SrF2 Crystals Research Articles

Absolute Light Yield Measurements on SrF$_2$ and BaF$_2$ Doped With Rare Earth Ions

Results of absolute light yield measurements on strontium and barium fluoride doped with PrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and CeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> are presented and compared with light yield of well-known scintillators s(NaI-Tl and CsI-Tl). For pure SrF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> crystal, we obtain the value of about 29,200 photons/MeV.

Nanostructured crystals of fluorite phases Sr1 − x R x F2 + x and their ordering: 9. The defect crystal and real structure of quenched fluorite phases Sr1 − x Ce x F2 + x (x = 0–0.5)

A Sr0.7Ce0.3F2.3 crystal (CaF2 type, sp. gr. \(Fm\bar 3m\)), obtained by quenching from melt, has been studied for the first time by X-ray diffraction. Fluorine vacancies and interstitial anions are found in the 8c and 32f sites, respectively. The defect ratio in the Sr0.7Ce0.3F2.3 structure corresponds to the tetrahedral cluster configuration of defects {Sr4 − nCenF26}. The defect structure of quenched (at a rate of ∼25 K/min) crystal differs from that of a crystal grown from melt (cooling at a rate of ∼3 K/min) by the displacement of some cations (presumably Ce3+) along the threefold axis to the 32f site and the anisotropy of thermal vibrations of ions in the cluster core (Fint(32f)3). The concentration dependence of the lattice parameters of quenched Sr1 − xCexF2 + x phases (x = 0–0.5) is described by a third-order polynomial: a = 5.80009 + 1.166518 × 10−3x − 1.124969 × 10−5x2 + 8.258155 × 10−8x3. The compositional dependence of microdistortions is also nonlinear; maximum microdistortions are observed in the SrF2 crystal. They decrease with an increase in the cerium concentration x to ∼ 0.35. The minimum in the range x = 0.30–0.35 correlates with a composition corresponding to the peak (at x ∼ 0.29) in the melting curves of the fluorite phase estimated from the phase diagram of the SrF2-CeF3 system (the method of thermal analysis).

Laser performance and thermal lens of diode-pumped Nd, Y-codoped:SrF2 single crystals

We demonstrate the laser performances of Nd, Y:SrF2 crystals with Nd3+ concentrations of 0.15 and 0.43 at.%. The sample with 0.43 at.% Nd3+ concentration yields a maximum output power of 1.023 W at 1056.9 nm with a slope efficiency of 53%. The focal length of the thermal lens is analyzed for the 0.15 at.% Nd3+-doped crystal sample. An improved cavity is designed considering the thermal lens. The maximum output power is 464 mW at 1056.9 nm, with a slope efficiency of 36.1%. The wavelength is tuned within the range of 1049.74-1059.13 nm.

Characteristics of a diode-pumped Yb:CaF2–SrF2 mode-locked laser using a carbon nanotube absorber

Yb:CaF2–SrF2 disordered crystals are successfully grown by the TGT method. By using a double-walled carbon nanotube saturable absorber (DWCNT-SA), the continuous-wave mode-locked (CWML) laser properties of Yb:CaF2–SrF2 crystals are demonstrated under diode pumping for the first time. The mode-locked laser delivers pulses as short as 5 ps at a center wavelength of 1045.5 nm without any dispersion compensation. The oscillator operating at a repetition rate of ∼87 MHz delivers 292 mW average output power.

Ab initio calculations for the F-center transfer and R centers in SrF2

We have simulated the F-center transfer and R center in SrF2 crystal by using density functional theory (DFT) with a hybrid B3PW description of exchange and correlation. Our calculations show that the F-center diffusion barrier is equal to 1.84eV. During the F-center transfer, the trapped electron is more delocalized than that in the regular F-center case, and the gap between defect level and conduction bands (CB) in the α-spin state decreases. The formation energy calculations of R center show the trend of F centers to aggregate in SrF2. During the F-center aggregation, a considerable covalency forms between two neighboring fluorine vacancies with trapped electrons. Three incompletely paired electrons trapped in the R center have an up-down-up spin arrangement and induce three defect levels in the gaps between valence bands (VB) and conduction bands for both of α- and β-spin polarized band structures, respectively. More defect bands lead to more complex electron transitions, which were classified into two F- and four M-like transitions. The DOS calculations clearly reveal the components of defect bands.

Scintillation properties of SrF2 and SrF2-Ce3+ crystals

This Letter presents the results of measuring scintillation properties of pure SrF2 crystals and crystals activated by various concentrations of Ce3+ ions. The light yield of these materials is compared to that of the known scintillators NaI-Tl and CaF2-Eu2+. Strontium fluoride crystals activated with Ce3+ ions are found to be characterized by high light yield and to be promising materials for use in scintillation detectors employed for γ-ray well logging.

Thermal conductivity of single crystals of M1−x M x ′F2 (M = Ca, Sr; M′ = Mn, Co) isovalent solid solutions

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.

Scintillation properties of pure and Ce3+-doped SrF2 crystals

In this paper results of scintillation properties measurements of pure and Ce3+-doped strontium fluoride crystals are presented. We measure light output, scintillation decay time profile and temperature stability of light output. X-ray excited luminescence outputs corrected for spectral response of monochromator and photomultiplier for pure SrF2 and SrF2-0.3 mol% Ce3+ are approximately 95% and 115% of NaI–Tl emission output, respectively. A photopeak with a 10% full width at half maximum is observed at approximately 84% the light output of a NaI–Tl crystal after correction for spectral response of photomultiplier, when sample 10 × 10 mm of pure SrF2 crystal is excited with 662 keV photons. Corrected light output of SrF2-0.3 mol% Ce3+ under 662 keV photon excitation is found at approximately 64% the light output of the NaI–Tl crystal.

Ceramic planar waveguide structures for amplifiers and lasers

Ceramic and crystalline weakly guiding optical fibres with the core – cladding refractive index difference of 10-2 – 10-4 are fabricated by a hot pressing method. The waveguides with one or several cores for operation in the spectral range 0.2 – 5 μm are produced. The waveguides are based on CaF2, SrF2, and BaF2 ceramics and crystals and their solid solutions doped with trivalent Pr, Nd, Tb, Dy, Yb, Ho, Er, and Tm ions, as well as on LiF ceramics and crystals with colour centres. The first results of investigation of the lasing properties of ceramic SrF2 : NdF waveguides under diode pumping are presented, and the prospects of further investigation are discussed.

Femtosecond laser-inscribed waveguides in Nd3+:Y3+:SrF2 crystals

Optical waveguides are fabricated in Nd3+:Y3+:SrF2 crystals by a 1-kHz femtosecond laser using the double-line approach. Waveguides with different separations (10, 15, and 20 \mm m) between two consecutive optical breakdown tracks are produced, and their optical performances are explored by end-fire coupling to 780-and 532-nm lasers. Propagation loss of the waveguide with 20-\mm m separation is estimated. The microphotoluminescence and micro-Raman spectra indicate that the original fluorescence and lattice structure of the Nd3+:Y3+:SrF2 crystals are well preserved in the waveguide. Therefore, the obtained waveguide structures are promising candidate for application in integrated waveguide lasers.

Spin-Hamiltonian parameters for the tetragonal [formula omitted] centers in CaF2 and SrF2 crystals

The spin-Hamiltonian parameters (g factors g//, g⊥ and zero-field splittings b20, b40, b44, b60, b64) of the tetragonal GdM3+–Fi- centers in CaF2 and SrF2 crystals at T≈1.8K are calculated from the diagonalization (of energy matrix) method based on the one-electron crystal field mechanism. In the calculations, the crystal field parameters used are estimated from the superposition model with the reported defect structural data obtained from the analyses of superhyperfire interaction constants at the same temperature. The calculated results are in reasonable agreement with the experimental values. It appears that the above defect structural data reported in the previous paper are suitable and the diagonalization (of energy matrix) method is effective to the studies of spin-Hamiltonian parameters for 4f7 ions in crystals.

Raman electron spin–lattice relaxation with the Debye-type and with real phonon spectra in crystals

Electron spin–lattice relaxation temperature dependence was measured for Ti2+ (S=1) and for Cu2+ (S=1/2) ions in SrF2 single crystal by electron spin echo method in temperature range 4–109K. The spin relaxation was governed by the two-phonon Raman processes. The relaxation theory is outlined and presented in a form suitable for applying with real phonon spectra. The experimental relaxation results were described using Debye-type phonon spectrum and the real phonon spectrum of SrF2 crystal. The Debye approximation does not fit well the results for SrF2 both at low and at high temperature. The relaxation rate is faster than that predicted by Debye-type phonon spectrum at low temperatures where excess of lattice vibrations over the Debye model exists but is slower at higher temperatures (above 50K) where density of phonon states continuously decreases when approaching to the maximal acoustic phonon frequency. The expected deviation from Debye approximation was analyzed also for Cu2+ in NaCl and MgSiO3 crystals for which phonon spectra are available. The fitting with the real phonon spectrum allowed us to calculate spin–phonon coupling parameter as 267cm−1 for Ti2+ and 1285cm−1 for Cu2+ in SrF2.

Eu‐concentration dependence of optical and scintillation properties for Eu‐doped SrF2 single crystals

AbstractSome Eu‐doped scintillators, such as Eu:SrI2, emit around 400‐nm wavelength that originates from 5d‐4f transition of Eu2+, while some Eu‐doped crystals have emissions from Eu3+ (f‐f emission). In this report, effect of Eu‐doping on optical and scintillation properties of SrF2 crystals with Core‐valence luminescence is discussed. The samples had 0.1, 0.2, 0.5, 1.0, and 2.0 mol% of Eu and were grown by an annealing method. As a result, 5D0‐7Fi (i =1, 2) transition (Eu3+) was detected in the SrF2, containing over 0.5 mol% of Eu, while emission from 0.1 and 0.2‐mol% Eu:SrF2 originated from 5d‐4f transition of Eu2+ and Core‐valence luminescence. The light output of SrF2 doped with 0.1, 0.2, 0.5, 1.0 and 2.0 mol% Eu was estimated to be 13,000 ± 1,000, 12,000 ± 1,000, 13,000 ± 2,000, 8,000 ± 1,000 and 7,000 ± 1,000 photons/MeV, respectively. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Studies of EPR spectra and defect structure for Er3+ ions in BaF2 and SrF2 crystals

The local lattice structure and electron paramagnetic resonance (EPR) spectra have been studied systematically by diagonalizing 364×364 complete energy matrices for a f11 ion in a trigonal ligand-field. By simulating the calculated Stark levels and EPR parameters to the experimental results, the shift parameters are determined for Er3+ ions in BaF2 and SrF2. The results show that the trigonal center is attributed to an interstitial F− ion located at the [111] axis of the cube, and the nearest ligand close to the charge compensator has a displacement towards central ion by 0.042Å for L center in BaF2:Er3+ and 0.026Å for J center in SrF2:Er3+, respectively. Moreover, the relationships between g-factors and shift parameter ΔZ as well as orbit reduction factor k′ are discussed.

Energy Transfer Mechanism in Pr-Doped SrF$_{2}$ Crystals

Results of investigation of energy transfer mechanism in SrF2 crystals doped with Pr3+ ions are presented. Temperature dependences of 5d-4f and exciton emission in this material under X-ray excitation, and thermostimulated luminescence (TSL) in temperature range 80-300 K are studied. It has been established that in SrF2-Pr3+ crystals the consecutive electron-hole capture mechanism of energy transfer takes place. Also we found that hole centers made a contribution to energy transfer from primary electrons and holes to Pr3+ ions.

Photochromism in Calcium and Strontium Fluoride Crystals Doped With Rare-Earths Ions

This paper reports the absorption spectra of photochromic centers in CaF2 and SrF2 crystals doped with Ce3+ and Gd3+ impurities and thermal decay of the centers in temperature range 80-500 K. The ionized photochromic color centers are generated in crystals under low-temperature X-rays irradiation. These centers are transformed into photochromic color centers upon heating of crystals. All color centers decay at temperature about 500 K.

Nanostructured crystals of Sr1−x R x F2+x fluorite phases and their ordering: 6. Microindentation analysis of crystals

Hardness, crack resistance, brittleness, and effective fracture energy have been studied for crystals of 24 fluorite phases Sr1 − x R x F2 + x (R are 14 rare earth elements (REEs); 0 < x ≤ 0.5) and SrF2 grown by the Bridgman method from a melt. These characteristics change nonlinearly with an increase in the REE content for Sr1 − x R x F2 + x (0 < x ≤ 0.5) with R = La, Nd, Sm, Gd, and Lu; it is maximum in the range x < 0.1 for all REEs. The changes in a number of REEs have been traced for an isoconcentration series of Sr0.90 R 0.10F2.10 crystals (R = La, Nd, Sm, Gd, Ho, Er-Lu, or Y) and crystals (similar in composition) with R = Tb and Dy. The hardness of Sr1 − x R x F2 + x crystals is higher by a factor of ∼2–3 than that of SrF2. The effect of decrease in microstresses in SrF2 crystals is confirmed by the isomorphic introduction of R 3+ ions into this crystalline matrix.

VUV-spectroscopy of Ce3+ -doped crystals with fluorite-type structure

The spectral properties of two-component Ca1−xSrxF2:Ce3+(0.05 mol%) (x = 0.75, 0.41, 0.25, 0.14) crystals in UV spectral region were investigated. It was shown that spectral properties of Ca1−xSrxF2:Ce3+(0.05 mol%) in UV spectral region have intermediate position between those for pure CaF2 and SrF2 crystals. The optical constants were calculated from reflectance spectrum for CaF2, SrF2, Ca1−xSrxF2:Ce3+(0.05 mol%) (x = 0.75, 0.25) crystals by use of Kramers-Kronig relations for complex reflection coefficient and Fresnel formulas.

The Refinement of Self-Trapped Excitons Structure in CaF$_{2}$ and SrF$_{2}$ Crystals: An Ab Initio Study

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.

Mode-locked operation of a diode-pumped femtosecond Yb:SrF_2 laser

Femtosecond mode-locked operation is demonstrated for the first time, to our knowledge, with a Yb:SrF(2) crystal. The shortest pulse duration is 143 fs for an average power of 450 mW. The highest average power is 620 mW for a pulse duration of 173 fs. Since Yb:SrF(2) corresponds to the longest-lifetime Yb-doped crystal with which the mode-locking operation has been achieved, a detailed analysis is carried out to characterize the quality of the solitonlike regime.