Engineering aperiodic spiral order for photonic-plasmonic device applications

Thesis (Ph.D.)--Boston UniversityDeterministic arrays of metal (i.e., Au) nanoparticles and dielectric nanopillars (i.e., Si and SiN) arranged in aperiodic spiral geometries (Vogel's spirals) are proposed as a novel platform for engineering enhanced photonic-plasmonic coupling and increased light-matter interaction over broad frequency and angular spectra for planar optical devices. Vogel's spirals lack both translational and orientational symmetry in real space, while displaying continuous circular symmetry (i.e., rotational symmetry of infinite order) in reciprocal Fourier space. The novel regime of "circular multiple light scattering" in finite-size deterministic structures will be investigated. The distinctive geometrical structure of Vogel spirals will be studied by a multifractal analysis, Fourier-Bessel decomposition, and Delaunay tessellation methods, leading to spiral structure optimization for novel localized optical states with broadband fluctuations in their photonic mode density. Experimentally, a number of designed passive and active spiral structures will be fabricated and characterized using dark-field optical spectroscopy, ellipsometry, and Fourier space imaging. Polarization-insensitive planar omnidirectional diffraction will be demonstrated and engineered over a large and controllable range of frequencies. Device applications to enhanced LEDs, novel lasers, and thin-film solar cells with enhanced absorption will be specifically targeted. Additionally, using Vogel spirals we investigate the direct (i.e. free space) generation of optical vortices, with well-defined and controllable values of orbital angular momentum, paving the way to the engineering and control of novel types of phase discontinuities (i.e., phase dislocation loops) in compact, chip-scale optical devices. Finally, we report on the design, modeling, and experimental demonstration of array-enhanced nanoantennas for polarization-controlled multispectral nanofocusing, nanoantennas for resonant near-field optical concentration of radiation to individual nanowires, and aperiodic double resonance surface enhanced Raman scattering substrates

Advanced VLBI Imaging

Very Long Baseline Interferometry (VLBI) is an observational technique developed in astronomy for combining multiple radio telescopes into a single virtual instrument with an effective aperture reaching up to many thousand kilometers and enabling measurements at highest angular resolutions. The celebrated examples of applying VLBI to astrophysical studies include detailed, high-resolution images of the innermost parts of relativistic outflows (jets) in active galactic nuclei (AGN) and recent pioneering observations of the shadows of supermassive black holes (SMBH) in the center of our Galaxy and in the galaxy M87. Despite these and many other proven successes of VLBI, analysis and imaging of VLBI data still remain difficult, owing in part to the fact that VLBI imaging inherently constitutes an ill-posed inverse problem. Historically, this problem has been addressed in radio interferometry by the CLEAN algorithm, a matching-pursuit inverse modeling method developed in the early 1970-s and since then established as a de-facto standard approach for imaging VLBI data. In recent years, the constantly increasing demand for improving quality and fidelity of interferometric image reconstruction has resulted in several attempts to employ new approaches, such as forward modeling and Bayesian estimation, for application to VLBI imaging. While the current state-of-the-art forward modeling and Bayesian techniques may outperform CLEAN in terms of accuracy, resolution, robustness, and adaptability, they also tend to require more complex structure and longer computation times, and rely on extensive finetuning of a larger number of non-trivial hyperparameters. This leaves an ample room for further searches for potentially more effective imaging approaches and provides the main motivation for this dissertation and its particular focusing on the need to unify algorithmic frameworks and to study VLBI imaging from the perspective of inverse problems in general. In pursuit of this goal, and based on an extensive qualitative comparison of the existing methods, this dissertation comprises the development, testing, and first implementations of two novel concepts for improved interferometric image reconstruction. The concepts combine the known benefits of current forward modeling techniques, develop more automatic and less supervised algorithms for image reconstruction, and realize them within two different frameworks. The first framework unites multiscale imaging algorithms in the spirit of compressive sensing with a dictionary adapted to the uv-coverage and its defects (DoG-HiT, DoB-CLEAN). We extend this approach to dynamical imaging and polarimetric imaging. The core components of this framework are realized in a multidisciplinary and multipurpose software MrBeam, developed as part of this dissertation. The second framework employs a multiobjective genetic evolutionary algorithm (MOEA/D) for the purpose of achieving fully unsupervised image reconstruction and hyperparameter optimization. These new methods are shown to outperform the existing methods in various metrics such as angular resolution, structural sensitivity, and degree of supervision. We demonstrate the great potential of these new techniques with selected applications to frontline VLBI observations of AGN jets and SMBH. In addition to improving the quality and robustness of image reconstruction, DoG-HiT, DoB-CLEAN and MOEA/D also provide such novel capabilities as dynamic reconstruction of polarimetric images on minute time-scales, or near-real time and unsupervised data analysis (useful in particular for application to large imaging surveys). The techniques and software developed in this dissertation are of interest for a wider range of inverse problems as well. This includes such versatile fields such as Ly-alpha tomography (where we improve estimates of the thermal state of the intergalactic medium), the cosmographic search for dark matter (where we improve forecasted bounds on ultralight dilatons), medical imaging, and solar spectroscopy

Excitations of Quasi-Particles in Nanostructured Systems

The excitation of quasiparticles, like the investigated excitons and plasmons here, are the optically most prominent responses of materials. In nanostructured system, the sample quality is crucial for quantitative investigations of these optical excitations. We used electron beam evaporation, nano-second laser dewetting, and electron metalorganic chemical vapor deposition techniques to prepare well-defined and “clean” transmission electron microscopy (TEM) samples. Electron energy-loss microscopy (EELS) performed in STEM mode was employed to investigate the structural and electro-optical properties. Quantifit software was used to analyze the EELS spectra quantitatively in terms of inelastic scattering probability, energy and lifetime. We found that the ferroplasmon originates from induced excitation by the Ag’s intrinsic dipole mode at low energy, and it has a redshift with increasing particle size. Because the bimetallic system is associated with one dipole mode only, the ferroplasmons is strongly dependent on geometry. Disc-skirt AgCo nanostructures also show ferroplasmons because plasmon excitation mode of Ag disc is similar in geometry to Ag spherical, while the nanotriangles and nanobowties did not show a ferroplasmon. The bulk plasmon (BP) did not have a significate change from the pure metals to the metals in the bimetallic systems, indicating that the electron density did not change through the contact of the metals. In semiconductors, high binding energy excitons were detected universally at room temperature by EELS for the first time. The states associated with these excitons were identified as molecular states. The singlet S0 state can be directly excited to the triplet T1 state by electrons, even though the transition is forbidden optically. The conclusion on molecular states was based on the fact that this excitation can be bleached with time, and recovered in minutes. Bandbending was observed when the semiconductor is in contacting with Au nanoparticles. This exciton has a signal reduction and blue shift introduced by the band bending. The higher energy exciton can be excited from the S0 state to the singlet S1 state when the band bending is large enough. The distribution of the point defects can be mapped with high precision through mapping the intensity of the exciton

Dilute-Nitride Low-Dimensional Nanostructures Formed on Non-Planar Substrates

The properties of semiconductors heterostructures of nanoscopic dimensions change from that of bulk material according to the rules of quantum mechanics. The planar quantum wells (QWs) are widely used in various diode and laser devices thanks to the relative ease of fabrication and to their improved electronic and optical performance compared to bulk materials. Quantum effects become more apparent when the charge carriers are confined in more than 1 spatial dimension. Much scientific interest was initially dedicated to the quantum wire (QWR) structures, which confine carriers into a quasi-1D space. But their sensitivity to disorder and the development of efficient fabrication methods of quantum dots (QD) shifted the attention to this latter system. The carrier confinement in the three directions of space confers to these structures a discrete spectrum of energy states, with the state occupancy ruled by the Pauli exclusion principle. The prospective applications are numerous in domains such as ultra-low threshold lasers, quantum cryptography, true random numbers generation, quantum electrodynamics experiments, etc. One of the prominent semiconductor growth techniques is the metalorganic vapour phase epitaxy (MOVPE). We used this method to produce ordered QWRs and QDs at the bottom of V-shaped and tetrahedral recesses, respectively. These nanostructures form by the complimentary actions of nano-capillarity and growth rate anisotropy in these recesses etched in GaAs substrates. This fabrication process offers some key advantages over other methods. The emission energy is very well controlled, with a narrow inhomogeneous broadening and a high uniformity across the wafer. Combined with the highly precise control on the formation site, this offers the possibility of high-yield integration of one or even several nanostructures, e.g., into photonic crystal devices. Other advantages of this approach are the impressive tunability of the electronic potential within the nanostructures and the possible use of well-defined intraband transitions. However, the emission energy of V-groove QWRs and pyramidal QDs studied so far is quite limited due to the formation mechanism imposing a low degree of strain. Incorporation of nitrogen has dramatic effects on the band structure of GaAs-based materials. Dilute concentrations ( 250 meV). The substrate miscut and the surface corrugations are shown to play an important role in the N incorporation efficiency: QWs grown on large substrate misorientations emit at longer wavelength than those grown on usual (100)-"exact" substrates, while exhibiting a comparable luminescence efficiency. The importance of a uniform N distribution within the QW is stressed, which appears difficult to achieve when the effect of surface corrugation is combined with that of In segregation. The second part of the work focuses on the N incorporation into V-groove QWRs. Important emission redshifts are achieved, in the ∼ 250 meV range. We first detail the emission spectrum and assert the 1D-character of the carrier wavefunctions. The influence of various growth and structural parameters is explored, leading to the fabrication of QWRs emitting at 1.3 μm at room temperature. The evolution of the polarization properties with temperature is also characterized. The third main topic and primary goal of this thesis is the nitrogen incorporation into QDs formed in inverted pyramids etched on (111)B GaAs substrates. A study is first conducted to understand the effects of several growth and structural parameters on the emission properties of InGaAs QDs. Nitrogen incorporation into QDs is then successfully demonstrated. The monitoring of the lateral QWR emission energy suggests a peculiar N incorporation pattern, or a significant perturbation of the formation of these lateral nanostructures. By contrast to what achieved with QWs and QWR structures, only limited emission redshifts were achieved (∼ 75 meV). The QD linewidths, degree of linear polarization and fine structure splitting are significantly deteriorated when compared to the InGaAs counterparts. These results cast serious doubts on the perspective of high-quality GaAs-based QDs in pyramids emitting at long wavelength. Our results demonstrate that nitrogen does not have the potential to shift the emission wave-length of InGaAs pyramidal QDs up to 1.3 μm, while simultaneously satisfying strict quality requirements. But other material systems may offer such opportunities. We briefly explore the possibilities of growing InGaAs/InAlAs nanostructures on patterned InP wafers. This ongoing project may open new possibilities for exploiting the pyramidal QD system. A kinetic Monte-Carlo numerical algorithm was implemented, reproducing by deposition and diffusion processes the evolution of the pyramidal template during growth. The numerical experiments were compared with post-growth AFM measurements of real samples. The recesses are observed to strongly affect the monatomic step flow on the neighboring (111)B surfaces. The simulations especially evidence the strong attraction of the pyramid apex on the atoms of the surrounding area, tending to elevate the QD formation site from the nominal C3V symmetry toward a hexagonal one

Optically active quantum dots in bottom-up nanowires

This thesis is dedicated to the discovery and progressive study of quantum emitters embedded in the shell of coaxial gallium arsenide/ aluminum gallium arsenide nanowires. The bottom-up core/shell nanowires were grown in a molecular beam epitaxy machine. During the shell growth, diffusion-driven phenomena lead to segregation effects. Gallium-rich regions are formed at the nanoscopic scale. The observation has been made that the reduced dimensionality of these regions provides true tridimensional confinement for the carriers. The recombination spectra of the electrons with the holes in what was coined shell quantum dots (shell-QDs) thus appear as sets of narrow, intense peaks. The formation of shell quantum dots is taking place on a large range of growth temperatures and nominal alloy fractions, giving freedom to engineer the growth process. The shell thickness plays an important role in the quantum dot density and total ensemble spectrum. In addition, the adjunction of an aluminum arsenide predeposition layer increasing the local curvature has been seen to foster the quantum dots formation. Single emitter spectroscopy reveals the few-particles electronic structure of quantum dots, with systematic signatures for the different degrees of occupation of the quantum dot. The shape anisotropy of the quantum dots leads to observable spin-spin interactions, which lift the degeneracy of the exciton level (one hole and one electron). Generally undesirable, this effect allows here to find that the orientation of the quantum dots in the nanowire is not hard-wired to the growth direction or to the nanowire long axis. This observation is confirmed by magneto-photoluminescence experiments. The energetic splitting and shift of the spin sublevels when an external magnetic field is applied also confirms the small size of the quantum dots. It is found that for GaAs in the strong confinement regime, the Landé coefficients of the electron and hole take opposite signs and are dependent on the angle at which the field is applied. These effects allow to tune the exciton composite Landé coefficient and could be used to reduce the splitting between the exciton spin sublevels or create optically degenerate coupled systems. Finally, the sub-nanosecond dynamics happening in the quantum dots are probed with time-correlated photon counting. It is shown that the carriers in the shell are quickly captured by the quantum dots. In addition, it is proposed that the electron population is reduced due to diffusion-assisted mechanisms or through electron-donor recombination

Superresolution Reconstruction for Magnetic Resonance Spectroscopic Imaging Exploiting Low-Rank Spatio-Spectral Structure

Magnetic resonance spectroscopic imaging (MRSI) is a rapidly developing medical imaging modality, capable of conferring both spatial and spectral information content, and has become a powerful clinical tool. The ability to non-invasively observe spatial maps of metabolite concentrations, for instance, in the human brain, can offer functional, as well as pathological insights, perhaps even before structural aberrations or behavioral symptoms are evinced. Despite its lofty clinical prospects, MRSI has traditionally remained encumbered by a number of practical limitations. Of primary concern are the vastly reduced concentrations of tissue metabolites when compared to that of water, which forms the basis for conventional MR imaging. Moreover, the protracted exam durations required by MRSI routinely approach the limits for patient compliance. Taken in conjunction, the above considerations effectively circumscribe the data collection process, ultimately translating to coarse image resolutions that are of diminished clinical utility. Such shortcomings are compounded by spectral contamination artifacts due to the system pointspread function, which arise as a natural consequence when reconstructing non-band-limited data by the inverse Fourier transform. These artifacts are especially pronounced near regions characterized by substantial discrepancies in signal intensity, for example, the interface between normal brain and adipose tissue, whereby the metabolite signals are inundated by the dominant lipid resonances. In recent years, concerted efforts have been made to develop alternative, non-Fourier MRSI reconstruction strategies that aim to surmount the aforementioned limitations. In this dissertation, we build upon the burgeoning medley of innovative and promising techniques, proffering a novel superresolution reconstruction framework predicated on the recent interest in low-rank signal modeling, along with state-of-the-art regularization methods. The proposed framework is founded upon a number of key tenets. Firstly, we proclaim that the underlying spatio-spectral distribution of the investigated object admits a bilinear representation, whereby spatial and spectral signal components can be effectively segregated. We further maintain that the dimensionality of the subspace spanned by the components is, in principle, bounded by a modest number of observable metabolites. Secondly, we assume that local susceptibility effects represent the primary sources of signal corruption that tend to disallow such representations. Finally, we assert that the spatial components belong to a class of real-valued, non-negative, and piecewise linear functions, compelled in part through the use of a total variation regularization penalty. After demonstrating superior spatial and spectral localization properties in both numerical and physical phantom data when compared against standard Fourier methods, we proceed to evaluate reconstruction performance in typical in vivo settings, whereby the method is extended in order to promote the recovery of signal variations throughout the MRSI slice thickness. Aside from the various technical obstacles, one of the cardinal prospective challenges for high-resolution MRSI reconstruction is the shortfall of reliable ground truth data prudent for validation, thereby prompting reservations surrounding the resulting experimental outcomes. [...

Proceedings Virtual Imaging Trials in Medicine 2024

This submission comprises the proceedings of the 1st Virtual Imaging Trials in Medicine conference, organized by Duke University on April 22-24, 2024. The listed authors serve as the program directors for this conference. The VITM conference is a pioneering summit uniting experts from academia, industry and government in the fields of medical imaging and therapy to explore the transformative potential of in silico virtual trials and digital twins in revolutionizing healthcare. The proceedings are categorized by the respective days of the conference: Monday presentations, Tuesday presentations, Wednesday presentations, followed by the abstracts for the posters presented on Monday and Tuesday