Robert Král

Growth and Luminescence Properties of Single Crystals Prepared by Modified Micro-Pulling-Down Method

This paper reports on the preparation and growth of Eu-doped strontium iodide (SrI₂) single crystals by modified micro-pulling-down method. Influence of the Eu^{2 +} and Eu^{3 +} (introduced in form of EuI₂ and EuCl₃) on the luminescence and scintillation properties is analyzed. Sufficient optical quality of prepared single crystals was obtained allowing further characterization of their absorption, radio- and photoluminescence spectra. Photoluminescence and scintillation decay time of grown SrI₂ crystals irradiated under γ-ray source were evaluated as well.

Phosphate content influence on structural, spectroscopic, and lasing properties of Er,Yb-doped potassium-lanthanum phosphate glasses

Abstract. The influence of the phosphorus pentoxide (P2O5) content on the material properties of Er,Yb-doped potassium-lanthanum phosphate glass was studied. Glass samples of the following nominal composition 35.0K2O-6.8Yb2O3-8.0La2O3-0.2Er2O3-50.0P2O5 (in mol%) were prepared from starting materials mixed with five additional amounts of P2O5 (0, 7.5, 15.0, 30.0, and 45.0 mol% related to the nominal glass composition). The P2O5 addition influence on properties of prepared glasses was studied using Raman, absorption, and fluorescence spectroscopy. The glass residual IR absorption and Judd–Ofelt intensity parameters together with absorption and emission cross sections were estimated. The results showed the increasing polymerization of glass and the P─O bond shortening with P2O5 content increase. The spectroscopy of Er3+ and Yb3+ ions was affected only marginally by the glass composition. It was found that fluorescence decay time corresponding to upper-laser-level I413/2 increased with the decrease of P2O5 content in the glasses, which was related to increasing OH− contamination of the glass. The laser action at 1.53  μm under 975-nm pulsed laser diode pumping was successfully demonstrated. Low threshold and laser slope efficiency up to 21% in respect to absorbed pumping power were obtained.

Er-doped ortho- and metha-phosphate glassy mixtures for 1.54 μm laser construction

The goal of our work was preparation and investigation of Er-doped potassium ytterbium lanthanum orthoand metha-phosphate glassy mixtures developed as a solid-state laser active medium. The tested samples were prepared by rapid quenching of molten mixture of starting K2CO3, YbPO4, LaPO4, YbPO4 and P2O5. Their cations molar concentration was as follows n(K) = 0.7, n(Yb) = 0.135, n(La) = 0.16 and n(Er) = 0.005 and it was the same in all tested samples. The additions of the P2O5 to the individual starting charges were batched so the following compositions of resulting glasses should be obtained: (i) pure meta-phosphate, (ii) mixture of 80 mol% of meta-phosphate and 20 mol% of ortho-phosphate, (iii) mixture of 50 mol% of meta-phosphate and 50 mol% of ortho-phosphate, (iv) mixture of 25 mol% of meta-phosphate and 75 mol% of ortho-phosphate, and (v) pure ortho-phosphate. The glassy samples were prepared in the form of discs about 8mm in diameter and 2mm in height. The absorption spectra were measured in broad range from 200 up to 2500 nm to identify possible impurities, mainly the residual OH-absorption and to calculate absorption cross-section for pumping and laser transition 4I15/2 → 4I11/2 and 4I13/2 → 4I15/2, respectively. For particular transitions fluorescence spectra and fluorescence decay time were recorded simultaneously. It was found that the fluorescence decay time, corresponding to upper laser level 4I13/2 depopulation, progressively increases with the content of orthophosphate in glass composition starting from 2ms for sample (i) up to 8ms for sample (iv). The laser action at 1.54 μm under 975nm pulsed laser diode pumping was successfully demonstrated using the sample with the longest upper laser level lifetime.

Luminescence and Scintillation Properties of Scintillators Based on Orthorhombic and Monoclinic BaLu

The rare-earth-doped orthorhombic BaLu₂F₈ vacuum-ultra-violet scintillator crystals have been studied. The fast emission around 185 nm with a decay time of several nanoseconds was due to the allowed 5d-4f transition of the Nd³⁺ ion. The high temperature phase BaLu₂F₈ orthorhombic crystals have been prepared by micro-pulling-down method. Unfortunately, no 5d-4f emission (neither from Nd³⁺ nor from Tm³⁺) was observed in the crystals under excitation by ionizing radiation at all. This was explained by preferential energy transfer from the host to the lattice defect states. Further improvement of the scintillation efficiency by facilitating the energy transfer from the host matrix to the Nd³⁺ luminescence center by Tm³⁺-codoping was attempted. The energy transfer from the Tm³⁺ ions to the Nd³⁺ ones has been proved, however, no improvement of the overall scintillation efficiency was observed.

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Glass matrix doped with rare-earth ions is a promising laser active medium for high power laser systems. Due to amorphous structure of glasses the absorption and emission spectra lines are broader in comparison with crystalline materials thus pumping radiation can be absorbed efficiently, moreover much broader gain bandwidth is suitable for generation of ultra-short pulses. Another advantage of the glass matrix is the possibility to fabricate large volume ingots and simultaneously preservation of sufficient optical quality. The lower thermal conductivity of glasses can be compensated by geometry of the active medium for instance shaped into fibres or discs. We present temperature dependence of spectroscopic and laser properties of newly developed Er, Yb - doped potassium-lanthanum phosphate glass, which is appropriate for generation of radiation at 1.53 μm. The sample of Er,Yb:KLaP glassy mixture was cut into disc shape with dimensions of 2.5 mm (thickness) and 5 mm (diameter) and its faces were polished plan-parallelly without being anti-reflection coated. The temperature dependence of the transmission and emission spectra Er,Yb:KLaP together with the fluorescence decay time were measured the temperature range from 80 to 400 K. The fluorescence lifetime of manifold 4I13/2 (upper laser level) prolonged and the intensity of up-conversion radiation decreased with decreasing temperature. The longitudinal excitation of Er,Yb:KLaP was carried out by a fibre-coupled laser diode (pulse duration 2 ms, repetition rate 10 Hz, pump wavelength 969 nm). Laser resonator was hemispherical, with flat pumping mirror (HR @ 1.5 μm) and spherical output coupler (R = 98 % @ 1.5 - 1.6 μm). The Er,Yb:KLaP glass laser properties were investigated in the temperature range 80 - 300 K. The highest slope efficiency with respect to absorbed pumped power was 6.1 % at 80 K. The maximum output of peak amplitude power was 0.71 W at 80 K, i.e. 1.2 times higher than at 300 K. Tunability of laser wavelength at 80 K in range 1528 - 1552 nm was obtained using MgF2 birefringent filter. From our measurement it can be concluded, that spectroscopic and laser properties of newly developed Er,Yb:KLaP glass are slightly temperature dependent.

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Phase transitions and crystal structures of η’’-Cu(3+x)Si and η’’’-Cu(3+x)Si Cinthia Antunes Correa1,2, Morgane Poupon1, Olivier Pérez2, Robert Král1, Petra Zemenová1, Jérôme Lecourt2, Jaromír Kopeček1, Petr Brázda1, Mariana Klementová1, Lukáš Palatinus1 1Department Of Structure Analysis Institute Of Physics Of The Czech Academy Of, Prague, Czech Republic, 2Department of Physics of Materials, Charles University in Prague, Ke Karlovu 5, Prague 2, Czechia E-mail: cinthiacac@gmail.com