Distributed MIMO (DMIMO) communications and specifically the idea of distributed transmit beamforming involves multiple transmitters coordinating among themselves to form a virtual antenna array and steer a beam to one or more receivers. Recent works have successfully demonstrated this concept of beamforming with narrowband, frequency-flat wireless channels. We consider the generalization of this concept to wideband, frequency selective channels and propose two Figures of Merit (FOMs), namely, communication capacity and received power to measure the performance of beamforming. We formulate the precoder design that maximizes the two FOMs as optimization problems and derive general properties of the optimal precoders. The two metrics are equivalent with frequency-flat channels, whereas, they result in vastly different optimal criteria with wideband channels. The capacity maximizing solution also differs from classical water-filling due to the per-transmitter power constraints of the distributed beamforming setting, whereas, the power maximizing solution involves the array nodes concentrating their power in a small, finite set of frequencies resulting in an overall received signal consisting of a small number of sinusoidal tones. We have not been able to derive closed-form solutions for the optimal precoders, but we provide fixed point algorithms that efficiently computes these precoders numerically. We show using simulations that solution to both these maximization problems can yield substantially better performance as compared to simple alternatives such as equal power allocation. The fixed point algorithms also suggest a distributed implementation where each node can compute these precoders on their own iteratively using feedback from a cooperating receiver. We also establish the relationship between various precoders. The idea of maximizing received power suggests a natural application of wireless power transfer(WPT). However, the large-scale propagation losses associated with radiative fields makes antennas unattractive for WPT systems. Motivated by this observation, we also consider the problem of optimizing the efficiency of WPT to a receiver coil from multiple transmitters using near-field coupling. This idea of WPT using near-field coupling is not new; however, the difficulty of constructing tractable and realistic circuit models has limited the ability to accurately predicting and optimizing the performance of these systems. We present a new simple theoretical model and take the more abstract approach of modeling the WPT system as a linear circuit whose input-output relationship is expressed in terms of a small number of unknown parameters. We present a simple derivation of the optimal voltage excitations to be applied at the transmitters to maximize efficiency, and also some general properties of the optimal solution. Obviously, the optimal solution is a function of unknown parameters, and we describe a procedure to estimate these parameters using a set of direct measurements. We also present a series of experimental results, first, with two transmitter coils and a receiver coil in a variety of configurations and then with four transmitter coils and two receiver coils to illustrate our approach and the efficiency increase achieved by using the calculated optimal solution from our model.

With the advent of wide bandgap (WBG) semiconductor devices, the electromagnetic interference (EMI) emissions are more pronounced due to high slew rates in the form of high dv/dt and high di/dt at higher switching frequencies compared to the traditional silicon technology. To comply with the stringent conducted emission requirements, EMI filters are adopted to attenuate the high frequency common mode (CM) and differential mode (DM) noise through the propagation path. However, self and mutual parasitic components are known to degrade the EMI filter performance. While parasitic cancellation techniques have been discussed at length in prior literature, most of them have focused mainly on single phase applications. As such this work focuses on extending the preexisting concepts to three-phase systems. Novel component placement, winding strategy as well as shielding and grounding techniques were developed to desensitize the influence of the parasitic effects on a three-phase multi-stage filter. The effectiveness of the three-phase filter structure employing the proposed methodologies has been validated via noise measurements at the line impedance stabilization network (LISN) in a 15kW rated motor drive system. Consequently, general design guidelines have been formulated for filter topologies with different inductor and capacitor form-factors. / Master of Science / The adoption of wide bandgap (WBG) semiconductor devices, such as Silicon Carbide (SiC) or Gallium Nitride (GaN) transistors, improves the power density with higher slew rates and switching frequencies compared to the traditional Silicon technology. However, the high switching speeds and high frequencies have generated higher electromagnetic interference (EMI) noise in the surroundings. To comply with the conducted emission requirements at the grid terminal, EMI filter is mandatory to attenuate the high frequency EMI noise that flows into grid. However, near field and the effect of parasitic components are known to degrade the filter performance at the higher end of frequency spectrum where the limit lines are typically stringent. While parasitic cancellation techniques have been discussed at length in prior literature, most of them has focused mainly on single phase applications. Therefore, this thesis aims to extend the pre-existing concepts to compensate the mutual and self-parasitic coupling components in a three-phase multi-stage filter. In this regard, novel component placement, winding strategy as well as shielding and grounding techniques were developed to compensate for the parasitic effects in a three- phase multi-stage filter. The effectiveness of the three-phase filter structure employing the proposed methodologies has been validated in a 15kW rated motor drive system. Consequently, general design guidelines have been formulated for filter design with minimal parasitic effects.

Nanometer-scale particles of the noble metals have been used for decades as contrast enhancement agents in electron microscopy. Over the past several years it has been demonstrated that these particles also function as excellent contrast agents for optical imaging techniques. The resonant optical scattering they exhibit enables scattering cross sections that may be many orders of magnitude greater than the analogous efficiency factor for fluorescent dye molecules. Biologically relevant labeling with nanoparticles generally results in aggregates containing a few to several tens of particles. The electrodynamic coupling between particles in these aggregates produces observable shifts in the resonance-scattering spectrum. This dissertation provides a theoretical analysis of the scattering from nanoparticle aggregates. The key objectives are to describe this scattering behavior qualitatively and to provide numerical codes usable for modeling its application to biomedical engineering. Considerations of the lowest-order dipole-dipole coupling lead to simple qualitative predictions of the behavior of the spectral properties of the optical cross sections as they depend on number of particles, inter-particle spacing, and aggregate aspect ratio. More comprehensive analysis using the multiple-particle T-matrix formalism allows the elaboration of more subtle cross-section spectral features depending on the interactions of the electrodynamic collective-modes of the aggregate, of individual-particle modes, and of modes associated with groups of particles within the aggregate sub-structure. In combination these analyses and the supporting numerical code-base provide a unified electrodynamic approach which facilitates interpretation of experimental cross section spectra, guides the design of new biophysical experiments using nanoparticle aggregates, and enables optimal fabrication of nanoparticle structures for biophysical applications. / text

The absorptance in vertical nanowire (nw) arrays is typically dominated by three optical phenomena: radial mode resonances, near-field evanescent wave coupling, and Fabry–Perot (F-P) mode resonances. The contribution of these optical phenomena to GaAs, InP and InAs nw absorptance was simulated using the finite element method. The study compared the absorptance between finite and semi-infinite nws with varying geometrical parameters, including the nw diameter (D), array period (P), and nw length (L). Simulation results showed that the resonance peak wavelength of the HE1n radial modes linearly red-shifted with increasing D. The absorptance and spectral width of the resonance peaks increased as L increased, with an absorptance plateau for very long nws that depended on D and P. Near-field coupling between neighbouring nanowires (nws) was observed to increase with increasing diameter to period ratio (D/P). The effect of F-P modes was more pronounced for shorter nws and weakly coupled light. Based on the collective observation of the correlation between nw geometry and optical phenomena in GaAs, InP, and InAs nw arrays, a periodic array of vertical InSb nws was designed for photodetectors in the low-atmospheric absorption window (λ = 3-5 μm) within the mid-wavelength infrared (MWIR) spectrum (λ = 3-8 μm). Simulations, using the finite element method, were implemented to optimize the nw array geometrical parameters (D, P, and L) for high optical absorptance (~0.8), which exceeded that of a thin film of equal thickness. The results further showed that the HE1n resonance wavelengths in InSb nw arrays can be tuned by adjusting D and P, thus enabling multispectral absorption throughout the near infrared (NIR) to MWIR region. Optical absorptance was investigated for a practical photodetector consisting of a vertical InSb nw array embedded in bisbenzocyclobutene (BCB) as a support layer for an ultrathin Ni contact layer. Polarization sensitivity of the photodetector was examined. Lastly, how light flux enters the nw top and sidewalls on HE11 resonance was investigated. / Dissertation / Doctor of Philosophy (PhD)