Resonances and Fundamental Bounds in Wave Scattering

In this thesis, we develop a framework for analyzing light-matter interaction by resonances, explore power concentration limits in wave scattering, and discover fundamental bounds of quantum state controls via pulse engineering. We use quasinormal modes to develop an exact, ab initio generalized coupled-mode theory from Maxwell’s equations. This quasinormal coupled-mode theory, which we de- note “QCMT”, enables a direct, mode-based construction of scattering matrices without resorting to external solvers or data. We consider canonical scattering bodies, for which we show that a conventional coupled-mode theory model will necessarily be highly inaccurate, whereas QCMT exhibits near-perfect accuracy. We generalize classical brightness theorem to wave scattering, showing that power per scattering channel generalizes brightness, and obtaining power-concentration bounds for systems of arbitrary coherence for general linear wave scattering. The bounds motivate a concept of “wave ́etendue” as a measure of incoherence among the scattering-channel amplitudes and which is given by the rank of an appropriate density matrix. The bounds apply to nonreciprocal systems that are of increasing interest, and we demonstrate their applicability to maximal control in nanophotonics, for metasurfaces and waveguide junctions. Through inverse design, we discover metasurface elements operating near the theoretical limits. We show that an integral-equation-based formulation of conservation laws in quantum dynamics leads to a systematic framework for identifying fundamental limits to any quantum control scenario. We demonstrate the utility of our bounds in three scenarios – three-level driving, decoherence suppression, and maximum-fidelity gate implementations – and show that in each case our bounds are tight or nearly so. Global bounds complement local- optimization-based designs, illuminating performance levels that may be possible as well as those that cannot be surpassed

Prediction of Spin Polarized Fermi Arcs in Quasiparticle Interference of CeBi

We predict that CeBi in the ferromagnetic state is a Weyl semimetal. Our calculations within density functional theory show the existence of two pairs of Weyl nodes on the momentum path $(0, 0, k_z)$ at $15$ meV} above and $100$ meV below the Fermi level. Two corresponding Fermi arcs are obtained on surfaces of mirror-symmetric (010)-oriented slabs at $E=15$ meV and both arcs are interrupted into three segments due to hybridization with a set of trivial surface bands. By studying the spin texture of surface states, we find the two Fermi arcs are strongly spin-polarized but in opposite directions, which can be detected by spin-polarized ARPES measurements. Our theoretical study of quasiparticle interference (QPI) for a nonmagnetic impurity at the Bi site also reveals several features related to the Fermi arcs. Specifically, we predict that the spin polarization of the Fermi arcs leads to a bifurcation-shaped feature only in the spin-dependent QPI spectrum, serving as a fingerprint of the Weyl nodes.Comment: 9 pages with 9 embedded figures. Supplemental material shortene

Coherent diffraction imaging for enhanced fault and fracture network characterization

Faults and fractures represent unique features of the solid Earth and are especially pervasive in the shallow crust. Aside from directly relating to crustal dynamics and the systematic assessment of associated risk, fault and fracture networks enable the efficient migration of fluids and therefore have a direct impact on concrete topics relevant to society, including climate-change-mitigating measures like CO2 sequestration or geothermal exploration and production. Due to their small-scale complexity, fault zones and fracture networks are typically poorly resolved, and their presence can often only be inferred indirectly in seismic and ground-penetrating radar (GPR) subsurface reconstructions. We suggest a largely data-driven framework for the direct imaging of these features by making use of the faint and still often underexplored diffracted portion of the wave field. Finding inspiration in the fields of optics and visual perception, we introduce two different conceptual pathways for coherent diffraction imaging and discuss respective advantages and disadvantages in different contexts of application. At the heart of both of these strategies lies the assessment of data coherence, for which a range of quantitative measures is introduced. To illustrate the versatility and effectiveness of the approach for high-resolution geophysical imaging, several seismic and GPR field data examples are presented, in which the diffracted wave field sheds new light on crustal features like fluvial channels, erosional surfaces, and intricate fault and fracture networks on land and in the marine environment

Amplitude and phase sonar calibration and the use of target phase for enhanced acoustic target characterisation

This thesis investigates the incorporation of target phase into sonar signal processing, for enhanced information in the context of acoustical oceanography. A sonar system phase calibration method, which includes both the amplitude and phase response is proposed. The technique is an extension of the widespread standard-target sonar calibration method, based on the use of metallic spheres as standard targets. Frequency domain data processing is used, with target phase measured as a phase angle difference between two frequency components. This approach minimizes the impact of range uncertainties in the calibration process. Calibration accuracy is examined by comparison to theoretical full-wave modal solutions. The system complex response is obtained for an operating frequency of 50 to 150 kHz, and sources of ambiguity are examined. The calibrated broadband sonar system is then used to study the complex scattering of objects important for the modelling of marine organism echoes, such as elastic spheres, fluid-filled shells, cylinders and prolate spheroids. Underlying echo formation mechanisms and their interaction are explored. Phase-sensitive sonar systems could be important for the acquisition of increased levels of information, crucial for the development of automated species identification. Studies of sonar system phase calibration and complex scattering from fundamental shapes are necessary in order to incorporate this type of fully-coherent processing into scientific acoustic instruments

Phase-driven optomechanics in exotic photonic media

Integrated photonics provide unique advantages in tailoring and enhancing optical forces. Recent advancements in integrated photonics have introduced many novel phenomena and exotic photonic media, such as photonic topological insulator, negative index material, photonic crystals, 2D material, and strongly-modulated time-dynamic systems. In my dissertation, I theoretically and numerically explore the novel properties and applications of optical forces in these systems. We propose guided-wave photonic pulling forces in photonic crystal waveguides. Photonic crystal waveguides offer great capability to define the mode properties, and can incorporate complex trajectories, leading to unprecedented flexibility and robustness compared to previous works in free space or in longitudinally uniform waveguides. With response theory, a virtual work approach, we establish general rules to tailor optical forces in periodic structures involved with photonic crystals: pulling forces arise from negative gradients in the phase responses of the outgoing modes, which corresponds to forward scattering on the Bloch band diagram with unit cell function corrections. We devise robust forward scattering, first, using topologically protected nonreciprocal chiral edge states, second, using backward (i.e. negative index) waves in a reciprocal system. The structures are tailored to accommodate only the necessary modes, which largely benefits the robustness. With these, we numerically demonstrate long range pulling forces on arbitrary particles through sharp corners. Our work paves the way towards sophisticated optical manipulation with single laser beam. We next explore the implication and applicability of momentum conservation in periodic media, which has been unclear due to the inhomogeneity and strong near field. We first quantify the linear momentum flux of Bloch modes under discrete translational symmetry, which is further understood from their plane wave composition. We then demonstrate through varies examples that the change in momentum flux predicts a total force distributed to both the scatterer and the media. However, one still need response theory to predict the forces on individual objects. Using response theory, we can predict more general forms of optical forces. We numerically demonstrate optical motoring effect due to singularity in the phase responses, and strong optical forces between graphene sheets due to large gradients in the phase responses. In particular, by combining the strong forces in graphene guided-wave system and the exceptional elastic properties of graphene, we can get an SBS gain that is four orders of magnitude stronger than in a silicon step-index waveguide, which may lead to smaller devices for RF signal processing.Physic

Synthetic aperture radar/LANDSAT MSS image registration

Algorithms and procedures necessary to merge aircraft synthetic aperture radar (SAR) and LANDSAT multispectral scanner (MSS) imagery were determined. The design of a SAR/LANDSAT data merging system was developed. Aircraft SAR images were registered to the corresponding LANDSAT MSS scenes and were the subject of experimental investigations. Results indicate that the registration of SAR imagery with LANDSAT MSS imagery is feasible from a technical viewpoint, and useful from an information-content viewpoint

Ultrasonic scattering from volumetric flaws in structural materials and their characterisation

This thesis is a theoretical and experimental study of ultrasonic scattering from volumetric flaws in structural materials and ultrasonic inversion techniques for nondestructive characterisation of such flaws. For forward scattering problems, the Method Of Optimal Truncation (MOOT) is studied. A large general purposed computer model is developed based on MOOT. The computer model can be used to simulate ultrasonic scattering from different shapes and sizes of voids, with only minor changes. Numerical results for a number of voids are presented in both the frequency and time domains to provide understanding of basic physical mechanism of scattering by volumetric flaws. The simulated forward scattering data are also used to test a new inversion technique developed in this study. A new ultrasonic inversion technique is developed for determining the geometrical features of a volumetric flaw in structural materials, by the inversion of the backscattered ultrasonic signal using the area function formula. The area function formula is derived from a weak scattering approximation, the Born approximation, but it is shown that the area function sizing technique works well for voids which are clearly strong scatterers. The technique extracts the flaw size from the shape of the area function which is evaluated from the backscattering signal. Unlike most of other ultrasonic inversion schemes, this technique has the advantage that it does not require the determination of the flaw centroid (zero-of-time problem). The technique is tested by the inversion of the numerical and experimental scattering data for estimating the sizes of a number of flaws. The results show very good agreement between the true sizes and the estimated sizes. The experimental work is carried out on simulated defects in the immersion and contact modes. Several techniques for processing experimental signals are investigated, including deconvolution techniques