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Site-selective time-resolved laser spectroscopy techniques, which provide high spectral resolution at low temperatures (4.2 K and 6.5 K), were used to clarify the formation mechanism, spectral and kinetic properties of the Nd3+ ion single-site optical centers formed in the fluorite-type BaF 2 single crystals doped with Nd3+. This formation occurs by heterovalent substitution of Ba2+ host cations with additional charge compensation by F− anions. To determine the lifetimes of the Stark 4F 3/2 (1) levels of the Nd3+ impurity centers, the fluorescence decay kinetics was measured using the gated photon counting technique. The data from spectroscopic experiments were combined with density functional theory (DFT) calculations to identify and analyze the local structure of Nd3+ cations in the Nd3+ ions doped BaF 2 crystal. Our studies have shown that several types of single-site Nd3+ optical centers can be distinguished in BaF 2 :Nd3+. We confirmed the existence of an optical center with high C 4v site symmetry and a long radiative lifetime of the 4F 3/2 (1) state τ = 14.3 ms. We also describe two other configurations of centers with local structure characterized by low site symmetry of C s , which were identified and described for the first time. The decrease in the tetragonal site symmetry from C 4v to C s was explained by the fact that the Nd3+ dopants, distributed over the equivalent cationic positions of the fluorite-type crystal lattice, coordinate to anionic motifs that essentially differ in the position of the additional charge compensator F−. This structural variability induced in the local coordination environment of the optical center affects its spectroscopic properties through a shift in the energy of the 4F 3/2 (1) Stark level of the Nd3+ ion and results in a different radiative lifetime τ. • Nd3+ ion single-site optical centers in BaF 2 crystals were studied. • DFT analysis showed four configurations of the Nd3+ ion sites in BaF 2. • Local geometries of the Nd3+ ion single-sites in BaF 2 were determined. • The existence of Nd3+ site in BaF 2 with high C 4v site symmetry was confirmed. • Two configurations of the Nd3+ sites with low symmetry of C s were revealed. [ABSTRACT FROM AUTHOR]

This work reports a microsphere laser at ∼2.0 μm in a Tm3+-doped ZrF 4 -BaF 2 -YF 3 -AlF 3 (ZBYA) fluoride glass. The ZBYA glass microsphere was fabricated by reflowing the tip of a 1.0 mol% Tm3+-doped ZBYA glass fiber with a CO 2 laser. By using a fiber taper for input and output coupling in the Tm3+-doped ZBYA microsphere, single and multi-mode laser outputs at around 2.0 μm were observed under 808 nm laser excitation. The radiation lifetime and emission cross-section of Tm3+ ions in ZBYA glass were calculated to be 6.42 ms and 5.86 × 10−21 cm2, respectively. Our results indicate that the Tm3+-doped ZBYA fluoride glass has potential applications for 2.0 μm lasers. [ABSTRACT FROM AUTHOR]

The stationary X-ray excited luminescence (XEL) spectra at 80 and 294 K as well as the thermally stimulated luminescence (TSL) in the 80–300 К range of a single crystal and nanoparticles of BaF 2 with various grain sizes of 20–113 nm were studied. Under the influence of size effects in BaF 2 nanophosphors, a short-wave displacement of self-trapped excitons emission band was observed compared to the corresponding single crystal, and its splitting into two components peaked at 324 and 305 nm for 294 K, and at 321 and 305 nm – for 80 K. A slight decrease of radiation resistance of BaF 2 nanophosphors was observed in comparison to single crystal, which led to additional band of X-ray excited luminescence at ∼410 nm that may be associated with emission of exciton localized in the vicinity of anionic vacancies of the matrix. At the same time, it was established that radiation resistance of BaF 2 nanophosphors is much higher compared to the corresponding CaF 2 nanophosphors. With the use of integral TSL curves and TSL spectra, it has been shown that V H -centers with delocalization temperature of ∼140 K are the main hole centers that occur in BaF 2 nanoparticles under X-ray irradiation influence, while in ВаF 2 single crystals – these are V k - and H -centers with delocalization temperatures at 113 and 198 К, respectively. Significant long-wave displacement of emission bands associated with (V H -F)- and (H-F)- recombinations, compared to the spectra of corresponding single crystal, was observed in BaF 2 nanophosphors. [ABSTRACT FROM AUTHOR]

Abstract Powder sample of Cubic phase BaF 2 :Ce phosphor was synthesized by chemical co-precipitation method and annealed at 500 °C for 2 h. The synthesized powder of BaF 2 :Ce phosphor was characterized by XRD, which shows FCC structure with 30 nm crystallite size. TEM images show a cubic structure and FTIR contains the existence of F-Ba-F, C-H, C˭C, and O-H bonds. Glow curve of the Thermoluminescence contains two major peaks, one at 175 °C and another at 275 °C. The intensity of the major dosimetric peak (175 °C) observed to be increased linearly from 1 Gy to 3 kGy for the doses of 60Co gamma rays and then saturates. Overall, the new BaF 2 :Ce phosphor shows good material properties for the high gamma dose measurement applications. [ABSTRACT FROM AUTHOR]

Luminescence properties of impurity-trapped excitons (ITEs) in LiBaF 3 :Eu 2+ crystals are investigated by optical and time-resolved laser-excitation spectroscopy. The three excited states of 4f 6 5d, 4f 7 ( 6 P 7/2 ) and ITE are identified in the excitation and emission spectra. The intensity of ITEs’ emission increases unexpectedly with increasing temperature and excitation energy. The initial intensity of the ITE emission just after pulsed-laser excitation increases continuously with temperature in the temperature range 7–600 K. The results are discussed in relation to radiative and nonradiative rates and excitonic processes in this system. The theoretical model that includes the ITE, 4f 7 ( 6 P 7/2 ) and 4f 6 ( 7 F J )5d(e g ) states is developed to describe and support the experimental observations. [ABSTRACT FROM AUTHOR]

2 at.% Dy3+: BaF 2 and 2 at.% Dy3+/2 at.% RE3+(RE3+ = Gd3+, Y3+, Lu3+): BaF 2 single crystals were grown by the Bridgman-Stockbarger method, and characterized by absorption, visible emission, and fluorescence decay curves. Also, relying on the excitation and emission spectra, decay curves at low temperature (77 K) for Dy3+: BaF 2 and Dy3+/RE3+(RE3+ = Gd3+, Y3+, Lu3+): BaF 2 crystals have been measured. We found that Dy ions have at least three lattice sites in BaF 2 crystal, and the lattice sites of Dy3+ ions can be reduced by co-doping with Gd3+, Y3+ or Lu3+ ions, where Dy3+: BaF 2 and Dy3+/RE3+(RE3+ = Gd3+, Y3+, Lu3+): BaF 2 single crystals can be considered as quasi-single lattice crystals. From visible fluorescence spectra, it was found that co-doping Gd3+ will increase the emission intensity of yellow light by 20 times, co-doping Y3+ will increase the emission intensity of yellow light by 30 times, and co-doping Lu3+ will increase the emission intensity of yellow light by 25 times. The calculated absorption cross-section, emission cross-section, and fluorescence lifetimes indicate that Dy3+: BaF 2 and Dy3+/RE3+(RE3+ = Gd3+, Y3+, Lu3+): BaF 2 crystals especially, Dy3+/Y3+: BaF 2 crystals are suitable to achieve yellow laser output. • Dy3+: BaF 2 and Dy3+/RE3+(RE3+ = Gd3+, Y3+, Lu3+): BaF 2 crystals are grown. • Dy3+ ion occupies a minimum of three lattice sites in Dy3+: BaF 2 crystal. • Dy3+ ion lattice sites can be reduced by co-doping Gd3+, Y3+, and Lu3+ ions. • Co-doped Gd3+, Y3+, and Lu3+ ions enhance yellow luminescence of Dy3+: BaF 2 crystal. [ABSTRACT FROM AUTHOR]

Luminescence properties of Pr 3+ ions in germanate glasses modified by BaF 2 were investigated. Several luminescence bands originating to transitions from the 3 P 0 state to the lower-lying states of Pr 3+ were registered under 450 nm excitation. The spectral analysis suggests that the positions of luminescence bands and their relative intensities are changed significantly with BaF 2 content. The relative integrated luminescence intensities of 3 P 0 → 3 H 4 transition (blue) to the 3 P 0 → 3 F 2 transition (red) of Pr 3+ ions strongly depend on fluoride modifier BaF 2 in glass composition. The luminescence bands of Pr 3+ ions are shifted to shorter wavelengths with increasing BaF 2 content. [ABSTRACT FROM AUTHOR]

Laser selective excitation and polarization studies of the dominant C 3v symmetry Pr 3+ centre in BaF 2 :0.1%Pr 3+ crystals have been carried out in the 10–75 K temperature range. Crystal-field energy levels for seven of the 12 multiplets of Pr 3+ have been derived and their irrep symmetry labels determined. A further two minor centres have also been identified; their fluorescence lifetimes for the 1 D 2 multiplet are much less than that of the C 3v symmetry centre. [ABSTRACT FROM AUTHOR]

Abstract: Absorption and site-selective laser spectroscopy of BaF2 single crystals homogeneously doped with Nd3+ ions as well as heterogeneously doped with Nd3+ and either Gd3+ or Yb3+ ions, performed at 10K, is presented. In each case a single centre, assigned to be of C3v symmetry, is dominant. A heterogeneous Nd3+–RE3+ (RE being either Nd or Yb) is also present in each of the double doped crystals. Energy levels for six multiplets of the single Nd3+ centre and for three multiplets for each Nd3+–RE3+ centre are presented together with the 4F3/2 (R) multiplet fluorescence lifetime for each centre. Calculations give comparable solution energies for C3v and C4v symmetry Nd3+ centres in BaF2; the C3v symmetry configuration has lower energy for the preceding lanthanides while C4v is favoured from Sm3+ onwards. [Copyright &y& Elsevier]

Abstract: An ab initio model potential embedded cluster study on the and 5d manifolds of the long range compensated, cubic, substitutional Tb3+ defects of Tb-doped BaF2 is presented here. 4f–4f and 4f–5d transition energies and 4f–5d absorption electric dipole oscillator strengths for the dipole allowed transitions are reported and comparisons with emission and excitation experimental spectra are presented. We find good agreement with the energies of the experimental 4f–4f green and blue emissions. The characteristic features of the 4f–5d excitation spectrum (spin allowed vs. spin forbidden transitions) are well reproduced by the calculations. The transition energies from the ground state to the different spin nonet and spin septet levels of the 5d configuration of this ion in the center of the lanthanide series show an error of around 10%, which is in line with previous calculations on other lanthanide ions with lower spin multiplicities and simpler manifolds at the beginning and at the end of the series. The calculation supports a recent assignment of experimental excitation bands between 54,000 and 66,000cm−1 as transitions. On the other hand, a new assignment is made of the highest band in the excitation spectrum at around 76,000cm−1: it corresponds to transitions to levels of the 5d configuration with the subshell in the excited 6 P term, rather than to a Tb-bound exciton. Preliminary studies on excitonic states of Tb-doped BaF2 indicate that energy levels that can be characterized as Tb-trapped excitons are found in the same spectral region as the spin forbidden transitions at ground state Tb–F distances, and their energy lowers very much at shorter Tb–F distances. [Copyright &y& Elsevier]

Abstract: BaF2:Ce nanophosphors with a peak emission at 355nm were synthesized using a chemical precipitation method with oleic acid as the capping ligands. The cerium doping concentration was optimized and it was found that the photoluminescence intensity reached a maximum at about 15mol% of Ce doping. Two distinct photoluminescence excitation peaks were observed from all samples at about 255 and 290nm. The nanophosphors were incorporated into an epoxy matrix to form bulk nanocomposite samples for potential scintillation applications. Index matching between the nanophosphor and the epoxy matrix is critical to obtain high transparency. [Copyright &y& Elsevier]

Abstract: We present the theoretical calculations of the core hole electronic and spatial structure and cross-luminescence spectrum in cluster. The ab initio Hartree–Fock calculation of cross-luminescence has been performed in embedded-cluster approach. The theoretical spectrum reproduces well the characteristics of the experimental one. The valence states corresponding to the 5.6, 6.3 and 7.1eV emissions is found to be localized. The low-energy tail is shown to be due to transitions between 5p core states of ion and 2p valence states of next-nearest ions. [Copyright &y& Elsevier]

This work reports the ellipsometric study of trivalent erbium (Er3+) doped monocrystalline barium yttrium fluoride (BaY 2 F 8), which has recently been shown to be one of the best photon upconversion (UC) materials available. This spans the BaY 2 F 8 applications in a large range of wavelengths, from ultraviolet (UV) to near-infrared (NIR). We detail the optical properties of BaY 2 F 8 : Er3+ (0.5 mol%, 10 mol%, and 30 mol%), measured via variable angle spectroscopic ellipsometry over a spectral range from 300 nm to 1800 nm, reporting for the first time the indices of refraction for BaY 2 F 8 :Er3+. The upconversion external photoluminescence quantum yield (e PLQY) of the BaY 2 F 8 :Er3+ samples have also been studied by exciting at λ = 1493 nm. The highest e PLQY of BaY 2 F 8 was found for the largest dopant concentration 30 mol% Er3+ reaching the value of 3.62% ± 0.01%, at an irradiance of (6.23 ± 0.45) × 10 − 2 W/cm2. The refractive index (λ = 589.3 nm) was determined to be 1.4808 ± 0.014 for 0.5 mol%, 1.4980 ± 0.003 for 10 mol%, and 1.5022 ± 0.006 for 30 mol%. Increasing Er3+ doping concentration increased the refractive index. All samples decreased monotonically with increasing wavelength. The Brewster angle of BaY 2 F 8 :Er3+ is observed to be ≈ 56 ∘ , whilst the Abbe number of the samples was found to be as high as 124.62. These findings provide valuable insight into the optical properties of BaY 2 F 8 :Er3+ in the wide range of frequencies that is has proven useful. • The Er3+ doped BaY 2 F 8 crystals have been optically characterized. • Refractive index and Abbe number of the crystals were measured by ellipsometry. • The NIR e PLQY (940 nm to 1040 nm) has been obtained under 1493 nm excitation. • BaY 2 F 8 with 30 mol% Er3+ doping was found to have a refractive index of 1.5022. [ABSTRACT FROM AUTHOR]

This paper describes the preparation and characterization of various concentrations of Nd3+ doped B 2 O 3 –BaF 2 –PbF 2 –Al 2 O 3 (BaPbAlFB) glasses in detailed. The glasses used in the present work are synthesized by using melt quenching process and characterized by measuring the absorption, photoluminescence (PL) and PL decay spectral profiles. The energies of the absorption spectral profiles called oscillator strengths are evaluated from the absorption spectra and subjected to Judd-Ofelt (J-O) theory to evaluate the J-O intensity parameters using least squares fit method. Under 808 nm diode laser excitation, the BaPbAlFBNd glasses exhibit three emission bands at near-infrared region around 887, 1063, 1336 nm from 4F 3/2 → 4I 9/2, 4I 11/2 and 4I 13/2 radiative transitions respectively. Among the three emissions, the one observed at 1063 nm is found to be more intense relatively. The lifetime of 4F 3/2 transition has been estimated experimentally through decay measurements recorded under 808 nm excitation wavelength. Among all the BaPbAlFB glasses investigated, the one with 1.0 mol% of Nd3+ ions is showing relatively high fluorescence quantum efficiency. The measured saturation intensity (I S) values shows relatively the low threshold values. The obtained results endorsed the potentiality of BaPbAlFBNd1.0 glass for the fabrication of fiber amplifiers and solid-state lasers that can act in NIR region at 1063 nm wavelength. • The J-O intensity parameters of the glass samples. • Experiment lifetimes are calculated from decay measurements. • The evaluation of absorption cross section, emission cross section and gain cross section. • The figure of merit and saturated intensity are calculated. • NIR laser emission near 1.063 μm was obtained. [ABSTRACT FROM AUTHOR]

The influence of γ-ray irradiation on the optical and structural properties of BaF 2 – Bi 2 O 3 –SiO 2 glasses has been studied. The absorption spectra of respective glasses are found to shift to higher wavenumber side as dosage rate of gamma irradiation increases. This is because of the formation of non-bridging oxygen hole centers (NBOHC) and electrons due to ionizing power of BaF 2. The optical band gap value decreases from 3.07 to 2.61eV with an increase in dosage rate as well as alkaline content. The Urbach energy of glasses increases due to gamma irradiation may be attributed to formation of non-bridging oxygen in the system. The electron paramagnetic resonance (EPR) spectra exhibit resonance signals at g = 4.17 and g = 2.17 due to triplet hole pairs localized on apical oxygens. There were no significant variations in Raman spectra and this may be due to high content of Bismuth that is known for shielding of gamma irradiation. Photoluminescence (PL) studies reveal that both pure and irradiated barium silicate glasses exhibit white light where red region is predominant than blue in irradiated glasses. • White light emitting pure BaF 2 –Bi 2 O 3 –SiO 2 glasses are successfully prepared. • High dosage γ-ray leads to formation of NBOHC and improves CIE color coordinates. • Life time spectra give the idea about no permanent changes in Bi oxidation states. • EPR study confirms the presence of triplet oxygen hole center in BBS glasses. [ABSTRACT FROM AUTHOR]