Henry Elder - January 17, 2024
Dissertation Title: Nonlinear Propagation of Orbital Angular Momentum Light in Turbulence and Fiber
Date and Time: Wednesday, January 17, 3:00 PM
Location: AVW 2460
Dissertation Committee Chair: Phillip Sprangle
Wendel Hill (Dean’s Rep)
Light that carries orbital angular momentum (OAM), also referred to as optical vortices or twisted light, is characterized by a helical or twisted wavefront ∝exp[imφ]. In contrast to spin angular momentum (SAM), where photons are limited to two states, OAM allows for, in principle, an infinite set of spatially orthogonal states. OAM-carrying light has found applications ranging from quantum key distribution in free space and guided-wave communication systems, particle trapping and optical tweezers, nanoscopy, and remote sensing. Understanding how OAM light propagates through complex environments, and how to efficiently generate particular OAM states, is critical for any such application.
In the first part of this dissertation, we describe how OAM light propagates through a turbulent atmosphere. We build analytic models which describe (1) the OAM mode mixing caused by turbulence, (2) the evolution of short, high-power OAM pulses undergoing the effects of self-phase modulation (SPM) and group velocity dispersion (GVD), and (3) the evolution of high-power Gaussian pulses including SPM, GVD, and turbulence. The models are compared to experimental data and nonlinear, turbulent pulse propagation simulation programs, which we have made freely available. We also explore how self-focusing can minimize certain deleterious effects of turbulence for OAM light.
The second part of this dissertation considers nonlinear effects of OAM light propagating in azimuthally symmetric waveguides. Such waveguides have so-called spin-orbit (SO) modes, which are quantized based on their total angular momentum (TAM). We develop a generalized theory of four wave mixing-based parametric amplification of SO modes and show that these processes conserve TAM, but under certain circumstances can be taken to conserve SAM and OAM independently. Our theory is validated against a nonlinear multimode beam propagation simulation program which we developed and, again, have made freely available.
Huan-Kuang Wu - January 8, 2024
Dissertation Title: Aspects of Unconventional Transport and Quasiparticle in Condensed Matter Systems
Date and Time: Monday, January 8, 1:00pm
Location: ATL 4402
Dissertation Committee Chair: Jay Sau
The Boltzmann transport equation (BTE) is a successful framework for describing transport in condensed matter systems. The assumption of BTE is the localized energy excitations, or quasiparticles, and it is applicable to length scales above the mean free path of the quasiparticles. In this dissertation defense, I will cover two example systems where the assumptions of BTE fail. One is a new proposed system for Majorana zero mode whose topological phase is controlled by the phases of three s-wave superconductors. When the time reversal symmetry is restored, it can be tuned to a class DIII topological superconductor, which exhibits helical Majorana edge states. The second system I will talk about is the Josephson junction chain in the insulating regime, whose high-frequency plasmon remains coherent while the low energy excitations are Anderson-localized due to charge disorder.
Sara Nabili - January 4, 2024
Dissertation Title: Search for boosted semi-visible jets in all hadronic final states with the CMS experiment at CERN
Date and Time: Thursday, January 4, 10:30 am
Location: PSC 3150
Dissertation Committee Chair: Sarah Eno
This dissertation represents the search for the dark sector beyond the standard model using the Compact Muon Solenoid experiment simulated Monte Carlo data of the Large Hadron Collider at CERN. The search is focused on the strongly coupled Hidden Valley models that couple with the standard model via a leptophobic (fully hadronic) Z′ mediator. The final state of resonant production consists of one large jet composed of both visible and invisible particles. This search focuses on the lower mediator mass range (mZ′ ≤ 550 GeV) with the boosted topology that recoils against the initial state radiation (ISR) jet such that its decay products are contained within a single large-diameter “semi-visible” jet. The main parameters of our model are the mediator mass, the mass of the dark mesons, and the fraction of invisible stable particles. In the event of no discovery, the exclusion limits for mediator mass of 275 to 550 GeV are expected.