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Journal of Applied Physics

Photoionization thresholds of rare-earth impurity ions. Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2

Christian Pedrini, F. Rogemond, D. S. McClure

Published Feb 15, 1986• 144 citations
Publisher

Abstract

The spectral dependence of the photoionization energy of Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2 systems have been measured and thresholds experimentally determined and compared with theoretical values calculated from electrostatic models. It is shown that the excited state absorption transitions or the persistent hole burning observed by other authors occur above the threshold energy of photoionization of the impurities and that the states of the crystal which form the bottom of the conduction band may play an important role in the strong probability of these processes. A review of thresholds now known is also given.

DOI
10.1063/1.336557

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The spectral dependence of the photoionization energy of Eu2+:CaF2, Ce3+:YAG, and Sm2+:CaF2 systems have been measured and thresholds experimentally determined and compared with theoretical values calculated from electrostatic models. It is shown that the excited state absorption transitions or the persistent hole burning observed by other authors occur above the threshold energy of photoionization of the impurities and that the states of the crystal which form the bottom of the conduction band may play an important role in the strong probability of these processes. A review of thresholds now known is also given.
It is demonstrated that persistent hole burning in the zero-phonon line of Eu2+ in SLs of CaF2:Eu/CdF2 with uniformly doped CaF2 layers is mainly due to tunneling of electrons from the 4f65d excited state of Eu2 to the conduction band of adjacent CdF2 layer. However in selectively doped suoerlattices (SLs), the two-step photoionization process dominates in the formation of spectral holes. Two mechanisms of spectral hole broadening were discussed. These are the Stark-shifting of the Eu2 f-d transition energy induced by internal electric field and phonon assisted transitions.
We report on investigations of the dependence of ionization spectral hole burning efficiencies on the location of impurity ion energy levels relative to both the host valence band and the conduction band. An ideal system is Cr-doped Li2Ge7O15,{\mathrm{Li}}_{2}{\mathrm{Ge}}_{7}{\mathrm{O}}_{15},Li2​Ge7​O15​, where the Cr3+{\mathrm{Cr}}^{3+}Cr3+ ions occupy three distinct noncentrosymmetric Ge4+{\mathrm{Ge}}^{4+}Ge4+ sites, which differ from one another by the location of their charge-compensating ions. Most important for our investigation is the large (\ensuremath{\sim}eV) and site-specific shift of the Cr3+{\mathrm{Cr}}^{3+}Cr3+ energy levels relative to the host valence and conduction bands. As a result, only one of the three sites shows persistent spectral hole burning. It is shown that this result can be understood from an analysis of the differences of the energy-level locations of the ground and excited states of the three Cr3+{\mathrm{Cr}}^{3+}Cr3+ centers relative to the host bands. Two of the centers have their energy levels sufficiently close to the host conduction band such that two-step ionization, and thus persistent spectral hole burning, can occur. However, for one of the centers the ground state lies so close to the valence band that an erasure process involving promotion of an electron from the valence band to the ionized center leads to the restoration of the initial charge state. This erasure process, clearly demonstrated in this paper, has to be considered when evaluating the hole burning potential of doped insulators, especially those with relatively narrow band gaps.
We have measured the pump-probe spectrum for CaF2:Sm2+ over a wide (360–1020 nm) spectral range. We observed gain throughout most of the 730-nm emission band, although there is some evidence of excited-state absorption (ESA) in this region. A much stronger ESA was observed near 500 nm. The temperature dependence of the ESA signal clearly indicates that the absorption is associated with divalent samarium. Because of the width and strength of this transition, we assign the ESA to a 4f55d → conduction band transition. We calculated the oscillator strength for this ESA band to be 0.10 and rationalized its magnitude based on an intensity-borrowing mechanism that involves the transition from the valence band to the conduction band.
Rare-earth ions play an important role in modern technology as optically active elements in solid-state luminescent materials. In many of these materials, interactions between the electronic band states of the host crystal and the rare-earth ion's localized 4f N and 4f N−1 5d states influence the material's optical properties. The importance of these interactions is discussed for material applications in photon-gated hole burning, quantum information and phosphors. Material dependent trends in the relative binding energies of the 4f N states and the host bands have been observed and are summarized. An empirical model for the ion dependence of the 4f electron binding energies is formulated in terms of atomic number and compared with previous models. These models are extended to describe the 4f N−1 5d states with one additional parameter. Improved estimates for the free-ion ionization potentials used in the model are also presented and discussed.
Summary form only given. Using persistent spectral hole burning, rare-earth ions doped in crystalline or glassy matrices may prove useful in optical data storage. In the case of Sm/sup 2+/, persistent spectral hole burning is due to photoionization: the Sm/sup 2+/ is oxidized to form Sm/sup 3+/, and the ejected electron is trapped in the matrix. For applications, it is important to understand the trapping mechanism, to identify the nature of the traps, and to see whether they are stable at room temperature.