CaF2 Single Crystals Research Articles

Excited-state absorption and fluorescence dynamics in Er:CaF2

Emission, ground- and excited-state absorption spectra of Er:CaF2 single crystals were registered from the visible to the mid-infrared. These spectra were calibrated in unit of cross section and analyzed to derive purely radiative lifetimes and branching ratios. Fluorescence decays were then recorded to determine effective emission lifetimes and branching ratios including non-radiative multiphonon relaxations. These fluorescence data along with population ratios and emission intensity measurements performed versus pumping rates were finally confronted with a rate equation model to derive energy transfer rates.

Revisit of the anisotropic deformation behavior of single-crystal CaF2 in orthogonal cutting

A resolved stress model is presented to re-discuss the anisotropic deformation mechanism of single-crystal calcium fluoride in plunge-cut tests. The authors have investigated the machinability of CaF2 in ultra-precision cutting. However, the influence of crystal anisotropy on the brittle–ductile transition was discussed in a qualitative manner using optical microscopy and white light interferometer. Moreover, only the primary {100}<110> slip system and {111} cleavage were considered as deformation mechanisms. In this study, the ductility and brittleness of CaF2 is semi-quantitatively re-discussed on the basis of computation of the resolved stresses, by considering a secondary slip system and cleavage. The surface morphologies of the machined surface were characterized using field-emission scanning electron microscopy. The primary {100}<110> slip system is assumed to be dominant for the ductility. The brittle fracture initiated by the {111} cleavages was observed in a different manner in dependency rwith cutting directions.

Heterogeneous scintillator geometries to maximise energy deposition for waterborne beta particle detection

Here the geometries that maximise detection efficiency of heterogeneous scintillators used to detect beta particles in aqueous solutions by maximising energy deposition are described. The determination of the geometry was achieved with the Monte Carlo code Geant4 using CaF2:Eu scintillator as a pertinent case study, and validated with experimental data using single crystal CaF2:Eu and heterogeneous CaF2:Eu scintillators. Both 2D and 3D structures composed of arrays of primitive unit cells of packed spheres were examined to find the optimal geometry to maximise detection of volumetric sources of tritium and aqueous Carbon 14 and Lead 210. The 2D structures were evaluated relative to a single crystal scintillator and results show the detection efficiency of the 2D structures is maximised when the sphere radius is c.a. 0.46x the maximum track length of the beta particle in the scintillator. Data for the 3D structures show that the efficiency is maximised when the sphere radius is minimised, but it is further shown that practical issues limit the minimum radius that can be used for transient radiological contamination monitoring.

Highly efficient CW laser operation in 4 at. % Tm3+ and 4 at. % Y3+ codoped CaF2 crystals

Tm:CaF2 and Tm,Y:CaF2 single crystals were prepared by the temperature gradient technique. The spectral properties of Tm,Y:CaF2 single crystals were investigated and compared with those of Tm:CaF2. It was demonstrated that codoping with Y3+ ions could efficiently improve the spectroscopic properties. Tm,Y:CaF2 crystals have larger absorption cross-sections at the pumping wavelength, larger mid-infrared stimulated emission cross-sections, and much longer fluorescence lifetimes of the upper laser level (Tm3+: H43 level) than Tm:CaF2 crystals. Continuous-wave (CW) lasers around 1.97 μm were demonstrated in 4.0 at. % Tm,4.0 at. % Y:CaF2 single crystals under 792 nm laser diode (LD) pumping. The best laser performance has been demonstrated with a low threshold of 0.368 W, a high slope efficiency of 54.8%, and a maximum output power of 1.013 W.

Deposition and properties of Fe(Se,Te) thin films on vicinal CaF2 substrates

We report on the growth of epitaxial Fe1+δSe0.5Te0.5 thin films on 0°, 5°, 10°, 15° and 20° vicinal cut CaF2 single crystals by pulsed laser deposition. In situ electron and ex situ x-ray diffraction studies reveal a tilted growth of the Fe1+δSe0.5Te0.5 films, whereby under optimized deposition conditions the c-axis alignment coincides with the substrate [001] tilted axis up to a vicinal angle of 10°. Atomic force microscopy shows a flat island growth for all films. From resistivity measurements in longitudinal and transversal directions, the ab- and c-axis components of resistivity are derived and the mass anisotropy parameter is determined. Analysis of the critical current density indicates that no effective c-axis correlated defects are generated by vicinal growth, and pinning by normal point core defects dominates. However, for H∣∣ab the effective pinning centers change from surface defects to point core defects near the superconducting transition due to the vicinal cut. Furthermore, we show in angular-dependent critical current density data a shift of the ab-planes maxima position with the magnetic field strength.

Refractive indices of CaF2 single crystals under elastic shock loading

Refractive indices and Hugoniots of CaF2 single crystals are investigated by laser displacement interferometry under shock loading below 5 GPa. Birefringence is observed for the [110] loading. We obtain the Hugoniot equation of states for [100], [110] and [111], and refractive indices for these orientations with consideration of their polarization. The measured refractive indices are in reasonable agreement with predictions based on the piezo-optic theory, and are used to refine the elasto-optic coefficients.

Thermal conductivity measurements of Yb:CaF2 transparent ceramics using the 3ω method

This paper reports on the measurement of the thermal conductivity of ytterbium doped CaF2 transparent ceramics using the 3ω method. Samples with different doping levels were studied in a temperature range of 10 K to 310 K. The results for doped samples are very similar to those obtained for Yb:CaF2 single crystals with the same doping levels, which demonstrates an important improvement in the thermal properties of transparent ceramics. However, the results obtained for non-doped ceramics and single crystals differ for the maximum conductivity due to the presence of grain boundaries inside the ceramic. Theoretical calculations were also conducted using Callaway's model and are in a very good agreement with experimental results. The fitting parameters found tend to indicate that the introduction of a doping element in the lattice has more impact on thermal conductivity than the presence of grain boundaries in the material.

Energy transfer study between Ce3+ and Tb3+ ions in a calcium fluoride crystal for solar cell applications

Energy transfer from Ce3+ to Tb3+ ions in a single crystal of CaF2 has been analyzed for down-conversion application in Si solar cells. The nanophosphor samples were successfully synthesized by the co-precipitation method. Results obtained with x-ray diffraction indicated that the crystallite size were in the range of 30–40nm. X-ray photoelectron spectroscopy confirmed the formation of the matrix and the presence of the dopants. The luminescence data obtained clearly indicated that energy transfer between Ce3+ and Tb3+ took place inside the crystalline matrix. The calculated energy transfer efficiency was relatively efficient at the high Tb3+ concentration. The results suggested that Ce3+ may therefore be used as an efficient sensitizer to feed the Tb3+ ions in a CaF2 crystal. UV light may be down shifted to the visible region for solar cell applications.

Performance and wavelength tuning of green emitting terbium lasers.

Wavelength tuning experiments in the green spectral range applying Tb3+-doped LiLuF4 and Tb3+/Na+-codoped CaF2 single crystals are presented. While in the former case almost 100 mW could be obtained throughout a total range of 7 nm, the inhomogeneously broadened emission features of Tb3+ in the CaF2-matrix allowed for a continuous tuning range exceeding 10 nm. For initial characterization, free running continuous wave laser experiments using Tb3+-doped LiLuF4, Na+:CaF2, and stoichiometric TbF3 crystals were carried out resulting in efficient continuous wave laser operation in the green spectral region. The density of active centers in TbF3 is among the highest that can be obtained in solid state gain materials. Nevertheless, a slope efficiency as high as 32% was obtained from this crystal. High Tb3+-densities are essential to compensate for the adherent low absorption cross sections at useful pump-wavelengths.

Spectroscopic properties of long-lifetime Tm3+ optical centers in Ca–Sr–Ba fluorides in the form of single crystals and ceramics at the 1G4–3H5 magnetic dipole allowed transition

The spectroscopic properties of new long-lifetime Tm3+ tetragonal optical centers at low (77 K) temperature were investigated using a site-selective time-resolved technique. The absorption and excitation spectra at the 3H6–1G4 transition and the fluorescence spectra at the 1G4–3H5 transition were measured in CaF2, SrF2, and BaF2 single crystals. The appearance of additional weak lines in the excitation and fluorescence spectra in hot-formed ceramics produced from the same crystals was observed. These lines were attributed to the recently observed long-lifetime tetragonal optical centers with a modified local environment formed in fluoride ceramics.

Electronic sputtering of LiF, CaF2, LaF3 and UF4 with 197 MeV Au ions. Is the stoichiometry of atom emission preserved?

Sputtering experiments with swift heavy ions in the electronic energy loss regime were performed by using the catcher technique in combination with elastic recoil detection analysis. Four different fluoride targets, LiF, CaF2, LaF3 and UF4 were irradiated in the electronic energy loss regime using 197MeV Au ions. The angular distribution of particles sputtered from the surface of freshly cleaved LiF and CaF2 single crystals is composed of a broad cosine distribution superimposed by a jet-like peak that appears perpendicular to the surface independent of the angle of beam incidence. For LiF, the particle emission in the entire angular distribution (jet plus broad cosine component) is stoichiometric, whereas for CaF2 the ratio of the sputtered F to Ca particles is at large angles by a factor of two smaller than the stoichiometry of the crystal. For single crystalline LaF3 no jet component is observed and the angular distribution is non-stoichiometric with the number of sputtered F particles being slightly larger than the number of sputtered La particles. In the case of UF4, the target was polycrystalline and had a much rougher surface compared to cleaved crystals. This destroys the appearance of a possible jet component leading to a broad angular distribution. The ratio of sputtered U atoms compared to F atoms is in the order of 1–2, i.e. the number of collected particles on the catcher is also non-stoichiometric. Such unlike behavior of particles sputtered from different fluoride crystals creates new questions.

Improved 2.79 μm continuous-wave laser performance from a diode-end pumped Er,Pr:CaF2 crystal

The spectral properties of lightly doped Er:CaF2 and Er,Pr:CaF2 single crystals were investigated. Benefited from particular clustering effect, 2.79 μm continuous-wave (CW) laser operation was demonstrated in lightly doped 3 at.% Er:CaF2 crystal. After co-doping small amount of Pr3+, the lifetime gap between Er3+:4I11/2 and Er3+:4I13/2 was shortened from 7.86 ms to about 1 ms. Moreover, the near-infrared and up-conversion fluorescence were significantly quenched. Thus, the CW laser performance was greatly improved. The slope efficiency was effectively improved from 8.0% to 15.8%.

Scintillation and dosimeter properties of CaF2 translucent ceramic produced by SPS

We have developed CaF2 translucent ceramics by spark plasma sintering (SPS) and investigated the scintillation and dosimeter properties, in comparison with a CaF2 single crystal. Under X-ray irradiation, light emission was observed at 270 and 300nm, due to self-trapped exciton (STE). The scintillation light yield of CaF2 translucent ceramic was measured to be about 6000photons/MeV under 137Cs irradiation, and this value was smaller than that of the single crystal by a factor of approximately 2. The afterglow level, after X-ray irradiation, of the ceramic sample was lower than that of the single crystal. Thermally stimulated luminescence (TSL) glow peaks were observed at 100 and 150°C in both ceramic and single crystal samples, however the TSL intensity was much stronger than that of the single crystal sample.

Radiation damage caused by swift heavy ions in CaF2 single crystals

The radiation-induced optical absorption in a wide spectral region of 1.5–10 eV have been investigated in nominally pure CaF2 single crystals irradiated at 295 K with 0.23 GeV Xe132 or 2.38 GeV Bi209 ions (fluences of 1011−1014 ions/cm2) providing an extremely high density of electronic excitations. The stepwise annealing of the radiation-induced optical absorption have been analysed at the heating of irradiated crystals up to 1023 K and accompanying thermally stimulated luminescence has been detected at 300–750 K. The origin of several absorption bands (incl. vacuum ultraviolet region), the possible creation mechanisms of the corresponding radiation defects as well as the difference in radiation damage caused by Xe132 or Bi209 ions have been considered.

Effects of Gd3+/Y3+ codoping on the spectral properties of Nd:CaF2 crystals

In the last few years, Nd3+ doped fluoride crystals have achieved some amazing laser performances by codoping buffer ions such as Y3+ and Gd3+ ions, which lead to the changing of local structure of Nd3+ ions. In this work, effects of doping concentration of Gd3+ and Y3+ ions on optical properties are discussed. The relationships between spectroscopic properties and the unit cells are also discussed. Nd, Y:CaF2 and Nd, Gd:CaF2 disordered crystals are grown by using temperature gradient technique (TGT). Among 0.5 at.%Nd, x at.%Gd(x=2, 5, 8, 10):CaF2 crystals, the crystal with Gd3+ of 2 at.% has the longest fluorescence lifetime (499 s). Increasing the concentration of Gd3+ up to 5 at.%, the crystal has a maximum absorption cross section of 1.910-20 cm2, and a maximum emission cross section of 1.910-20cm2. The crystal with Gd3+ of 8 at.%has a maximum emission bandwidth of 29.03 nm(FWHM). Among 0.6 at.%Nd, x at.%Y(x=2, 5, 8, 10):CaF2 crystal, the crystal with Y3+ of 5 at.%has the biggest absorption cross section (2.4110-20 cm2), and the biggest emission cross section (3.1710-20 cm2), when the concentration of Y3+ is 5 at.%. When the Y3+ concentration increases up to 10 at.%, the crystal has a longest fluorescence lifetime of 359.4 s and maximal emission bandwidth of 26 nm(FWHM).The different concentrations of codoping ions have different effects on the Nd:CaF2 crystals, for the formations of different optical centers. In order to study the effects of local structure around Nd3+ on the optical properties in a set of Nd:CaF2 single crystals with different codoping concentrations of Gd3+ and Y3+, the unit cell parameters are investigated by X-ray diffraction. With different concentrations of Gd3+ and Y3+ ions in Nd:CaF2 crystal, the local structure of Nd3+ changes, which leads to different optical properties. The relevant details will further be explained in this paper.

The Bridgman method growth and spectroscopic characterization of calcium fluoride single crystals

It must be noted that the main objective of this study was to obtain single crystals of calcium fluoride - CaF2, and after that the crystals were characterized with various spectroscopic methods. The crystals were grown using the Bridgman technique. By optimizing growth conditions, &lt;111&gt; oriented CaF2 crystals up to 20 mm in diameter were grown. Number of dislocations in CaF2 crystals was 5?104 - 2?105 per cm2. Selected CaF2 single crystal is cut into several tiles with the diamond saw. The plates were polished, first with the silicon carbide, then the paraffin oil, and finally with a diamond paste. The obtained crystal w?s studied by Raman and infrared -IR spectroscopy. The crystal structure is confirmed by X-ray diffraction (XRD). One Raman and two IR optical modes predicted by group theory are observed. A low photoluminescence testifies that the concentration of oxygen defects within the host CaF2 is small. All performed investigations show that the obtained CaF2 single crystal has good optical quality, which was the goal of this work.

On the mechanism of asymmetric ductile–brittle transition in microcutting of (111) CaF2 single crystals

In contrast to forming an amorphous deformed layer, lattice rotation induced asymmetric ductile–brittle transition (aDBT) is first identified in the microcutting of (111) CaF2 single crystals through a systematic experimental study of the machined surface profile, cutting force, crack morphology, and material microstructural changes. Crystal plasticity finite element method simulation has been developed to elucidate the underlying mechanism of variation in macroscopic cutting forces, meso-scale dislocation activities, and lattice rotation under the microcutting induced stress field. The characterised aDBT evokes a reflection on determining the critical chip thickness which plays an essential role in ductile-regime microcutting of brittle materials.

Nd,Y:CaF2laser crystals: novel spectral properties and laser performance from a controlled local structure

A series of Nd,Y:CaF2 single crystals grown using the TGT method were studied systematically by absorption, excitation and luminescence spectroscopy, and the extended X-ray absorption fine structure (EXAFS) technique. The incorporation of Y3+ substantially manipulated the local structure of Nd ions in the CaF2 crystal lattice. Subsequently, the changes in the local structure of the Nd3+ ions induced substantial variations of the photoluminescence parameters, including FWHM, cross sections, lifetimes and quantum efficiency. Preliminary laser experiments were carried out with 0.5% Nd,10% Y:CaF2, 0.8% Nd,5% Y:CaF2 and 0.8% Nd,10% Y:CaF2 crystals under LD pumping; true CW laser operation was achieved with slope efficiencies of 30.1%, 19.3% and 24.9%, respectively.

Crystal growth and frequency conversion of BaMgF4 single crystal by temperature gradient technique

The BaMgF4 single crystal was grown by the temperature gradient technique. Highly pure MgF2 and BaF2 powders were used as raw materials and CaF2 single crystal was chosen as a seed to induce BaMgF4 single crystal grow along 〈001〉 orientation. The short cut-off wavelength of BaMgF4 single crystal was determined to be 130nm which is consistent with the previously reported result. We realized second harmonic generation from 1064nm to 532nm laser, and 800nm to 400nm femtosecond laser in single domain BaMgF4 by using birefringent phase matching. 355-nm-wavelength laser was produced by Sum-Frequency Generation from 1064-nm- and 532-nm-wavelength laser, which is the shortest coherent radiation obtained in the UV in the BaMgF4 crystal.

Phase relations in the system Ca–Ta–O and thermodynamics of calcium tantalates in relation to calciothermic reduction of Ta2O5

Phase equilibria of the system Ca–Ta–O is established by equilibrating eleven samples at 1200K for prolonged periods and phase identification in quenched samples by optical and scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Four ternary oxides are identified: CaTa4O11, CaTa2O6, Ca2Ta2O7 and Ca4Ta2O9. Isothermal section of the phase diagram is composed using the results. Thermodynamic properties of the ternary oxides are measured in the temperature range from 975 to 1275K employing solid–state galvanic cells incorporating single crystal CaF2 as the solid electrolyte. The cells essentially measure the chemical potentials of CaO in two-phase fields (Ta2O5+CaTa4O11), (CaTa4O11+CaTa2O6), (CaTa2O6+Ca2Ta2O7), and (Ca2Ta2O7+Ca4Ta2O9) of the pseudo-binary system CaO–Ta2O5. The standard Gibbs energies of formation of the four ternary oxides from their component binary oxides Ta2O5 and CaO are given by:ΔGf(ox)o(CaTa4O11)(±482)/Jmol-1=-58644+21.497(T/K)ΔGf(ox)o(CaTa2O6)(±618)/Jmol-1)=-55122+21.893(T/K)ΔGf(ox)o(Ca2Ta2O7)(±729)/Jmol-1=-82562+31.843(T/K)ΔGf(ox)o(Ca4Ta2O9)(±955)/Jmol-1=-126598+48.859(T/K)The Gibbs energy of formation of the four ternary compounds obtained in this study differs significantly from that reported in the literature. The thermodynamic data and phase diagram are used for understanding the mechanism and kinetics of calciothermic and electrochemical reduction of Ta2O5 to metal.