FLORESCENCE AND unconversion IN CAF[2]:HO[3+]

This work is a report on laser excitation studies of CaF2:Ho3+ , The fluorescence and unconversion of the crystal were observed for the first time using a Quanta-Ray DCR-3 Nd:YAG laser at the second harmonic excitation wavelength, The spectra of CaF2 doped with Ho3+ concentration ranging from 0.01 % to 10 % were systematically recorded at liquid nitrogen temperatures. Lifetimes of certain new fluorescence lines were determined. From the lifetime data, the corresponding single-ion sites of the fluorescence lines were· assigned. A concentration dependence study of the fluorescence intensities of a manifold and an unconverted manifold was carried out, From the study, various energy transfer mechanisms responsible for the intensities of fluorescence were analysed. We begin in Chapter 1 with a brief description of the lanthanides. A review of past work on trivalent holmium was given. We then describe, in particular, the past and present spectroscopic work on CaF2: Ho3+. Chapter 2 ls a discussion of the theoretical background of rare-earth spectroscopy. The free-ion energy levels of Ho3+ and the effects of crystal field environment on the ion are described. An in depth discussion of the Judd-Ofelt Theory, which forms the backbone of the theory of rare-earth spectra, is given. The application of the theory to transition probabilities, line strengths and lifetimes of transitions is presented. A description of the various energy transfer processes, such as multiphonon relaxation, ion-pair relaxation, unconversion and two-photon absorption follows. The different crystallographic sites of Ho3+ in CaF2 as a result of charge compensation are also given. In Chapter 3, the experimental procedure is presented. A description of the experimental set-up for fluorescence detection and lifetime measurement is given. The crystal samples are described. The final part of the chapter is a discussion and estimation of the experimental errors encountered. The experimental results are presented in Chapter 4. The wavelengths of the absorption, fluorescence and unconversion spectra were tabulated. Figures of the various spectra are also given. The plots of the relative fluorescence intensities against concentration are shown. In Chapter 5, a discussion of the results is given. The effects of various energy transfer mechanisms on the fluorescence intensities are analysed. From the concentration dependence of the fluorescence intensities, the dominant energy transfer mechanism in CaF2:Ho3+ is identified. The conclusion is presented in Chapter 6.

Author dc:creator

• KEE SIEW CHENG