published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

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Thesis (Ph. D.)--University of Hong Kong, 2008. / Also available in print.

Hydrogen, being the simplest and most abundant element in the Universe, is of fundamental importance to condensed matter sciences. Through advances in high pressure experimental technique, hydrogen (and its isotope deuterium) has been contained and studied using in situ optical spectroscopy to 315 (275 GPa) at 300 K, pressure and temperature conditions previously thought to be inaccessible. At 200 GPa, hydrogen undergoes a phase transformation, attributed to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental mode, new low-frequency phonon excitations, and a dramatic softening and broadening of the first vibrational fundamental mode. To impose constraints on the P-T phase diagram, the temperature stability of phase IV is investigated through a series of low temperature experiments, where the phase IV-III transformation is observed. Analysis of the Raman spectra suggests that phase IV is a mixture of graphene-like layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules. Isotopic comparisons reveal spectral differences between the phase IV-III transition of hydrogen and deuterium, which strongly indicates the presence of proton tunnelling in phase IV. Optical transmission spectra of phase IV reveals an overall increase of absorption and a closing band gap reaching 1.8 eV at 315 GPa. No differences between the isotopes were observed in absorption studies, resulting in identical values for the band gap. Extrapolation of the band gap yields 375 GPa as the minimum transition pressure to a metallic state of hydrogen (deuterium).

The strong light-matter interaction in the atomically thin transition metal dichalcogenides (TMDCs) has allowed the use of optical spectroscopy to investigate these materials in great depth. It has been shown that optoelectronic properties of ultrathin TMDCs are remarkably different from their bulk counterparts. Among them, this dissertation focuses on ultrathin MoTe2 (molybdenum ditelluride) and ReS2 (rhenium disulfide). We first introduce the fundamental properties of the two material systems, MoTe2 and ReS2, investigated in this dissertation. Specific experimental methods for optical spectroscopy of 2D materials, 2D sample preparation, and related optics calculations are presented. Absorption and photoluminescence measurements are applied to demonstrate that semiconducting MoTe2, an indirect band gap bulk material, acquires a direct band gap in the monolayer limit. Furthermore, strain-tuned optical spectroscopy on MoTe2 shows that tensile strain can significantly redshift its optical gap and partially suppress the intervalley exciton-phonon scattering. This suppression results in a narrowing of the near-band excitonic transitions. We also discuss the effect of strain on the transport properties of MoTe2 due to this reduction in scattering. We investigate monolayer ReS2 as a TMDC system exhibiting strong in-plane anisotropy. These properties are explored by polarization-resolved spectroscopy. We show how the accessible optical properties vary with optical polarization. We find that the near-band excitons in ultrathin ReS2, absorb and emit light along specific polarizations. We also show that purely non-contact, optical techniques can determine the crystallographic orientation of ReS2.

Made available in DSpace on 2014-06-12T18:02:18Z (GMT). No. of bitstreams: 2 arquivo5666_1.pdf: 7107729 bytes, checksum: 331daa72875ae82bd7eecdcd35436b14 (MD5) license.txt: 1748 bytes, checksum: 8a4605be74aa9ea9d79846c1fba20a33 (MD5) Previous issue date: 2011 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Tomografia por coerência óptica (OCT) é uma técnica de imageamento não invasiva que usa radiação infravermelho para sondar alguns milímetros the profundidade de um alvo com um resolução de poucos micrômetros. Aqui, nós expomos a base teórica para entender a técnica. O texto cobre as duas variedades de OCT domínio temporal e domínio da frequência e descreve três aplicações da técnica em odontologia: a) Um na avalição the propagação rachaduras em polímeros reforçado com fibra usado em restauração dental; b) O imageamento da sobra de dentina e cavidade pulpar após excavação da dentina, com o propósito de medir a espessura da dentina, e c) uma avaliação clínica da integridade de restaurações dentais. Em todas essa aplicações, OCT gerou imagens marcantes e forneceu informações semiquatitativas sobre a estrura dentária. Com o objetivo de desenvolver um sistema de tomografia óptica integrada em um chip. Nós expomos a base teórica da plataforma de fotônica integrada. Após uma revisão literária, nós descobrimos que não existe espectrômetro integrado com a especificações necessárias para uso em OCT. Nós, então, desenvolvemos um espectrômetro com a características necessárias. Isso foi possível devido a uma nova arquitetura de espectrômetro baseada na combinação de um ressoador em anel e um espectrômetro de grade de difração

This thesis describes the computational tools that allow the simulation of polymers made up of an arbitrary number of three-level systems, the study of such systems and comparisons to experimental nonlinear optical spectra. The three-level system generator (3LSG), is designed to automatically generate the operators that describe the system, whether it is a closed system or an open quantum system (OQS) in contact with a bath, with just a few input parameters. A user is free to specify each 3LS's energy levels and transition dipoles between said levels, site couplings between the different units of the polymer and in the case of open systems, the rates and couplings describing the different relaxation processes taking place in an OQS, using the Redfield formalism. In either cases, the 3LSG is then capable of generating the Hamiltonian 𝐻₀ describing the closed system or the Liouvillian 𝓛₀ describing the open system from the various inputs. The Ultrafast Spectroscopy Suite (UFSS) is an open-source software suite used to perform the nonlinear optical spectroscopies simulations. It contains 4 main modules, one of which is the Hamiltonian/Liouvillian Generator (HLG), a module previously designed to model simpler two-level systems. The 3LSG is a sub-module of the HLG. The three-level system generator is used to replicate a theoretical model describing a copolymer model made of many identical pairs of squaraine monomers, where each monomer is a three-level system interacting with its neighbouring sites and a surrounding bath. The system automatically generated by the 3LSG is used, along with other spectroscopic calculation tools, to simulate high-order transient absorption (TA) spectroscopies and study the long-time behaviour of the 3rd-order to 13th-order excited state absorption (ESA) peaks in the TA signals. The 3LSG is used in conjunction with spectroscopic calculations tools as it was originally intented, though it may also be used by itself to study Hamiltonians and Liouvillians of electronic three-level systems.

Fluorescence spectra and UV-Vis absorption spectra are collected on cerium oxide nanocrystalline particles. While CeO2 is the stable form of bulk cerium oxide, ceria nanoparticles exhibit a nonstoichiometric composition, CeO2-γ, due to the presence of oxygen vacancies and the formation of Ce2O3 at the grain boundaries. The Ce(III) ions, which are more reactive and therefore more desirable for various applications, are created by oxygen vacancies, which act as defects in the CeO2-γ crystal lattice. These defects form trap states in the band gap of CeO2, which can be seen in the absorption spectra. Ce(III) is required for fluorescence of the ceria nanoparticles while Ce(IV) is involved in only nonradiative transitions. The optical spectroscopy results show that the ceria samples have different ratios of Ce(III) ions to Ce(IV) ions, which is verified by x-ray photoemission spectroscopy (XPS). / Master of Science

Given their unique structural properties, quantum rings (QRs) structures have recently been of particular interest for investigating quantum interference which is called the neutral charged exciton optical Aharonov-Bohm (AB) effect. A delocalized wavefunction around the rim is a prerequisite for the AB effect, but asymmetry and anisotropy seem to have been overlooked in the spectroscopy of QRs. In this thesis, the presence of a localized state in a single GaAs QR is presented.