Comparison of fluorescence spectra of Yb:Y3Al5O12 and Yb:YAlO3 single crystals

Yb:Y₃Al₅O₁₂ (Yb:YAG) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 25 at.%, 50 at.%, 100 at.% and Yb:YAlO₃ (Yb:YAP) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 30 at.% were grown by the Czochralski process. The fluorescence spectra of these crystals and the effects of self-absorption on the shape of the fluorescence spectra were studied. Through comparing the fluorescence spectra of Yb:YAG and Yb:YAP, all results indicate that the effects of self-absorption on the fluorescence spectra of Yb:YAP are remarkably stronger than that of Yb:YAG at the same Yb concentration.     

Spectroscopic and laser characterization of Yb,Tm:KLu(WO4)2 crystal

We report on a comprehensive spectroscopic and laser characterization of monoclinic Yb,Tm:KLu(WO₄)₂ crystals. Stimulated-emission cross-section spectra corresponding to the ³F₄ → ³H₆ transition of Tm³⁺ ions are determined. The radiative lifetime of the ³F₄ state of Tm³⁺ ions is 0.82 ms. The maximum Yb³⁺ → Tm³⁺ energy transfer efficiency is 83.9% for 5 at.% Yb – 8 at.% Tm doping. The fractional heat loading for Yb,Tm:KLu(WO₄)₂ is 0.45 ± 0.05. Using a hemispherical cavity and 5 at.% Yb – 6 at.% Tm doped crystal, a maximum CW power of 227 mW is achieved at 1.983–2.011 μm with a maximum slope efficiency η = 14%. In the microchip laser set-up, the highest slope efficiency is 20% for a 5 at.% Yb– 8 at.% Tm doped crystal with a maximum output power of 201 mW at 1.99–2.007 μm. Operation of Yb,Tm:KLu(WO₄)₂ as a vibronic laser emitting at 2.081–2.093 μm is also demonstrated.     

Optical and scintillation properties of SrI2:Yb2+

The luminescence and scintillation properties of SrI₂:0.5%Yb²⁺ have been investigated. SrI₂:Yb single crystals were grown by the vertical Bridgeman method from the melt. They showed a light yield of 38,400 ph/MeV and energy resolution of 12.5% for the 662 keV full absorption peak. Yb²⁺ photoluminescence intensity and decay time were studied between 78 and 600 K. Two emission bands centered at 418 and 446 nm were observed and ascribed to spin-allowed and spin-forbidden Yb²⁺ 5d-4f transitions, respectively. Their corresponding room-temperature decay time constants are 710 ns and 77 μs. Both, the emission intensities and the decay time constants vary with temperature. The obtained results were interpreted using a model of self-absorption of Yb²⁺ emission and a model of non-radiative relaxation of the electron from the low spin to the high spin 4f¹³5d Yb²⁺ excited states. The radiative lifetime of the low spin Yb²⁺ excited state was determined as 400 ns.     

Determination of light output function and angle dependent correction for a stilbene crystal scintillation neutron spectrometer

In addition to liquid NE213 scintillators also stilbene solid crystals are applied traditionally for fast neutron spectrometry. A proper evaluation of experimental data provides a precise determination of the nonlinear light output function for the given scintillator/photomultiplier combination, and for stilbene additionally an adequate correction of the anisotropy effect. Calibration experiments with monoenergetic neutrons (1.2, 2.5, 5.0, 13.95, 14.8, 19.0 MeV) and various neutron incidence angles were carried out at the accelerator facility of the PTB Braunschweig for two cylindrical scintillators (∅30 mm×25 mm, ∅10 mm×10 mm). An improved analytic light output function as well as an adequate angle dependent correction function were derived.     

Growth,thermal and optical properties of Yb:GdYAl3(BO3)4 crystal

Single crystal of Yb:GdYAl₃(BO₃)₄(Yb:GdYAB) has been grown by the flux method. The structure of Yb:GdYAB crystal has been determined by X-ray diffraction analysis. The experiment show that the crystal has the same structure as that of YAl₃(BO₃)₄ crystal and its unit cell constants have been measured to be a = 9.30146 Å, c = 7.24164 Å, Vol = 542.59 Å³. The absorption and fluorescence spectrum of Yb:GdYAl₃(BO₃)₄ crystal have also been measured at room temperature. In the absorption spectra, there are two absorption bands at 938 nm and 974 nm, respectively, which is suitable for InGaAs diode laser pumping. In the fluorescence spectra, there are two fluorescence peaks at 992 and 1040 nm. The thermal properties of Yb:GdYAl₃(BO₃)₄ crystal have been studied for the first time. The thermal expansion coefficient along c-axis is almost 5.4 times larger than that along a-axis. The specific heat of the crystal has been measured to be 0.77 J/g °C at room temperature. The calculated thermal conductivity is 5.26 Wm⁻¹ K⁻¹ along a-direction.     

Efficient tri-wavelength actively Q-switched Yb:GAGG laser

An efficient acousto-optic Q-switched Yb-doped Gd₃Al_xGa_5−xO₁₂ (GAGG) (x = 0.5) laser is demonstrated. Under the absorbed pump power of 7.4 W, the maximum average output power of 1.4 W is obtained at the pulse repletion rate of 1 kHz , with the slope efficiency as high as 32%. The pulse width of 40 ns is achieved with the pulse energy and peak power of 1.4 mJ and 35 kW, respectively. What’ more, the output spectrum shows itself tri-wavelength in either CW or Q-switching mode. To our knowledge, this is the first time for realizing simultaneous tri-wavelength Yb:GAGG laser actively Q-switched operation.     

Luminescence properties of CsI:Eu crystals

The work is aimed to research of CsI:Eu single crystals’ luminescence at different excitations in the wide temperature range (from 8 to 300 K) and admixture concentration from 10⁻⁴% up to 10⁻¹% of Eu²⁺ ions. This study was directed to separation of different luminescence mechanisms and luminescence quenching explore. It is found that due to the temperature quenching the room temperature yield is decreased in order of magnitude even for heavy Eu doped CsI crystals. Intense low temperature luminescence band at 441–448 nm connected with Eu²⁺ 4f⁶ 5d → 4f⁷ radiative transition is observed. The results obtained show that the excitonic mechanism plays the main role in energy transfer process to Eu²⁺ ions and this is the reason for relatively low light yield of this emission at RT. The nature of the additional emission bands (410, 450, 480 and 500 nm) are apparently caused by the significant non-isomorphism of cations and/or by the presence of oxygen-containing admixtures.     

Growth and characterization of polycrystalline lanthanide halide scintillators

We are exploring a novel time- and cost-efficient approach to produce robust, large-volume polycrystalline lanthanide halide scintillators using a hot wall evaporation (HWE) technique. To date, we have fabricated LaBr₃:Ce and LaCl₃:Ce films (slabs) measuring up to 7 cm in diameter and 7+ mm in thickness (20–25 cm³ in volume) on quartz substrates. These polycrystalline scintillators exhibit very bright emissions approaching those exhibited by their melt-grown crystal counterparts. Scanning electron micrographs (SEMs) and X-ray diffraction analysis confirm polycrystalline growth with columnar structures, both of which help in light piping, thereby contributing to the observed high light yields. The new scintillators also exhibit good energy resolution for γ-rays over the tested range of 60 keV (²⁴¹Am) to 662 keV (¹³⁷Cs), although they have not yet reached that of the corresponding crystals. The measured response linearity over the same energy range is comparable for both our HWE synthesized films and melt-grown commercially-available reference crystals. Similar consistency in response is also observed in terms of their decay time and afterglow behaviors. The data collected so far demonstrate that our HWE technique permits the rapid creation of scintillators with desired structural and compositional characteristics, without the introduction of appreciable defects, and yields material performance equivalent to or approaching that of crystals. Consequently, the deposition parameters may be manipulated to tailor the physical and scintillation performance of the resulting structures, while achieving a cost per unit volume that is substantially lower than that of crystals. In turn, this promises to allow the use of these novel scintillation materials in such applications as SPECT, PET, room-temperature radioisotope identification and homeland security, where large volumes of materials in a wide variety of shapes and sizes are needed. This paper describes our growth and testing of polycrystalline LaBr₃:Ce scintillators and provides comparative characterizations of their performance with crystals.     

Comparative study of ceramic and single crystal Ce:GAGG scintillator

Recent study revealed that single crystal Ce:Gd₃(Al,Ga)₅O₁₂ (Ce:GAGG) showed good scintillation response under γ-ray exposure. We discover here that ceramic Ce:GAGG scintillator exhibited better performance than the single crystal counterpart. We developed Ce 1% doped ceramic and single crystal GAGG scintillators with 1 mm thick and compared their properties. In radioluminescence spectra, they showed intense emission peaking at 530 nm due to Ce³⁺ 5d–4f transition. The ¹³⁷Cs γ-ray induced light yields of ceramic and single crystal resulted 70 000 ph/MeV and 46 000 ph/MeV with primary decay times of 165 and 143 ns, respectively. At present, the observed light yield was the brightest in oxide scintillators.     

Fabrication of (Ca + Yb)- and (Ca + Sr)-stabilized α-SiAlON by combustion synthesis

In this paper, (Ca + Yb)- and (Ca + Sr)-stabilized α-SiAlON powders were fabricated by combustion synthesis. The influence of Ca²⁺ incorporation on the phase composition and grain morphology of combustion products was discussed. The experimental results showed that with the incorporation of Ca²⁺ well-developed rod-like (Ca + Yb) α-SiAlON crystals could be produced. It was also found that, when only Sr²⁺ was used as stabilizing cation, the reaction product was (α + β)-SiAlON composite, in which β-SiAlON was the predominant phase and the relative content of α-SiAlON was low. With the incorporation of Ca²⁺, however, both the relative content and the lattice parameters of α-SiAlON were clearly increased. These results indicated that the incorporation of Ca²⁺ could assist Sr²⁺ into the α-SiAlON lattice structure.     

Growth of CuInTe2 single crystals by iodine transport and their characterization

The single crystals with stoichiometry close to 1:1:2 of CuInTe₂ (CIT) have been grown by chemical vapor transport (CVT) technique using iodine as the transporting agent at different growth temperatures. Single crystal X-ray diffraction studies have confirmed the chalcopyrite structure for the grown crystals and the volume of unit cell is found to be the same for the crystals grown at different conditions. Energy dispersive X-ray (EDAX) analysis of CIT single crystals grown shows almost the same stoichiometric compositions. Scanning electron microscope (SEM) analysis reveals kink, step and layer patterns on the surface of CIT single crystals depending on the growth temperatures. The optical absorption spectra of as-grown CIT single crystals grown at different conditions show that they have same band gap energies (1.0405 eV). Raman spectra exhibit a high intensity peak of A₁ mode at 123 cm^?1. Annealed at 473 K in nitrogen atmosphere for 40 h CIT single crystals have higher hole mobility (105.6 cm²V^?1s^?1₎ and hole concentration (23.28 × 10¹⁷ cm^?3) compared with values of hole mobility (63.69 cm² V^?1 s^?1) and hole concentration (6.99 × 10¹⁵ cm^?3) of the as-grown CIT single crystals.     

Ultralow-concentration Sm codoping in CsI:Tl scintillator: A case of little things can make a big difference

CsI:Tl scintillators were hindered from computer tomography and high-speed imaging applications by a serious afterglow problem. In this study, the effects of ultralow-concentration Sm codoping on the scintillation characteristics of CsI:Tl were investigated. Pulsed X-ray excited afterglow after 50 ms in 0.005 mol% Sm-codoped CsI:Tl was lowered by over one order of magnitude in comparison with Sm-free one. The beneficial effects of ultralow-concentration Sm codoping also appeared to be maintaining the light yield and energy resolution. The light yield and energy resolution after 0.005 mol% Sm codoping were 71,700 ± 6000 photons/MeV and 6.9% at 662 keV, respectively.     

Effect of processing parameters on structural,morphological and optical Y2O3:Yb3+/Ho3+ powders characteristics

Rod-like and flake-like up-converting Y₂O₃:Yb³⁺/Ho³⁺ particles which are composed of nanoparticles with size less than 100 nm, are prepared by a simple hydrothermal processing at 473 K (3 h) followed by additional thermal treatment at 1373 K (3 and 12 h). The effect of precursor pH value on the formation of Y₂O₃:Yb³⁺/Ho³⁺ is followed through X-ray powder diffractometry (XRPD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Structural refinement confirms formation of the cubic bixbyte structure (S.G. Ia-3) with the non-uniform accommodation of dopants at C₂ and S₆ cationic sites. Under 978 nm laser excitation, strong green (530–570 nm) up-conversion is observed in all samples. The emission shows a decrease in intensity with an increase in external temperature, indicating FIR (fluorescence intensity ratio) based temperature sensing behavior of 0.52% for the ⁵F₄ → ⁵I₈/⁵S₂ → ⁵I₈ transitions.     

Creep deformation and crack growth rates of a super α2 titanium aluminide alloy

The influences of stress and temperature on creep deformation behavior and the creep crack growth rates of the super α₂ Ti₃Al alloy were investigated with respect to its safe application at high temperatures. In a temperature range of 1033–1093 K at low applied stress levels, the stress exponent was equal to 1.5. At an intermediate stress range (10^?3 < σ/E < 3 × 10^?3), a stress exponent of 3.3 was observed. As the applied stress was increased, the stress exponent changed from 3.3 to 4.4. The high temperature crack growth rate of the Ti₃Al alloy can be correlated with stress intensity factor K rather than C1 at 1033 K due to environmental embrittlement.     

Transcrystallization kinetics at the poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/hemp fibre interface

Transcrystallization of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) on hemp fibres was investigated using polarized optical microscopy. Nucleation and growth of crystals of the transcrystalline region (TCR) were measured during isothermal treatments of samples located in a hot stage. The nucleation rate, nucleation density at saturation and growth rate of the TCR, were determined at crystallization temperatures between 320 and 363 K.The growth rate of crystals in the TCR is the same as that of spherulites in the bulk. Nucleation at the hemp fibres was properly described in terms of the heterogeneous nucleation theory and the transcrystalline growth agrees with the kinetics theory of polymer crystallization. From the empirical data we determined that the interfacial free energy difference function of PHBV on hemp fibres is Δσ = 4.4 erg/cm², lower than the one in the bulk. This indicates that the substrate is not neutral but contributes to the formation of the TCR.From the intensity of the depolarization light, measured under different cooling rates, we determined the activation energies for crystallization in the bulk and transcrystallization.     

Synthesis of Dy doped Yb0.1Ca0.9MnO3 ceramics with a high relative density and their thermoelectric properties

Perovskite-type Yb_0.1Ca_0.9?xDy_xMnO₃ ceramics were synthesized by solid state reaction. Their microstructures were characterized and the thermoelectric properties were evaluated between 300 K and 1100 K. Each of sample exhibits single phase with orthorhombic structure. All samples have high relative densities, and their values are between 95% and 97%, which is consistent with the SEM image. The electrical resistivity shows a typical metallic conductivity behavior. Lowest electrical resistivity 2.36 mΩ cm is achieved at room temperature, and the variation of electrical resistivity is not evident in whole measured temperature range. The Seebeck coefficients are negative, indicating an n-type conduction. The highest power factor 310 μW/(K² m) is obtained for the sample with x = 0.02. The thermal conductivity is decreased by the difference in the mass between the Ca²⁺ and Dy³⁺ ions, especially in the heavy doped samples. The highest figure of merit is 0.11 at 1069 K for the sample with x = 0.02.     

Optical and EPR spectroscopy of Zn:Cr:ZnSe and Zn:Fe:ZnSe crystals

Optical and EPR characterization of Cr and Fe doped ZnSe crystals annealed in Zn vapor revealed a strong bleaching of the divalent state of transition metal ions. Photo induced EPR kinetics were studied in 20–80 K temperature range. Analysis of time-dependent data reveals Cr¹⁺ signal rise time decreases with increasing temperature. The non-exponential decay of Cr¹⁺ concentration were analyzed using Auger-type recombination process. The photoluminescence quantum yield of Cr²⁺ ions at ⁵E(D) → ⁵T₂(D) mid-IR transition excited via chromium ionization process was measured to be close to 100%.     

Xe irradiation-induced defects in CuInSe2 by phase resolved photoacoustic spectroscopy

We report a study on the optical properties of 40 keV Xe⁺ implants with a dose of 5 × 10¹⁶ ions/cm² into p-type conducting CuInSe₂ single crystals using the phase resolved method of the photoacoustic spectroscopy (PAS) technique. Photoacoustic spectra have been measured in the photon energy range 0.7 < hν < 1.4 eV prior and after implantation at various phase angles using a high resolution fully computerized spectrometer. Once the spectra separation is carried out, an analysis on the impact of Xe⁺ on the defect structure of CuInSe₂ is presented. The results obtained here are discussed in the light of current reported literature.     

First observation of the reversible O3 ↔ P2 phase transition: Crystal structure of the quenched high-temperature phase Na0.74Ni0.58Sb0.42O2

According to thermal expansion data, O3-type phase Na_xNi_(1+x)/3Sb_(2−x)/3O₂ (x ≈ 0.8) undergoes at ca. 1270 K a reversible transition to a less dense form. The high-temperature phase quenched to liquid nitrogen belongs to P2 type, space group P6₃/mmc (no. 194), a = 3.0123 Å(2), c = 11.2264 Å(7) for x ≈ 0.74 at 298 K. The stabilisation of P2 versus O3-type structure at high temperatures seems to be due to alkali distribution over greater number of sites thus increasing entropy and decreasing Na⁺–Na⁺ repulsion.     

Optical functions and time-resolved luminescence of lithium hydride single crystals upon far-ultraviolet excitation

Low-temperature reflection spectra of lithium hydride (LiH) single crystals cleaved in ultrahigh vacuum (3 × 10⁻¹⁰ Torr, T = 10 K), were recorded using synchrotron radiation in vacuum ultraviolet spectral region. Based on the obtained experimental data, the optical functions of LiH in the energy range from 3.7 to 35 eV were analyzed using the Kramers–Krönig relations. Time-resolved photoluminescence excitation spectra were studied in detail for the near edge free exciton-phonon luminescence at 4.67 eV and photoluminescence at 2.4 eV due to the Bi³⁺ impurity centers. The effect of multiplication of electronic excitations due to inelastic scattering of hot photoelectrons and hot photoholes was revealed at photon energies above 15 eV (more than 3E_g). It was found that the radiative lifetime for free excitons in LiH at 4.67 eV is less than 1 ns as low temperatures as at 10 K. The interpretation of the electronic band structure of lithium hydride in the ultraviolet and vacuum ultraviolet spectral regions were carried out on the basis of the present experimental results with the involvement of the available band structure calculations.