integrated circuit, the generation of illumination patterns, the TIRF-based sample excitation, the optical filter and the monolithic integrated CMOS imager. The process of excitation and emission of the fluorophores is modeled, and a simulation is included to determine the radiation pattern of a fluorophore close to a planar interface. Fluorophores close to the interface were found to have a highly structured radiation pattern strongly affecting spatial distribution of light on the image sensor. The model developed in this thesis provides deep insight in the important design parameters and speeds up the prototyping of a microscopy technique that will revolutionize the field of fluorescence microscopy. Furthermore it provides a flexible framework that can be used to characterize other waveguide-based microscopy techniques.

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Confocal Raman imaging is proposed as a label-free circulating tumor cells (CTCs) detection method, that can be used as an alternative to the existing detection methods which potentially exclude CTC sub-populations, frequently lead to cell death, and require labels leaving the isolated CTCs incompatible for further characterization. It is investigated in this study whether cancer types can be distinguished from white blood cells and from each other based on spontaneous single-cell Raman scattering. Raman measurements are performed on 1500 single cells, i.e. 300 breast (MDAMB-231), 300 prostate (LNCaP), 300 ovarian (SKOV3), 300 skin (MM047) and 300 mononuclear white blood cells from healthy donors. The recorded Raman spectra were preprocessed using existing in-house algorithms from which the normalization algorithm was improved, and where several background removal methods were investigated to study the difference between calculation- and experiment-based methods. After preprocessing, the different cell spectra were compared using principal component analysis and classification of the cells using k-nearest neighbor (kNN), which was preferr linear discriminant analysis (LDA) as better classification efficiencies could be achieved. It was not possible to distinguish any of the cancer types and mononuclear white blood cells from each other as the obtained classification efficiencies could not be used. Analysis of the most important principal components and their corresponding eigenvectors showed that for every used background removal methods the obtained classification efficiencies were due to artificial contributions and not based on the biological differences between cells. Hence, classification achieved with these methods does not relate to the biology of the cells and isn't suited for the purpose of cancer and blood cell identification.

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The nitrogen-vacancy (NV) center is one of the many point defects in the diamond lattice. NV centers have proven themselves the last years as very promising magnetic field sensors. Quantum mechanical interactions of the spin states allow for magnetic resonance imaging with high sensitivity and subnanometer resolution in ambient conditions. The underlying measuring mechanism is optically detected magnetic resonance (ODMR). In this master thesis, NV centers in diamond nanoparticles are integrated in a photonic circuit to present a hybrid Si3N4 waveguide - diamond nanoparticle photonic circuit for quantum magnetic field sensing. A recent progress for low-loss photonic waveguides is the fixed beam moving stage (FBMS) feature in electron beam lithography (EBL), allowing stitching error-free waveguides. The goal of this master thesis is twofold. The first goal is the demonstration of ODMR mediated by evanescent field excitation of a Si3N4 waveguide. The second goal is the development of a fabrication process of FBMS waveguides on the new electron beam lithography tool at the Leuven nanocenter. To achieve these goals, an ODMR setup was designed, starting from a confocal microscope. Next, optimal waveguide parameters are obtained using simulations in Lumerical FDTD and ModeSolutions and an optimization of the EBL Si3N4 FBMS waveguide writing process is performed. A set of writing parameters is obtained, but unfortunately no FBMS waveguides were available in the measurements. However, successful ODMR measurements mediated by evanescent field excitation are presented on Si3N4 waveguides fabricated by DUV lithography at imec which where made available for this master thesis. A sensitivity of 27 μT/Hz^0.5 is shown for evanescent field excitation, which is comparable to the sensitivities in literature. This provides a basis for further research on the integration of magnetometry and photonic circuits.