One- and two-exciton states of pair centers of Kramers Nd3+ions in Nd-doped CaF2 and SrF2 crystals, and their possible use as qubits

We report on a comparative study of the spectroscopic and thermo-mechanical properties and of the laser slope efficiencies and laser tuning ranges of three Yb3+ doped CaF2, SrF2 and BaF2 single crystals grown in the same conditions.

One- and two-exciton cooperative states in ion pairs in Nd3+ doped CdF2, CaF2, and SrF2 crystals are studied. Strong resonance electrical exchange interaction with spin-flip determines the luminescence excitation spectra of the one-exciton entangled state.

The ionic conductivity of pure as well as NaF- and YF3-doped CaF2 and SrF2 crystals was investigated between room temperature and 900°C. The absorption of the crystals was measured before and after X-ray irradiation. The results are in accordance with an anti-Frenkel disorder. While for CaF2 only the formation energy of the defects, their activation energy of the mobility, and their association energy for the bonding to an aliovalent foreign ion could be obtained, in the case of SrF2 it was also possible to determine the density of the anti-Frenkel defects, their jump frequencies and mobilities as well as the entropy changes associated with the activation processes. In addition, the oscillator strength for the 213 nm band of O− in SrF2 could be calculated. Die Ionenleitfähigkeit reiner und mit NaF bzw. YF3 dotierter CaF2- und SrF2-Kristalle wurde zwischen Zimmertemperatur und 900°C untersucht. Die Messung der Absorption der Kristalle erfolgte vor und nach Röntgenbestrahlung. Die Ergebnisse stehen mit einer Anti-Frenkel-Fehlordnung in Einklang. Während für CaF2 nur die Bildungsenergie der Defekte, deren Aktivierungsenergie der Beweglichkeit und ihre Assoziationsenergie für die Bindung an ein anderswertiges Fremdion bestimmt werden konnten, war es bei SrF2 zusätzlich möglich, die Dichte der Anti-Frenkel-Defekte, deren Sprungfrequenzen und Beweglichkeiten sowie die mit den Aktivierungsprozessen verbundenen Entropieänderungen anzugeben. Außerdem konnte die Oszillatorenstärke für die 213 nm-Bande des O− in SrF2 berechnet werden.

The absorption, luminescence, and excitation spectra of CaF2, SrF2, and BaF2 crystals with EuF3 or YbF3 impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT1) were observed at energies below the 4fn-4fn − 15d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT2) with energies 1.5–2 eV lower than those of the CT1 bands have been found in the spectra of CaF2 and SrF2 crystals with EuF3 or YbF3 impurities. The fine structure of the luminescence spectra of CaF2 crystals with EuF3 impurities has been investigated under excitation in the CT bands. Under excitation in the CT1 band, several Eu centers were observed in the following luminescence spectra: C4v, Oh, and R aggregates. Excitation in the CT2 bands revealed luminescence of only C4v defects.

Theoretical study of self-trapped hole diffusion in CaF2, SrF2, BaF2 crystals

High-power 2.8 μm lasing in a lightly-doped Er:CaF2 crystal

Tm3+ codoping for mid-infrared laser applications of Dy3+ doped CaF2 crystals

Analysis of site symmetries of Er3+ doped CaF2 and BaF2 crystals by high resolution photoluminescence spectroscopy

The absorption spectra of photochromic centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC+) centers are generated in La- and Y-doped CaF2 crystals and in a La-doped SrF2 crystal. It is revealed that, upon heating of the CaF2-LaF3 crystal, PC+ centers are transformed into photochromic color (PC) centers. In the SrF2-YF3 crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF2-YF3 crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC+ centers.

The polarised two-photon (IR) and one-photon (visible) luminescence excitation spectra of Nd3+ nanoclusters in CaF2 and SrF2 crystals are measured at 10 K using a :LiF colour centre laser tunable in spectral ranges 1090 — 1230 nm and 545 — 615 nm with an emission linewidth of ~0.02 — 0.03 cm-1, an average output power of ~55 mW, and a pulse repetition rate of 10 Hz. The two-photon excitation spectra at the 4I9/2 → 4G5/2 transition reveal the structure, which is absent upon one-photon excitation, which can be explained by different selection rules for some Stark — Stark transitions upon one- and two-photon absorption.

Absorption by colour centres (i. e. the Ψ‐centre comprising an anion vacancy with a trapped electron) is observed in pure alkaline earth fluoride crystals after subtractive coloration at temperatures of 300, 84, and 37 °K. Colorability increases with decreasing temperature and increasing cation radius of the crystal hosts. The coloration spectra of the CaF2 and SrF2 crystals are similar; they consist of a strong double band, called the Ψ‐band, a weak ultraviolet band, probably caused by hole imperfections, called Ω‐centres, and an increase in absorption at 200 nm (Ω‐centre: collective term for all hole imperfections occuring in alkine earth fluoride crystals [8]). The peak of the Ψ‐band occurs at 375 nm for CaF2 and at 434nm for SrF2. BaF2 crystals exhibit two different coloration spectra depending on whether they are coloured at room temperature or at lower temperatures. Both the coloration spectra of BaF2 are dissimilar to the type of spectrum observed for CaF2 and SrF2. On heating the CaF2 and SrF2 crystals, strong discoloration is only observed at relatively high temperatures (between 125 and 150 °K); on the other hand, coloration increased when it is performed at 37 instead of 84 °K. Saturation of the coloration of the BaF2 crystals takes place at 84 °K whereas their discoloration commences already between 85 and 100 °K. The discussion of the results considers special models and uses published results for additive coloration spectra, electron spin resonance and theoretical calculations.

We present the results of ab initio calculations of self-trapped excitons (STE) in CaF2 and SrF2 crystals performed with BHHLYP density functional in embedded cluster approach. As a motivation for this theoretical study, we also report some previously unpublished experimental results on STE luminescence. They concern new luminescence bands of STE approximately 1 eV higher then the main band. The new bands have much lower intensities in pure crystals, however they remain almost unaffected by impurity doping while the main band is quenched by impurities. As possible candidates for excitonic configuration responsible for these high-energy bands, four possible configurations of the off-center STE are considered and rejected. For the on-center STE, i.e., VK + e, stable configuration has been found. The calculated luminescence energy of on-center STE is 6.57 eV in CaF2 and 6.31 eV in SrF2 crystal. On the basis of these energies (much higher than energies of new excitonic luminescence bands) and low stability of on-center STE, it is also rejected as a candidate for new bands. Finally, the hypothesis of electron self-trapping is considered. We have found electron self trapping in CaF2 and SrF2 crystals to be energetically favorable, however the value of energy gain lies within the possible calculation error. It was also found that there is a barrier about 0.3 eV for electron self-trapping. Self-trapped electron is a center with trigonal symmetry and can be considered as perturbed F-center. Calculated isotropic hyperfine couplings of self-trapped electron are given. On the basis of these results, we tentatively suggest that self-trapped electrons are the precursors of yet unknown excitonic configuration responsible for the new luminescence bands.

CW lasing is obtained in Er3+(5%) : CaF2 and Er3+(5%) : SrF2 crystals near 2.75 μm with 0.4 and 2 W of output powers, respectively, upon transverse diode laser pumping into the upper 4I11/2 laser level of erbium ions at 980 nm. Continuous tuning of the laser wavelength between 2720 and 2760 nm is realised in the Er3+ : SrF2 crystal.

A Photosystem two (PS II) core preparation containing the chlorophyll a binding proteins CP 47, CP 43, D1 and D2, and the non-chlorophyll binding cytochrome-b559 and 33 kDA polypeptides, has been isolated from PS II-enriched membranes of peas using the non-ionic detergent heptylthioglucopyranoside and elevated ionic strengths. The primary radical pair state, P680(+)Pheo(-), was studied by time-resolved absorption and fluorescence spectroscopy, under conditions where quinone reduction and water-splitting activities were inhibited. Charge recombination of the primary radical pair in PS II cores was found to have lifetimes of 17.5 ns measured by fluorescence and 21 ns measured by transient decay kinetics under anaerobic conditions. Transient absorption spectroscopy demonstrated that the activity of the particles, based on primary radical pair formation, was in excess of 70% (depending on the choice of kinetic model), while time-resolved fluorescence spectroscopy indicated that the particles were 91% active. These estimates of activity were further supported by steady-state measurements which quantified the amount of photoreducible pheophytin. It is concluded that the PS II core preparation we have isolated is ideal for studying primary radical pair formation and recombination as demonstrated by the correlation of our absorption and fluorescence transient data, which is the first of its kind to be reported in the literature for isolated PS II core complexes from higher plants.

The thermal conductivity of single crystals of the Ca0.99Co0.01F2, Ca0.97Co0.03F2, Sr0.99Mn0.01F2, Sr0.989Mn0.01Co0.001F2, and Sr0.995Co0.005F2 solid solutions has been measured in the temperature range 50–300 K. The results demonstrate that doping of CaF2 and SrF2 crystals with small amounts of isovalent transition metal impurities is accompanied by a significant reduction in thermal conductivity.