Problems in Scattering and Imaging

Abstract

Technology advances are always driven by the discovery of new materials, better understanding of their properties and improvements in processing power. This trend is reflected in this work, where I will demonstrate how new science and applications of both scattering and imaging are enabled by these frontiers.

This thesis explores a broad spectrum of topics associated with the problems of scattering and imaging. The first topic concerns the fundamental study of the symmetry breaking and the nonlinear light scattering in the system of gold nanorod. In the most recent experiments, the intrinsic electrostatic asymmetry of gold nanorods was investigated by Ji-Young et al. using a variety of microscopy techniques, and the associated optical asymmetry was immediately demonstrated through the nonlinear optical experiments. The understanding of the symmetry breaking of gold nanorods, motivated the development of a model where the second order longitudinal plasmon resonance mode scatters with the electron gas and accounting for the plasmon damping effect. The new microscopic description self-consistently explains all the main features of the nonlinear optical components, and provides a fresh look that beautifully aligns with the recent observations of the nonlinear optical properties of nanorods.

Next, we demonstrate an optical system that enables the control of monochromatic light transmission through highly scattering media, with Complex Semi-Definite Programming (SDP) introduced as a novel approach to solve the associated phase retrieval problem. In contrast to the conventional approach that employed an interferometric design which is vulnerable to system vibration, a simple optical setup without the need for a reference beam is proposed by Moussa et al. The SDP algorithm allows computation of the complex transmission matrix of the system from a sequence of intensity speckle patterns generated with phase-modulated wavefronts. We showed that once the transmission matrix is determined, optimal wavefronts can be computed that focuses the incident beam to any position on the far side of the scattering medium, without the need for subsequent measurements or wavefront shaping iterations.

Finally, the optical properties and applications of graphene were explored. As a true 2D material, graphene has a unique electronic band structure and has been demonstrated by various research groups to be an interesting photonic building block. At first, we focused on the absorption saturation in optically excited graphene. The microscopic theory that includes Coulomb-scattering as the dominant relaxation mechanism at high carrier densities was developed and then verified by the optical transmission experiment. Then, we showed a novel scheme of a light field camera using a focal stack proposed by a team at the University of Michigan. The key enabling technology is the highly transparent graphene photodetector fabricated by Che-Hung et al., where graphene is used both as the photoconductive gain material and the circuit interconnects. Physically, we built the prototype single-pixel light field camera and demonstrated its operation through optical experiment. Computationally, a synthetic camera system was designed based on the Fourier slice analysis and the framework for the model-based light field reconstruction was provided.