Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3051. Adviser: Richard Martin. Includes bibliographical references (leaves 128-130) Available on microfilm from Pro Quest Information and Learning.

Semiconductor nanowires are of great interest as active components in numerous optoelectronic devices. Therefore, accurate characterisation and control of the nanowire transport properties is of paramount importance for the realisation of nanowire-based devices. With this aim in mind, this thesis presents THz spectroscopy as an ideal, non-contact technique for probing the nanowire electrical conductivity and carrier dynamics, with particular focus on the effect of doping and crystal structure on key device parameters, such as carrier mobilities and lifetimes. Firstly, the effect of 'bulk' n-type and p-type shell doping is investigated in GaAs nanowires. For the first time using an optical pump terahertz probe technique, high extrinsic carrier concentrations on the order of 10¹⁸cm^-3 are extracted for these doped nanowires. An increase in carrier lifetime is demonstrated as a direct result of doping-induced bandbending, highlighting controlled doping as a method for reducing parasitic surface recombination in optoelectronic nanowire-based devices. This result is particularly promising for the development of nanowire solar cells and nanowire lasers, where long carrier lifetimes are required. However, this 'bulk' shell doping technique is synonymous with a reduction in the carrier mobility within the nanowire by over an order of magnitude in comparison to an undoped reference, as a direct result of increased impurity scattering due to doping. As a solution to this inherent reduction in electron mobility associated with 'bulk' doping, modulation doping in GaAs/AlGaAs core-shell nanowires is presented. Enhanced carrier lifetimes are again observed, as dopant electrons passivate trap states at the core-shell interface. Yet, for this doping technique, a lower extrinsic carrier concentration of 10¹⁶cm^-3 is extracted. More importantly, a minimal reduction in the electron mobility is observed compared to an undoped reference sample. By physically separating the donor ions from the photoexcited electrons, impurity scattering is reduced and a high electron mobility maintained. Temperature-dependent terahertz and photoluminescence measurements confirm that the dominant scattering mechanism affecting the electron mobility in these modulation doped nanowires is longitudinal optical phonon scattering, with impurity scattering reduced in comparison to an undoped reference. From these measurements, the dopant activation energy in these nanowires is extracted for the first time via the terahertz spectroscopy, coinciding with literature values for the donors in bulk AlGaAs. An increase in carrier lifetime and radiative efficiency was observed with increasing temperature above the dopant ionisation temperature. This demonstrates the suppression of non-radiative recombination routes in these nanowires, as dopants act to passivate trap states at the core-shell interface, making modulation doped nanowires promising candidates for use in nanowire-based optoelectronic devices. Secondly, the effect of crystal structure in InAsSb nanowires is investigated. Antimony incorporation in InAs nanowires is presented as a method for achieving catalyst-free growth of quasi-pure phase nanowires, where the transport properties of the nanowire are unaffected by defects in the nanowire crystal structure. Utilising an optical pump terahertz-probe technique, an increase in carrier lifetime with increasing antimony content is demonstrated for the first time, which directly correlates with a reduction in defect density due to antimony incorporation. The electron mobilities are also extracted and an increase in mobility with increasing antimony content is observed. This is a direct result of the reduced electron effective mass at higher antimony concentrations, as well as the reduction of interface and defect scattering, associated with decreased defect density at high antimony concentrations. As interface and defect scattering dominates at low temperatures, further enhancement of the electron mobility is expected at low temperatures. Finally, from the knowledge gained from these studies of the nanowire carrier dynamics, two applications of III-V nanowires in terahertz devices are explored. Single-nanowire terahertz detectors based on InP nanowires are demonstrated, with a broad detection bandwidth of up to 2THz and signal to noise ratio of 40, comparable to bulk InP terahertz receivers. An ultrafast terahertz polarisation modulator based on GaAs nanowires is also demonstrated for the first time with picosecond optical switching speeds, a high extinction ratio of 18%, modulation depth of -8dB and dynamic range of -9dB. The performance of these nanowire-based terahertz modulators are comparable to graphene-based terahertz modulators and far surpasses those based on carbon nanotubes, providing a nanoscale platform for ultrafast THz wireless communication.

Part I of this thesis deals with a brief historical review on diamond together with the studies made on crystal growth with the existing information on diamond. Part II deals with the experimental techniques employed for the present investigation including an interference technique whereby, it has been shown for the first time, that very sharp multiple-beam interference fringes can be produced with a high power 3 mm. lens of numerical aperture 0.95. The optical conditions necessary to secure high definition in the fringes are discussed together with the arrangement used which can resolve a lateral extension about 0.7 and a depth of 50 A0. Volume elements as small as 5 x 10 cc. can be resolved. Part III deals with the optical studies of growth features on the octahedron faces of some diamonds. Some features of particular interest are described namely :-1 - A number of anomalies in trigons. These are discussed in detail.2 - An occasional mode of growth sheets is shown to operate on the octahedral faces of diamond which leads to the formation of six-sided growth features containing alternate angles of approximately 90[degrees] and 150[degrees]. It is shown that the edges of these are effectively parallel to the directions 431.These may possibly arise through intersections of (221) with (111)3 - A remarkable case of multiple linear processes has been observed on five faces of a well formed good octahedron. This has been well interpreted as due to crystallographic slip in diamond. Part IV deals with the microstructure on the dodecahedral faces of some diamonds. Some new surface structures are described. A mosaic structure exists. At least three main different structures for the (110) faces can possibly arise during the growth of diamond namely :-1 - Striated surfaces either smooth or coarse.2 - A network structure.3 - A parallelogram structure related to 2.These structures are described and discussed in detail.

Measurements of the Faraday rotation have been made, as a function of thickness, applied magnetic field, and crystal orientation, for light of wavelength 5461A in films of nickel 100 to 400A in thickness. Reviews are given of the phenomenon of the Faraday rotation with particular reference to ferromagnetic films; of epitaxy, especially of metallic layers; and of thin film ferromagnetism. Techniques were developed with the aim of producing nickel specimens in the form of pure, parallel-sided, monocrystalline films, so as to obtain measurements which were true properties of the nickel lattice Using electron microscopy and diffraction, conditions were established for the epitaxial growth of nickel by condensation from the vapour phase in vacuo, on the principal planes of rock salt and copper. Oriented films grown on rock salt were generally aggregated or discontinuous, prominently twinned on {111} planes, and contained numerous crystal faults. Films grown on (110) faces displayed features attributable to twinning on (111) and (111) planes, and an elongation of the crystallites in the 001 direction. Oriented films grown on coppersubstrates were continuous down to 100A thickness and relatively free from faults. The magneto-optic rotations were measured by means of a photoelectric polarimeter with a precision of better than 1/3'. The construction and use of the instrument are described in the thesis. Film thicknesses were determined by measurement of the area and mass of each film, assuming bulk density. A discontinuity in the graph of rotation versus applied magnetic field for films grown on copper was found to be explicable in terms of the bulk lattice properties. No significant dependence of rotation on thickness was observed for such films. The specific rotations for the 100, 110, and 111 directions were found to be respectively +79,000 + 2,000/cm.,+88,000+ - 3,000/cm., and+97,000+ 3,000/cm.

In the last fifteen years there has been a considerable increase in the knowledge of the optical properties of metal layers less than 1000 A thick, and the use of layers produced by evaporation or cathodic sputtering has become increasingly common for the determination of the optical constants of metals. In this work the interferometric method, originally due to Tolansky, for the determination of the relative phase changes occurring on reflexion at the surface of a metal layer has been further developed, and, coupled with a simple photoelectric device for the measurement of intensity changes, has been applied to the study of reflexion at a number of surfaces. The method requires slightly transparent layers of the metal to form a transmission interferometer, and it is therefore of particular importance to determine within what thickness limits the optical properties of these layers can be taken as characteristic of the metal in bulk form. To this end, a study of the variation with thickness of the characteristics of non-normal reflexion at thin silver layers has been made for thicknesses between 100 and 1000 A. Measurements have also been made using the technique of the optical constants of copper, tin, speculum, and a new tin-nickel alloy. Some interesting anomalies in the opticalproperties of evaporated layers of tin have been observed. The thesis commences with a treatment of the theory of reflexion at metal surfaces and layers, and the previous work in this field is reviewed, with particular reference to measurements on evaporated layers, and the variation of their properties with thickness. The evaporation equipment and procedure used are described, and the measurement of layer thicknesses critically discussed. The optical techniques are then described, the interferometric technique being analysed in detail. Finally the results of the experiments are mentioned and discussed.

A historical review of the use of powder patterns, polarized light and intorferometry to examine small vibrations is given. Suitably cut quartz crystals are used for an examination by these methods of the longitudinal vibrations of thin isotropic discs. Previous calculations of the frequencies and displacements for these modes of vibration are extended and the distribution of stress is computed numerically. The theory underlying the use of the three methods of observation Is discussed with particular reference to the examination of these stresses and displacements. It Is found that for sufficiently thin discs the primary stresses and displacements are substantially in agreement with theory, except for the presence in all cases of a coupled flexural mode having a particular symmetry dependent upon the symmetry of the longitudinal mode. Moreover, for those modes which do not have circular symmetry about the disc axis, there are always two frequencies of vibration for which the orientations of the vibratory systems have a particular relation, again dependent upon the particular modal symmetry. A theory is given whichadequately explains the coupling phenomena by reference to the cross shear strains which are due to the particular crystal symmetry of quartz and are not possible for isotropic media. In connection with the polarized light experiments, a theory of the dependence of the apparent rotatory power of quartz upon stress is developed and agrees well with an experimental test; some phenomena observed by earlier workers are explained on the basis of this theory. It is shown that there is good correlation between the three methods of observation but that the information yielded by any one method separately is likely to give an oversimplified representation of the state of vibration.

Surface structure of the spiral is studied employing multiple beam interferometric techniques. The spiral has a slope perpendicular to a radius vector equal to h/2piA per radian, where h is the step height of the spiral. The surface of the spiral is highly curved; the curvature decreasing with distances away from the dislocation. The angle of inclination of the growth surface with the underlying (0001) face of SiC crystal is determined at various sections and is found to be greater than the theoretical radial slope Tan-1 h/a where d is the distance between tv/o consecutive loops of the spiral. The hollow dislocations predicted by Prank are associated with surface distortions of the sinking in type. Sometimes the large stress accompanying a giant dislocation produces a percussion figure on the crystal surface. Employing interferometric, X-ray diffraction and electron-microscopic techniques, it is shown that multi-molecular steps do exist in SiC and that they are sharp, single and vertical. Analysing the structure of multi-molecular steps, it is shown that the average distance between two kinks along the step edge should be about 1000A in SiC. It is established that at least in SiC, the step height is determined by the Burgers vector of the dislocation. In addition to the movement of unit dislocation causing unit slip, evidence for the movement of giant dislocations has been obtained for the first time. The experimental evidence for the operation of a modified cone source as an alternative to the Frank-Read source which is also common in SiC is presented. In this type of source, the slip plane of the sweeping dislocation rotates as the dislocation spirals. This surface study has also revealed the nature of dislocations present in SiC. In addition to the pure screw dislocations with Burgers vector perpendicular to the crystal surface, inclined dislocations which can he resolved into edge and screw components have been observed. In the light of these inclined dislocations, a modification to Frank's theory of polytypism is suggested. This explains the production of almost all the rhombohedral polytypes in SiC without invoking periodic break in the stacking sequence. Dislocations having screw component usually equal to 15 A have been observed on the step lines of the growth spiral. Deep channels terminating on these dislocations have been observed along the step lines. These dislocations cause among other things, fault lines, dissociation and grouping of steps and disorder in rhombohedral class of crystals. This latter has been confirmed by X-rays. Optical and interferometric study of the dendrites of germanium and tin grown from undercooled melt and by the action of Zn on SnCl2 solution respectively has been carried out. In the case of tin, tv/o types of crystals, viz: (1) flat dendrites with perpendicular branching and (2) needles without branching have been found to occur. It has been possible both in the case of Ge and Sn to correlate the surface features with their space lattice. Since most of the observations have been made employing the multiple beam interferometric techniques, the contouring efficiency of thin films used in these techniques is studied. It is shown for the first time that silver films accurately contour the underlying topography. Dielectric multilayers can be used for qualitative work but for quantitative measurements they do not seem to satisfy contouring requirements. The thesis commences with a brief account of the theories of crystal growth. The previous observations are reviewed in brief in the light of the present investigation.

This thesis reports on recent results which pave the way for future experiments in the emerging field of quantum magnonics. Chapter 1 presents a brief outline of the field of magnonics, which provides the context in which quantum magnonics has begun to develop. Chapter 2 provides an introduction to the theory of spin waves, which is necessary to understand the experiments reported in the thesis. In Chapter 3, the experimental methods and materials used to carry out the investigations in the thesis are described. Chapter 4 describes the coupling of resonant magnon modes in a sphere of yttrium-iron garnet to photon modes in a coplanar-waveguide resonator. Strong coupling is achieved to multiple magnon modes, and a theoretical model is used to identify the magnon modes which couple most strongly to the photon mode. In Chapter 5, the behaviour of propagating magnon modes is investigated in a waveguide formed from a thin film of yttrium-iron garnet. Two different configurations are investigated supporting different types of propagating mode, namely backward-volume and surface spin waves. Simulations are performed which reproduce the main features of the data. Chapter 6 characterises the effect of the gadolinium-gallium garnet substrate on propagating spin waves. The magnitude of this effect is dependent on both the orientation and temperature of the sample. Finally, Chapter 7 provides a short summary of the results of the thesis, and speculates on how they may inform future work in the field.

PURPOSE: To compare white matter neuron density across 3 regions, prefrontal, temporal, and posterior parietal (PFC, TE, PP) in macaque monkey, with further analysis of subdivisions within the gyral white matter. METHODS: Histological tissue from three adult macaque monkeys, previously prepared with the neuron-specific pan-neuronal marker Neuronal-N, was used for analysis. Tissue was digitized and processed electronically to investigate cross- and intra-regional differences in the distribution of white matter neurons. RESULTS: Statistical analysis showed significant differences across all regions sampled and across most intra-regional subdivisions, although the more conservative post-hoc tests failed to find significant differences between specific regions. CONCLUSIONS: The results of the current study support regional differences. Further studies using a larger sample size may help elucidate the relatively unknown properties of white matter neurons.

<p> Metamaterials operating at terahertz (THz) region of the electromagnetic spectrum have remained have remained a promising area of study not only for realizing technologies in a historically underdeveloped spectral regime, but also as a scientific tool for exploring and controlling fundamental physical phenomena at meV energy scales in a variety of condensed matter systems. In this thesis, I will present several projects in which metamaterials and more traditional condensed matter systems are integrated into hybrid metamaterial systems. We leverage these systems to realize new practical THz devices, as well as to couple to and control quantum phenomena in condensed matter systems. I will begin with an introduction to the conceptual, numerical, and experimental techniques in the THz metamaterial toolbox. The first research endeavor I will discuss is a metamaterial system that incorporates perhaps the simplest material; air. This metamaterial perfect absorber with a continuously tunable air dielectric layer allows for comprehensive exploration of metamaterial absorber systems, and demonstrates some unique phenomena owing to its lossless dielectric layer. Next I will introduce an applications oriented device; an electrically actuated broadband terahertz switch which transitions from a non-reflective, transmissive state to a fully absorptive state. It employs an all dielectric metamaterial layer to suppress reflections and trap light, and an electrically actuated phase change material, <i>VO</i>₂ to transition between states. The final section of this dissertation will explore strong coupling effects between a metamaterial and the superconducting c-axis Josephson plasmon in the layered cuprate, <i>La_2&ndash;xSr_xCuO₄</i>. Preliminary measurements are first presented then followed by high field THz measurements in which complex nonlinear behavior is observed.</p><p>