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    Free-Electron Laser and Synchrotron Spectroscopy of Fundamental Excitations in Ytterbium-Doped Fluoride Lattices (2015)

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    Type of Content
    Theses / Dissertations
    UC Permalink
    http://hdl.handle.net/10092/10581
    http://dx.doi.org/10.26021/6331
    
    Thesis Discipline
    Physics
    Degree Name
    Doctor of Philosophy
    Publisher
    University of Canterbury. Physics & Astronomy
    Collections
    • Science: Theses and Dissertations [4657]
    Authors
    Hughes-Currie, Rosa
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    Abstract

    The spectroscopy of wide-bandgap fluoride materials doped with divalent ytterbium is presented. The structure of impurity-trapped excitons is explored, vacuum ultraviolet excitation is used to investigate the transfer processes between excitations, and the effect of confinement on self-trapped excitons is studied.

    The excited-state structure of impurity-trapped excitons is measured in the multisite system NaMgF₃:Yb²⁺. A two-colour ultraviolet-infrared pulsed photoluminescence enhancement technique is employed to probe the interlevel transitions and dynamics of impurity-trapped excitons in doped insulating phosphor materials. NaMgF₃:Yb²⁺ exhibits emission from two charge-compensation centres with peaks at 22 300 cm⁻¹ (448 nm) and 24 000 cm⁻¹ (417 nm). The observed photoluminescence enhancement is caused by a combination of intra-excitonic excitation and electron trap liberation. The electron traps are inferred to have a depth of approximately 800 cm⁻¹.

    Time-resolved VUV spectroscopic studies of emission and excitation spectra of CaF₂:Yb, NaMgF₃Yb and MgF₂:Yb are presented to investigate excitation and relaxation mechanisms of both impurity-trapped excitons and intrinsic excitons in each fluoride host. Host-to-impurity energy transfer mechanisms leading to formation of impurity-trapped excitons are discussed. The 4f¹⁴ → 4f¹³5d CaF₂:Yb²⁺ absorption bands are successfully modeled with a semi-empirical effective Hamiltonian calculation for NaMgF₃:Yb²⁺ and MgF₂:Yb²⁺. The excitation and emission spectra of all studied materials are compared.

    Results on VUV spectroscopy of 3 and 5 monolayer CdF₂–CaF₂ superlattices show the change in optical behaviour of the self-trapped exciton in CdF₂ when it is confined and give an indication of the radius of the exciton. The decay of the emission is modeled with three components, corresponding to three self-trapped exciton states. Results on the VUV spectroscopy of CdF₂–CaF₂ superlattices show that the confinement effect seems to equally influence the energy of excitonic and bandgap absorption in 3 and 5 monolayer superlattices. At the same time, as the self-trapped exciton is more confined, the emission is blue-shifted by 1600 cm⁻¹ indicating that the effective excitonic radius is about three monolayers.

    Keywords
    lanthanide spectroscopy; impurity-trapped excitons; self-trapped excitons; bulk fluoride crystals; ytterbium
    Rights
    Copyright Rosa Hughes-Currie
    https://canterbury.libguides.com/rights/theses

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