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Journal of Applied Physics

Frequency non-degenerate sequential excitation of the impurity trapped exciton in strontium fluoride crystals doped with ytterbium

Pubudu S. Senanayake, Jon‐Paul R. Wells, Michael F. Reid, Rosa B. Hughes-Currie, Giel Berden, Roger J. Reeves, Andries Meijerink

Published Apr 7, 2015• 5 citations
Publisher

Abstract

We model the dynamic behaviour observed for impurity-trapped excitons in SrF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF2. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states.

DOI
10.1063/1.4916375

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We model the dynamic behaviour observed for impurity-trapped excitons in SrF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF2. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states.
Impurity trapped excitons (ITEs) occurring in divalent ytterbium doped calcium and strontium fluoride crystals have been investigated by exploiting the radically different radiative decay rates of the lowest exciton state and higher excited states, utilizing a novel two-colour transient photoluminescence enhancement experiment. The ITE energy levels have been directly measured with the observation of sharp transitions occurring from the changes of states of the localized hole and broad bands associated with changes of state of the delocalized electrons. The dynamic behaviour under excitation by time delayed ultra-violet (UV) and infrared (IR) pulses has been observed allowing for the identification of excitation and decay pathways between the ITE states. The position and transition intensities of the sharp lines within the IR excitation spectrum have been successfully matched using a semi-empirical effective Hamiltonian crystal field model. In CaCaF₂:Yb²⁺ the lines occurring at 249 and 1145 cm⁻¹ were matched with the crystal field parameter B4 = 800 cm⁻¹ and the exchange parameter G3(fs) = 5900 cm⁻¹. In SrCaF₂:Yb²⁺ these lines were observed at 178 and 1284 cm⁻¹ and matched with B4 = 600 cm⁻¹ and G3(fs) = 7278 cm⁻¹. Local heating and direct absorption by intra-excitonic transitions are found to be the causes of the broad band observed in the spectrum and have been deconvolved by studying the dynamic behaviour of the monitored emission at different IR excitation frequencies. Through this modeling, higher lying ITE states have been identified occurring at 785 cm⁻¹ in SrCaF₂:Yb²⁺ and in between 740 - 820 cm⁻¹ in CaCaF₂:Yb²⁺. The dynamic model developed successfully simulates the temporal behaviour of the emission under IR excitation under a variety of parameters including IR fluence, excitation frequency, sample temperature and UV - IR pulse delay. Examination of the SrCaF₂:Yb²⁺ dynamic behaviour over a time scale of 100 ms shows UV driven trap population at a rate of approximately 3% per pulse, which are liberated and recycled to the Yb²⁺ ground state by the IR pulse. The two-colour technique is applied to MgCaF₂:Yb²⁺, a candidate for possible ITE emission. Temperature dependent emission spectra obtained under UV excitation indicates the possibility of an ITE state, independently populated from the 5d level of the Yb²⁺. Typical 5d emission is also observed from this system. Under IR excitation, liberation of shallow traps and possible local heating is observed. No ITE emission is conclusively found with IR probing.
We demonstrate a direct measurement of the energy levels of impurity-trapped excitons in CaF2{}_{2}2​:Yb2+{}^{2+}2+. The radically different radiative decay rates of the lowest exciton state and higher excited states enable the generation of a transient photoluminescence enhancement measured via a two-step excitation process. We observe sharp transitions arising from changes of state of localized electrons, broad bands associated with changes of state of delocalized electrons, and broad bands arising from trap liberation.
The photoluminescence spectra of Yb-implanted InP samples were studied under pulsed and cw excitations using an Ar+-ion laser (above band-gap excitation) at different temperatures and excitation intensities. The photoluminescence spectrum of the 4f transitions 2F5/2–2F7/2 consists of a sharp peak at 1001 nm and broader peaks in the spectral range between 1002.5 and 1010 nm. Time-resolved spectra were recorded at different temperatures. Using a tunable Ti:sapphire laser (above and below band-gap excitation) the photoluminescence excitation spectra were investigated for different emission lines. Study of the rise and decay time under intrinsic excitation (band to band) show that the rise time is a function of excitation intensity, which reflects an indirect excitation process for Yb3+. The experimental data are explained using a kinetics model of energy transfer from the host lattice to the localized core excited states of rare-earth isoelectronic structured traps. The numerically simulated luminescence rise and decay times show a good general quantitative agreement with experimental data, over a wide range of generation rates. The photoluminescence spectra and decay time were also studied as a function of temperature. A quenching mechanism of ytterbium luminescence involving Yb and Fe ions is proposed. Finally, the electric-field quenching of InP:Yb photoluminescence is investigated.
A brief review is given of lifetime and line broadening studies with the free electron laser at FOM-Rijnhuizen (FELIX), emphasising work on far infrared (FIR) modulated photoluminescence (PL) of bound-to-bound transitions in InAs/GaAs quantum dots and mid-infrared (MIR) four wave mixing experiments on localised modes of H/sup -/ ions in calcium fluoride crystals. We have made new far/near infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The FIR resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots, and analysis of the FIR double resonance linewidth enables us to show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation. In addition we have made time resolved studies of local modes in ionic crystals, where nonradiative decay plays an important role in the optical pumping cycle of laser gain media. We have made pump-probe studies of the local modes created upon the introduction of a light impurity, in particular the H/sup -/ ion, in CaF/sub 2/ in the spectral region 700 to 1200 cm/sup -1/. We have also measured the free induction decay of this mode using a noncollinear degenerate four wave mixing geometry. The observed coherent transient is striking in having large quantum beat modulations at negative time which are associated with vibrational ladder climbing.
This work presents new results of experimental studies of exciton relaxation in GaAs/AlGaAs multiple quantum well materials, using transient nonlinear laser spectroscopy techniques. In the low excitation regime, effective optical Bloch equations similar to the density matrix formalism of a two-level system describe the response. Solutions of these equations appropriate for three-pulse scattering studies are presented, along with a discussion of new experimental techniques used in this work. Non-ideal conditions present in the molecular beam epitaxy growth process introduce monolayer fluctuations in the quantum well thickness, leading to inhomogeneous broadening and exciton localization at sites of minimum energy. Localized excitons are separated in energy from extended, quasi-delocalized states by the mobility edge. Studies of localized exciton relaxation indicate that these excitons relax on a much slower timescale than delocalized excitons. The temperature dependence of the localized exciton relaxation rate shows that above 15K, the excitons are thermally activated to extended states above the mobility edge. Below 15K the excitons relax by phonon assisted migration between localization sites. For temperatures above 5K, and for exciton energies greater than 1 meV below line center, an extra dephasing mechanism characterizes the exciton relaxation. The inhomogeneous broadening of the exciton leads to the emission of stimulated photon echoes in the experimental geometry used in this work. The time-resolved emission demonstrates the stimulated photon echo nature at low excitation densities. At higher excitation densities, the decay dynamics is more complex. The time-resolved emission shows the presence of a stimulated photon echo and a free polarization decay. This unusual result suggests that both a homogeneous and an inhomogeneous exciton distribution comprise the exciton resonance. Exciton relaxation at low excitation densities represents the interaction of the exciton with the lattice. At higher excitation densities, exciton-exciton and exciton-free carrier interactions modify the relaxation rates. The dependence of the exciton relaxation rate on excitation density shows that many-body effects play an important role in the exciton physics. Free carrier interactions are more efficient at increasing the exciton relaxation rate; both types of interactions are more important in quantum well systems than in bulk materials.