BaWO4 Crystal Research Articles

Scheelite-type BaWO4 doped with Ho2O3 oxide as a promising lead-free shield for gamma rays: Structural, optical properties, and radiation attenuation efficiency

Humans are constantly exposed to ionizing radiation and radioactive substances that exist naturally within the earth, their bodies, and their homes. Therefore, people and their surroundings must be protected from high levels of ionizing radiation. In general, different materials for radiation protection are used in diverse fields and for specific cases. In this work, a new ceramic-based material containing barium tungstate-doped with different amounts (x= 0–75%) of holmium oxide Ho2O3 was proposed as a γ-radiation shield. The structural and optical studies were performed based on X-ray diffraction technique, Fourier transform infrared spectrum, and diffuse reflectance spectroscopies. Radiation attenuation coefficients were determined using the Monte Carlo simulation. The structural analyses indicated that the BaWO4 crystal belongs to the scheelite structure and a lattice distortion was shown due to the effect of Ho2O3 doping. The band gap energy of the material increased progressively with increasing the content of the dopant in the host material. The radiation shielding coefficients reduced with increasing the concentration of Ho2O3 into BaWO4, where the linear attenuation coefficient reduced by 13% from 0.386 cm−1 to 0.338 cm−1 at 0.662 MeV when the concentration of Ho2O3 increased from 0% to 75%, respectively. Simultaneously, the lead equivalent thickness increased from 3.22 cm to 3.67 cm with raising the Ho2O3 concentration.

Stimulated Raman scattering of chirped Ti:sapphire laser pulse in BaWO4, SrMoO4 and Ca3(VO4)2 crystals

Transient stimulated Raman scattering (SRS) of chirped, temporally stretched up to 45 ps, Ti:sapphire laser pulses was studied in three different Raman-active crystals (BaWO4, SrMoO4 and Ca3(VO4)2) at the same experimental conditions. BaWO4 and SrMoO4 crystals had the same SRS “threshold” energy/intensity, which was associated with their similar integrated cross sections for spontaneous Raman scattering. The highest energy SRS efficiency was obtained in SrMoO4 crystal and reached 8%, which was two-times higher than one in BaWO4. Higher SRS efficiency in SrMoO4 was observed due to lower nonlinear absorption. The Ca3(VO4)2 crystal had significantly higher SRS “threshold” and lower efficiency despite the lowest nonlinear absorption losses. A simple expression of exponential gain for transient SRS of chirped laser pulses was proposed and verified by comparison with experimental results. This expression was used to estimate dephasing time T2 of Ca3(VO4)2 crystal which was 8.5 ± 1.0 ps.

Highly efficient transient stimulated Raman scattering on secondary vibrational mode of BaWO4 crystal due to its constructive interference with self-phase modulation

An exceptionally high stimulated Raman scattering (SRS) conversion efficiency to the first Stokes component associated with the secondary (low-frequency and low intensity) vibrational mode ν2 (∼330 cm-1) was observed in a BaWO4 crystal in a highly transient regime of interaction. The effect takes place in the range of pump pulse energy from ∼0.1 to ∼0.5 µJ with maximum energy conversion efficiency up to 35% at 0.2 µJ. The nature of the observed effects is explained by interference of SRS and self-phase modulation, where the latter is related to a noninstantaneous orientational Kerr nonlinearity in the BaWO4 crystal.

Spectral Broadening of Chirped Laser Pulse Caused by Four-Wave Mixing in BaWO4 Crystals

The spectral broadening of a chirped, temporally stretched (up to 200 ps) Ti:sapphire laser pulse propagating together with a broadband low-power seed pulse through BaWO4 crystals was experimentally and numerically studied. Our analysis demonstrated that self-phase modulation could not have affected the pulse spectrum under the experimental conditions. The spectral broadening of the Ti:sapphire laser pulse was found to be due to four-wave mixing, which resulted in transferring the power of the central wavelength to the spectral wings. To the best of our knowledge, this is the first demonstration of such origin of laser pulse spectral broadening in crystals.

Frequency down-conversion of a chirped Ti:sapphire laser pulse with BaWO4 Raman shifter and second-order nonlinear crystal

We experimentally carried out frequency conversion of a chirped and stretched up to 200 ps near-IR 90 fs Ti:Sapphire laser pulse into the mid-IR through difference frequency generation in either GaSe or LiGaS2 crystal. The crystal was pumped by laser radiation that passed through a BaWO4 crystal, where Stokes frequency-shifted radiation (signal wave) was generated through SRS. The generated ∼60 nJ mid-IR pulse had a central wavelength of 11.1 μm and FWHM spectral width of about 0.2 μm, which made it possible to achieve a pulse duration of ∼0.9 ps after its compression. The mid-IR pulse generation efficiency was ∼10−3% at SRS efficiency of 3%. To the best of our knowledge, this all solid-state scheme combining Raman shifting and second-order nonlinear crystals achieves the frequency down-conversion of femtosecond near-IR laser pulses to ∼11 μm radiation for the first time.

Measurement of Multi-Stokes Ultrashort Pulse Shapes of Synchronously Pumped Stimulated Raman Scattering on Combined Vibrational Modes in a BaWO4 Crystal

Multi-Stokes ultrashort pulse shapes and their relative positions of synchronously pumped stimulated Raman scattering (SRS) on combined primary and secondary vibrational modes in a BaWO4 crystal are investigated. An original method of its simultaneous measurement with the help of a streak camera has been developed. The structure of SRS pulses at the pulse shortening effect down to the pulse duration, close to the dephasing time of the secondary Raman mode of the BaWO4 crystal, is registered and analyzed for the detuning of the Raman laser cavity length.

Stimulated Raman Scattering of Chirped Ti:Sapphire Laser Pulses in BaWO4 Crystal

Stimulated Raman Scattering of Chirped Ti:Sapphire Laser Pulses in BaWO4 Crystal

Efficient SRS of chirped Ti : sapphire laser pulses in BaWO4 crystals

We report an experimental study of SRS of chirped 40-ps Ti : sapphire laser pulses with a transform-limited duration of 0.35 ps and centre wavelength of 925 nm in BaWO4 crystals. The highest generation efficiency for the ∼925 cm−1 shifted Stokes signal of SRS by the ν 1 vibrational mode of BaWO4 crystals has been reached using three sequentially located crystals with a total length of 3.3 cm. The position of the crystals after a focusing mirror has been optimised, which has ensured a pump-to-Stokes conversion efficiency of ∼50 % in terms of spectral brightness and ∼20 % in terms of pulse energy.

Microstructure investigation and fluorescence properties of europium-doped scheelite crystals in glass-ceramics made under different synthesis conditions

Glasses of the composition 8 BaO·8 SrO·34 ZnO·46 SiO2·4 WO3 doped with 1·1020 Eu3+/cm³ were melted either in air or under continuous bubbling with forming gas (90% N2 and 10% H2). UV–vis spectra of both glasses show absorption lines of Eu3+. In the glass melted under reducing conditions, an additional broad absorption band in the 300–600 nm range is observed, typical for the occurrence of Eu2+. Thermal treatment of the samples lead to the crystallization of (Sr, Ba)WO4 and LEAZit (Low expansion alkaline earth zinc silicate with the composition (Sr, Ba)Zn2Si2O7). The incorporation of rare earth oxides into glass-ceramics containing LEAZit have not been described before. As shown by scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy, europium is incorporated in (Sr, Ba)WO4, to a lesser extend in (Sr, Ba)Zn2Si2O7 crystals, and also in the residual glass phase. Fluorescence spectra showed solely emissions due to Eu3+. Emissions by Eu2+ were not observed, neither in the crystallized samples nor in glass samples bubbled with forming gas. In the samples melted in air, the emission intensities due to Eu3+ were ten times larger than in the samples bubbled with forming gas. This is supposedly due to the higher absorption of Eu2+ in the reduced samples which does not result in Eu3+ luminescence. In comparison to the emission spectra of the glasses, the intensity ratios of the Eu3+ emission peaks change during the crystallization process. This indicates the incorporation of Eu3+ into the (Sr, Ba)WO4 crystals, most probably at (Sr, Ba) sites.

Asymmetric spectral broadening of sub-picosecond laser pulse in BaWO4 crystal: interplay of self-phase modulation, stimulated Raman scattering, and orientational Kerr nonlinearity.

Spectral broadening of 0.3 ps 515 nm laser pulse in a highly Raman-active BaWO4 crystal and fused silica demonstrates significantly different behavior with the incident pulse energy. While the broadening in fused silica is fairly symmetric with respect to the pump laser pulse wavelength, the Stokes wing broadening in the BaWO4 crystal is 2 times wider than that of anti-Stokes wing, the former demonstrating a step-like increase with the pulse energy. To the best of our knowledge, the obtained data are the first clear evidences of the following facts: (i) stimulated Raman scattering with sufficiently high efficiency of conversion to Stokes components slows down spectral broadening induced by self-phase modulation, and (ii) the mechanism of Kerr nonlinearity, which is responsible for self-phase modulation in BaWO4, is of orientational nature. The nonlinear refraction coefficient and its decay time following from our experiments with BaWO4 were estimated as n2≈6.4.10-15cm2/W and τNL≈0.35ps.

Вынужденное комбинационное рассеяние чирпированных импульсов титан-сапфирового лазера в тандеме кристаллов BaWO4

Stimulated Raman scattering of chirped Ti:Sapphire laser pulses in BaWO4 crystal A spectrum transformation of a chirped Ti:sapphire laser pulse stretched to 0.2 ns (with initial transform limited pulse duration of 90 fs) was experimentally studied in a tandem of BaWO4 crystals. To increase stimulated Raman scattering efficiency, broadband laser emission was used as a seed of this process. Efficiency of the Stokes component generation corresponding to the BaWO4 crystal phonon mode ν1(Ag)≈925 cm-1 reached ~ 10%. Generation of the Stokes component corresponding to the weaker phonon mode ν3(Eg)≈795 cm-1, and second Stokes components of ν1(Ag) and ν3(Eg) modes were also observed. Mechanisms reducing the stimulated Raman scattering efficiency for the ν1(Ag) mode are discussed.

Two-color emission of BaWO4 crystals pumped by 472 nm 0.11 ps laser pulses for frequency conversion into the longwave IR domain

The frequency conversion of Ti:Sapphire laser second harmonic pulses into two-color emission using a tandem of BaWO4 Raman crystals was experimentally demonstrated with an internal efficiency of ∼17%. The output emission contained symmetrical Stokes and anti-Stokes spectral components separated by 902 cm−1 with a full-width half-maximum bandwidth of ≥110 cm−1 each. The applicability of this two-color emission was numerically verified for the production of longwave infrared ultrashort pulses suitable for seeding a high-pressure CO2 laser amplifier via difference frequency generation in LiGaS2 crystal.

Ablation of BaWO4 Crystal by Ultrashort Laser Pulses

Ablation of BaWO4 Raman crystals with different impurity concentrations by ultrashort laser pulses was experimentally studied. Laser pulses with duration varying from 0.3 ps to 1.6 ps at wavelengths of 515 nm and 1030 nm were applied. A single-pulse optical damage threshold of the crystal surface changed from 1.3 J/cm2 to 4.2 J/cm2 depending on the laser pulse parameters and BaWO4 crystal purity. The optical damage threshold under multi-pulse irradiation was an order of magnitude less.

Highly efficient stimulated Raman scattering of sub-picosecond laser pulses in BaWO4 for 10.6 µm difference frequency generation.

Transient stimulated Raman scattering (SRS) of 0.3 ps 515 nm laser pulses in ${\rm BaWO_4}$BaWO4 crystal was experimentally demonstrated with efficiency up to ${\sim}{20}\% $∼20% for the Stokes component with a wavenumber of ${\sim}{925}\;{{\rm cm}^{ - 1}}$∼925cm-1 in a simple single-pass geometry. This anomalous high efficiency was obtained due to the laser pulse self-phase modulation resulting in spectral broadening and seeding the SRS. The applicability of seed pulse production for a high-pressure sub-picosecond ${{\rm CO}_2}$CO2 laser amplifier via difference frequency generation in ${{\rm LiGaS}_2}$LiGaS2 crystal was numerically verified.

Control of the spectral parameters of neodymium ions in anisotropic crystals with a scheelite structure

The spectral parameters of neodymium ions in anisotropic YLiF4, SrMoO4, PbMoO4, CaMoO4, SrWO4, and BaWO4 crystals with a scheelite structure are experimentally studied in the region of 1 μm. The luminescence spectra of neodymium ions at the 4F3/2 → 4I11/2 transition in these crystals exhibit pronounced polarisation dependences. It is shown that, using different orientations of the active laser element and separating a particular radiation polarisation, one can control the spectral dependence of the active medium gain, switch the laser wavelength, and create dual-wavelength lasers. A dual-wavelength Nd3+ : YLiF4 laser emitting at λ = 1047 and 1053 nm with identical linear polarisations and good temporal and spatial matching is demonstrated.

Site Symmetries of Cerium Ions in BaWO4 Single Crystals Codoped with Sodium Ions

Barium tungstate crystals are an interesting and relatively new medium for stimulated Raman scattering for applications in Raman shifters of laser radiation. Good quality BaWO4 crystals can be grown by Czochralski technique and doped with rare-earth ions. Doping with trivalent ions requires charge compensation which may be provided, for example, by structural defects or proper codoping with alkaline metal ions. Crystals possess scheelite-like structure with the space group I41/a. Results of the electron paramagnetic resonance (EPR) studies of BaWO4:Ce crystals and crystals codoped with Na are presented in this paper. EPR study was performed in the low temperature range 3 K < T < 50 K for all three planes of single crystals. The EPR spectra were attributed to cerium ions with a fictitious spin S = 1/2. The spin Hamilton (SH) parameters were determined. The values of SH parameters indicate the occurrence of paramagnetic centers in axial and low symmetry. The number of observed EPR lines depends on the selected plane. Based on the roadmap, we have found that one center with axial symmetry and at least two centers with low symmetry appear. The linewidth, ∆B, vs temperature dependence revealed increasing exponential tendency with increasing temperature. It shows 1 phonon at the lower temperatures and Raman + Orbach effect at the higher temperatures. Exponential change of the ΔB could be connected with the spin–lattice relaxation processes involving excited states of Ce3+ ions.

Limits and performances of a BaWO[formula omitted] single crystal

A Barium Tungstate single crystal (BaWO4) was produced using the low thermal gradient Czochralski technique. In this paper the results concerning its light emission and radioactive contaminants are presented. The aim of this work is to investigate the possibility to realize BaWO4 crystals with suitably features to study the double beta decay of 130Ba and 132Ba isotopes by the “source=detector” approach. The results show the limitations of a BaWO4 crystal as a scintillator and give some idea on how to overcome them in order to take profit from the potentiality of this single crystal.

Continuous-wave intracavity YVO4/BaWO4 Raman laser pumped by a wavelength-locked 878.9 nm laser diode

In this paper, the composite Nd:YVO4 laser crystal is in-band pumped by a wavelength-locked laser diode at 878.9 nm, with the purpose of reducing thermal effects and improving pump absorption simultaneously. By using the YVO4 and BaWO4 crystals as Raman media, the influences of crystal properties, resonator structure and stability on the performance of continuous-wave intracavity Raman laser are investigated experimentally and theoretically. The results show that the resonator stability greatly affects laser performance due to the long cavity length of intracavity Raman laser. By choosing the Raman medium with high Raman gain, we can not only obtain higher Raman conversion efficiency, but also reduce the thermal effect to a certain extent. Furthermore, the smaller the curvature radius of the output mirror in the plano-concave cavity structure, the greater the power density of the fundamental laser in the Raman crystal is and the wider the dynamic stability region of the resonator, and hence the higher output power of the Raman laser can be achieved. Finally, by using 30-mm BaWO4 crystal as Raman medium, a highest Raman output of 3.02 W is obtained at a pump power of 25.1 W, corresponding to a diode-to-Stokes optical conversion efficiency of 12%.

In Situ TEM and AFM Investigation of Morphological Controls during the Growth of Single Crystal BaWO4

Barium tungstate (BaWO4) is a widely investigated inorganic optical material due to its attractive emission properties. Because those properties strongly depend on crystal structure and morphology, numerous approaches to controlling growth have been pursued. However, an understanding of the growth mechanisms that lead to the wide range of morphologies obtained to date is largely lacking, and most attempts to develop that understanding have been based on post-growth analyses. Significantly, such analyses have led to the conclusion that certain BaWO4 crystal morphologies result from a nonclassical growth process of oriented attachment. In this work, we systematically varied the morphology of BaWO4 crystals by adjusting the relative concentrations of solute, water, and ethanol. We then explored the growth mechanism leading to the observed range of morphologies through in situ TEM and in situ AFM. We find that even the most complex BaWO4 morphologies occur through purely classical growth mechanisms largely controlled by the content of solute and ethanol. The latter acts as an impurity to poison growth at low concentrations and low solute levels, but leads to development of growth instabilities and eventual dendritic growth at high alcohol and moderate solute concentrations by driving up the supersaturation. These findings also highlight the necessity of in situ experiments to interpret ex situ observations of crystal growth and decipher the controlling mechanisms.

(Invited) Studies of Luminescence Efficiency of BaWO4:Ce Crystals

Barium tungstate crystals are interesting and relatively new medium for stimulated Raman scattering for applications in Raman shifters of laser radiation [1]. Good quality BaWO4 crystals can be grown by Czochralski technique and doped with rare-earth ions. Doping with trivalent ions require charge compensation which may be provided, for example, by structural defects or proper codoping with alkaline metal ions. Crystals posses sheelite structure with the space group I41/a. Results of studies of BaWO4:Ce crystals and crystals codoped with Na are presented in this work. Optical absorption in the vis-UV and infrared range and Raman spectra has been studied. In the UV the crystals shows typical absorption for Ce3+ with two bands peaked at 320 nm and 285 nm, which are associated with lowest energy 4f-5d transitions. However only very weak Ce3+ luminescence is observed even at 10 K, contrary to some reports on this subject [2]. In the mid-infrared range several absorption bands are visible which may be associated with lattice absorption. Besides of lines associated with single TO phonon modes in the range up to 1000 cm-1 the additional lines appear around 1700 cm-1 and 2600 cm-1. Origin of these lines is discussed in this work. Raman spectra at room temperature show very sharp lines with the highest energy line at 927 cm-1. Some additional much broader lines are observed above 1200 cm-1 [3]. Their origin is most probably associated with combination of various phonon modes. In addition to that very sharp lines due to the 4f-4f transitions in the Ce3+ ions are visible around 2200 cm-1 and 2350 cm-1. There are at least four of these lines in each group which are related to the multisite structure of RE doping in these crystals. The crystal field analysis allows identifying nature of these lines. The transitions associated with Ce3+ ions are very strongly coupled with the lattice and they undergo very important broadening with increase of temperature. This is explained as a result of electron-phonon coupling, enhanced by the matching inter-level spacing of the 2F7/2 state with phonon energies. BaWO4 has the largest energy gap among several tungstates with scheelite structure [4]. It is in good agreement with the values of ionic radii of cations constituting the host. The conduction band is dominated by the W6+ 5d states. On the other hand the luminescence efficiency of Ce3+ dopand in BaWO4 is the lowest among the (Ca, Sr,­ Ba)WO­4, which have smaller band-gap than BaWO4. It may be associated with resonant position of the lowest 5d state of Ce3+ in the conduction band or with the thermally activated crossover between the 4f 5d state and the 4f states. The large band gap of BaWO4 rather shows that the latter mechanism is operative as the luminescence quenching mechanism. High pressure luminescence studies in diamond anvil cell, which should help to resolve this question will be presented in this work. Acknowledgements: This work was supported by the Polish National Science Center (project 2015/17/B/ST5/01658)