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

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.