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Photo- and thermally stimulated luminescence characteristics, as well as photostimulated defects creation processes are investigated in the 85–510 K temperature range for the Li+ co-doped single crystal of Gd3(Ga,Al)5O12:Ce (GAGG:Ce). The results for GAGG:Ce,Li are compared with the corresponding characteristics of the GAGG:Ce and GAGG:Ce, Mg single crystals of similar host composition. The influence of Li+ and Mg2+ ions on the Ce3+ - related emission band position, temperature dependence of the photoluminescence intensity, efficiency of the Gd3+ → Ce3+ energy transfer, intensity and decay kinetics of the afterglow, and thermally stimulated luminescence characteristics is revealed. The presence of the Ce3+ → Ce4+ conversion in GAGG:Ce,Li is confirmed. The conclusion is made that, unlike GAGG:Ce,Mg, the compensation of the Li+ excess negative charge in GAGG:Ce,Li occurs due to both the Ce3+ → Ce4+ conversion and the effective formation of intrinsic crystal lattice defects. The origin of these defects as well as the origin of the electron and hole trapping centers in the GAGG:Ce,Li crystal are discussed.

LiAlO2 can be considered as a good candidate for neutron scintillator host matrix due to the high Li content and high energy deposit of reaction of 6Li isotope with neutron. Low density (2.75 g/cm3) is of advantage to achieve low sensitivity of background gamma radiation in mixed radiation fields. Ti-doped LiAlO2 shows relatively high neutron light yield comparable to that of the Li-glass standard neutron scintillator. At room temperature the Ti4+ charge transfer (CT) luminescence at 375 nm with the 2.5 µs decay time is the leading emission process. Besides the CT luminescence host emission at shorter wavelengths (350 nm), defect-related emission at 440 nm and impurity Fe3+ luminescence at 740 nm are observed. Their emission spectra, excitation spectra and decay kinetics including temperature dependences were studied. Luminescence mechanism in the material and interplay between the luminescence processes are discussed together with possible role of nonstoichiometry and Mg codoping.

Photoluminescence, thermally stimulated luminescence (TSL) and EPR characteristics of the Ce3+-doped single crystals of multicomponent Y1Lu2GaxAl5-xO12 and Lu3GaxAl5-xO12 garnets with different Ga contents (x = 0, 1, 2, 3, 4, 5) excited in the Ce3+-related absorption bands are investigated in the 9–500 K temperature range. The distribution of Ga3+ and Al3+ ions in the crystal lattice is determined by the NMR method. The relative number of Ga3+ ions in the tetrahedral crystal lattice sites, the maxima positions of the TSL glow curve peaks and the corresponding trap depths are found to decrease linearly with the increasing Ga content. At the same time, the reduction of the activation energy Ea of the TSL glow curve peaks creation under irradiation in the 4f–5d1 absorption band of Ce3+ is strongly nonlinear. To explain this effect, the suggestion is made that Ea is the energy distance between the excited 5d1 level of Ce3+ and a defect level located between the 5d1 level and the bottom of the conduction band and arising from the Ga3+ ion perturbed by the nearest neighboring Ce3+ ion. The electrons thermally released from the excited Ce3+ ions are suggested to be trapped at the perturbed Ga3+ ions resulting in the appearance of electron Ga2+ centers. In spite of the fact that the paramagnetic Ga2+ ions were not detected by EPR, the process described above has been found for Fe3+ impurity ions, namely, the electron transfer from the 5d1 excited level of Ce3+ to Fe3+ is directly detected by EPR.

Solid state lanthanide doped lasers primarily operate in the ultraviolet, visible, near infrared and short-wavelength infrared out to around 2.1 µm. At longer wavelengths, the transitions in conventional oxide crystal and glass materials become susceptible to multiphonon quenching due to their relatively large phonon energy. The use of low phonon materials can minimize the nonradiative quenching, opening up possibilities for solid state lanthanide lasers operating in the mid-infrared (MIR). This provides motivation to study the spectroscopy of lanthanide ions in bromide, chloride and fluoride materials, which have relatively low phonon energies. In this article, the MIR spectroscopy or praseodymium ions in five different host materials is studied, specifically KPb 2 Br 5 (KPB), LaF 3 , KYF 4 (KYF), BaY 2 F 8 (BYF) and YLiF 4 (YLF) host crystals. The MIR emission cross sections have been measured from 3 to 6 µm and reciprocity of absorption and emission is utilized to validate the results. The lifetime dynamics in the MIR are covered for various pump and emission wavelengths. Results are also presented on MIR emission from 6.5 to 8.5 µm in a Pr: KPB crystal, which, to the best of the authors knowledge, is the first such measurement of luminescence in this wavelength range that has been published. © 2018 Elsevier Science. All rights reserved

Non-linear absorption spectroscopy in pump and probe configuration has been used to test the population of non-equilibrium carriers in Ce-doped Y$_{3}$Al$_{5}$O_${12}$ (YAG), Lu3Al5O12 (LuAG), and Gd3AlxGa(5-x)O12 (GAGG) crystals with and without codoping by Mg2+ ions. A faster rise time of the induced optical density has been observed in all crystals codoped with Mg with respect to that in Mg-free samples. A significant difference in the time evolution of the differential optical density in GAGG with respect to YAG and LuAG crystals has also been measured. In both GAGG:Ce and GAGG:Ce,Mg an absorption band with maximum in the blue-green range and a decay time of 1.4 ps is present. This band is due to the absorption by free electrons before they are trapped or re-captured by Ce3+ ions. A broad absorption band in the yellow-red region with very short rise time and a decay time longer than 150 ps has been observed in all the Ce-doped garnets under study and can be attributed to the absorption from the Ce3+ excited states.

This research dealt with the investigation of luminescence properties of undoped and La doped Lu3Al5O12 single crystals using synchrotron radiation excitation. The influence of La3+ doping on the exciton emission and decay kinetics properties of Lu3Al5O12 single crystals was studied at 7 K and room temperature. Undoped and La3+ doped samples showed bright luminescence in the UV spectral range. In undoped Lu3Al5O12 crystal, the UV luminescence was associated with LuAl antisite defects, which acted as luminescence centers in the band peaked at 295 nm La3+ doping in Lu3Al5O12 crystals caused the formation of LaLu luminescence centers and increased the concentration of LuAl antisite defects. Therefore, the luminescence of La doped Lu3Al5O12 crystals was a superposition of the emission coming from both LaLu and LuAl centers. Furthermore, La3+ doping significantly enhanced the intensity and shifted the UV luminescence of Lu3Al5O12 crystals towards higher energy due to the dominant contribution of LaLu luminescent centers in the band peaked at 275 nm. The decay time of heavily La3+ doped LuAG crystals was also significantly accelerated due to the prevailing luminescence of LaLu centers.

The influence of the Mg 2+ codoping and annealing in the air on the scintillation and luminescence characteristics of Pr-doped lutetium-aluminum garnet crystals (LuAG) was studied to find a possible positive effect of Pr 4+ . The overall scintillation efficiency under X-ray excitation of the annealed Pr- doped samples decreased with increasing Mg concentration. This was explained by increased overlap of the Pr 3+ 5d-4f emission with the charge-transfer (CT) absorption band of the Pr 4+ ion stabilized by Mg 2+ . This absorption caused even greater decrease of the light yield, as the light is collected from the whole sample volume in the pulse-height spectrum measurement. Electron centers based on oxygen vacancies (F + centers) which can have negative influence on scintillation mechanism were revealed by the electron paramagnetic resonance (EPR) experiments. The O - hole center was detected as well and the temperature dependences of the EPR spectra showed that its thermal stability was decreased by Mg codoping. Negligible changes in the decay time of Pr 3+ 5d-4f photoluminescence exclude the non-radiative energy transfer from relaxed 5d 1 excited state of Pr 3+ towards CT absorption transition of Pr 4+ and suggest that simple reabsorption of the emitted light takes place. Unlike for the Ce-doped analogs, promoting the Pr 3+ luminescence center to the tetravalent state by divalent ion co-doping and air annealing did not improve scintillation properties of Pr-doped LuAG.

The influence of annealing in air at elevated temperatures on the absorption, luminescence and scintillation characteristics was studied for a set of Ce-doped aluminum garnet and perovskite single crystals. Positive effects consisting mainly in increase of light yield and decrease of afterglow were found to varying extent in all the materials. It is explained by the positive role of created stable Ce 4+ center in scintillation mechanism and by decrease of deep trap concentration based on oxygen vacancies.

The photopeak and escape peak processes were studied in heavy Ce3+-doped LuAG and GGAG, Pr3+-doped LuAG and undoped BGO scintillating crystals. Energy resolution measurements were performed with 511 keV photons of 22Na radioisotope. In the pulse-height spectra the escape peaks were resolved clearly on thin samples (up to 3 mm). If sample thickness increases the escape peak cannot be resolved anymore (for thicknesses approx. above 5 mm, especially). Consequently, energy resolution increases by about 40% compared with that measured at 1 mm thick samples.

Pr doped (Lu,Gd) 3 (Ga,Al) 5 O 12 single crystals were grown by the micro-pulling down (μ-PD) method. The crystals were greenish and transparent with 3.0 mm in diameter, 10–30 mm in length. Neither visible inclusions nor cracks were observed. Luminescence and scintillation properties were measured. The substitution at the Al 3+ sites by Ga 3+ in garnet structure has been studied. The Pr 3+ 5d–4f emission is observed within 300–380 nm wavelength superposed with 312 nm emission line of Gd 3+ . Pr0.2%:Lu 2.5 Gd 1 Ga 3 Al 2 O 12 shows highest emission intensity. The light yield of Pr0.2%:Lu 2.5 Gd 0.5 Ga 2 Al 3 O 12 sample with diameter 3 mm×1 mm size was around 8000 ph/MeV. Two-exponential approximations of scintillation decay showed 39.6 ns (30.6%) and 151 ns (69.4%) decay times.

The optical and scintillation properties of Ce 3+ -doped YGd 2 Al 5− x Ga x O 12 ( x =2,3,4) single crystals were investigated. With increasing Ga content in the garnet host, the 5d 1 absorption and emission bands shift toward higher energy due to the decrease in the crystal field splitting of the 5d levels. Temperature dependences of absorption and emission spectra and of photoluminescence decays of both prompt and delayed components were measured. Light yield (LY) and its dependence on an amplifier shaping time were measured under excitation with γ-rays. High LY value of 38,000 photons/MeV was obtained for a YGd 2 Al 2 Ga 3 O 12 :Ce sample. Scintillation decay was measured with an extended dynamical and temporal scale under the nanosecond-pulsed soft X-ray excitation. The relative content of the fastest component in the scintillation response increases with increasing Ga content.

C 3i , C 2 sites and also pairs of Nd 3+ in Lu 2 O 3 ceramics and crystal as laser potential sesquioxides are analyzed.

This research deals with the luminescence and physical properties of Gd3(Ga,Al)5O12:Ce single crystal set and it is a complementary study to analogous (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. Those three groups of materials show similar luminescence and physical properties. In this research, the influence of substitution Ga for Al in Gd3(Ga,Al)5O12:Ce single crystals on the luminescence and scintillation characteristics as well as thermodynamical stability of the garnet phase is studied, and it is compared to (Y,Gd)3Al5O12:Ce and (Gd,Lu)3Al5O12:Ce single crystal sets. The unbalanced substitution Ga for Al makes garnet phase thermodynamically unstable and leads to the formation of the multiphase system with luminescence centers in UV and visible spectral ranges. The wavelength-dispersive X-ray spectroscopy, X-ray powder diffraction analysis, photoluminescence and decay kinetic measurements reviled all phase types within multiphase crystals. Temperature dependent measurements were applied to determine the quenching mechanism for Ce3+ luminescence. The timing characteristics are studied as the function of Ga content.

Low temperature thermoluminescence of GAGG:Ce and LuAG:Pr scintillator crystals has been studied by means of the T max – T stop method. It is shown that the glow curves of both materials are superpositions of discrete glow peaks and broad bands due to quasi-continuous Gaussian distributions of trapping levels. A model function has been built and trap parameters have been evaluated.

The Nd-doped, Gd-doped and doubly Nd, Gd-codoped BaLu1.2Y0.8F8 (BaLuYF) and LuF3 single crystals were grown by a micro-pulling-down method. Their photoluminescence, radioluminescence and alpha-ray-excited pulse-height spectra were studied in the vacuum-ultra-violet (VUV) region to confirm their possible application as fast VUV scintillators. Complementary photoluminescence measurements in the ultra-violet/visible spectral region were performed as well. The possibility of scintillation performance improvement by Gd codoping was tested. The overall scintillation efficiency of BaLuYF:Nd1% was comparable to that of the LaF3:Nd8% reference sample and only slight improvement was observed after Gd1% codoping. The positive effect of Gd codoping was confirmed for higher dopant concentrations. This was explained by an energy transfer from the high-energy Gd3+ 4f 2G(0)7/2 level to the Nd3+ 5d state. On the other hand, pulse-height spectra showed degradation of the light yield under alpha-ray excitation, which was explained by introduction of slow components due to Gd codoping. On the other hand, no positive effect of Gd codoping was found for the overall scintillation efficiency of Nd-doped LuF3, despite the same energy transfer process was confirmed. Slight improvement of the light yield under alpha-ray excitation was confirmed.

Crystal growth by micro-pulling-down, Czochralski, and floating zone methods and scintillation properties of Ce:Gd 3 (Ga,Al) 5 O 12 (Ce:GGAG) multi-component oxide garnets, and Ce:Gd 2 Si 2 O 7 (Ce:GPS) or Ce:(La,Gd) 2 Si 2 O 7 (Ce:La-GPS) pyro-silicates are reviewed. GGAG crystals demonstrated practically linear dependences of some of the parameters including lattice constant, emission wavelength, and band gap on Ga content. However, emission intensity, light yield and energy resolution showed maxima for intermediate compositions. GGAG crystals had the highest light yield of 56,000 photon/MeV for Ga content of 2.7 atoms per garnet formula unit. Similarly the light yield and energy resolution of La-GPS showed the highest values of 40,000 photon/MeV and 4.4%@662 keV, respectively, for La-GPS containing 10% of La. Moreover, La-GPS demonstrated stable scintillation performance up to 200 °C.

Rare-earth sesquifluorides with no absorption in visible spectral region, such as LaF 3 , GdF 3 , LuF 3 , YF 3 , ScF 3 , are the topic of intense study as a host for luminescence materials. However, except Nd:LaF 3 , they are not studied as a host for laser materials. The main obstacle troubling further study of GdF 3 , LuF 3 , YF 3 , ScF 3 single crystal is the fact that there is first-order phase transition (LaF 3 type↔beta-YF 3 type for GdF 3 , alpha-YF 3 type↔beta-YF 3 type for the rest) between the room and melting temperature.To prevent the phase transition, first of all, we have tried to make solid solution between GdF 3 and YF 3 in such a way that the average cation radii can be shifted to the size that does not have phase transition. Ce 3+ perturbed luminescence was observed in the Ce- and Sr-codoped GdF 3 –YF 3 system. Similar solid solution concept was applied to the combination between GdF 3 and YbF 3 . The emission spectrum of Yb 3+ that exhibits broad bands around 1 μm was observed. Room temperature up-conversion luminescence spectra of Pr 3+ -doped Gd 1− x Yb x F 3 were studied and visible emission from Pr 3+ was obtained under infrared laser pumping in the Yb 3+ broad absorption band at 935.5 nm.

The temperature dependences of photoluminescence spectra of Pr 3+ in Lu 2 SiO 5 (LSO) and Y 2 SiO 5 (YSO) hosts under excitation into the 5d 1 excited state are reported within 4-450 K together with measured decay times of the fast and slow delayed recombination decay processes. Ionization processes occurring from the 5d, relaxed state are evaluated using the mentioned decay measurements. The simple kinetic model is created to provide quantitative parameters of energy barriers and frequency factors, which govern the ionization process in the 5d 1 Pr 3+ excited state in both hosts. The ionization barriers are interpreted as the separation between the conduction band and the 5d, excited state. The temperature dependences of the Pr 3+ 5d 1 -4f and 4f-4f luminescence intensities point to the thermally stimulated transition from 5d 1 state towards the 3 P 2 and other lower lying 4f levels.

The spectroscopic properties of high-quality Czochralski grown 20% Yb3+-doped Li6Y(BO3)3 single crystal as new promising laser material are presented. The crystal was seeded-grown in the 〈0 1 0〉 direction and its crystallinity was measured using X-ray rocking curve analysis. Low temperature transmission spectrum exhibits broad bands in a short range of wavelengths and two sharp lines at 972.5 and 978 nm, interpreted as two zero-lines of two nonequivalent Yb3+ centers inside the lattice. The fluorescence lifetimes associated to these two intense lines are different: 0.867 and 1.33 ms. An attempt of determination of the Stark sublevels energies of the 4F5/2 and 4F7/2 manifolds of the two Yb3+ nonequivalent ions is given. The polarized absorption and emission spectra were also recorded at room temperature and we conclude that the most favorable emission line for laser application could be around 1042 nm in ng polarization.

Multicomponent garnets (Y 3− x Gd x Al 5− y Ga y O 12 ) doped with Ce 3+ ions are promising scintillators with a high density, fast response time and high light yield. To deepen the knowledge about the transfer stage of scintillation mechanism we discuss the energy migration and energy transfer processes in the set of undoped and Ce 3+ activated multicomponent garnet single crystals. Temperature dependence of Gd 3+ emission intensities as well as decay kinetics in Y 3− x Gd x Al 5− y Ga y O 12 ( x , y =1,2,3) crystals point to the Gd 3+ →Gd 3+ nonradiative energy migration, which is diffusion limited. Concentration quenching of Gd 3+ emission occurs by energy migration to accidental impurities and/or structure defects. Temperature dependence of photoluminescence emission intensities and decay time measurements of Gd 3+ as well as Ce 3+ ions in Gd 3 Ga 3 Al 2 O 12 :Ce 3+ single crystal reveal nonradiative energy transfer Gd 3+ →Ce 3+ (including migration through Gd 3+ sublattice) which is responsible for slow Ce 3+ fluorescence decay component.

Vacuum ultraviolet (VUV) luminescence from a Nd 3+ :(La 1− x ,Ba x )F 3− x ( x =0.1) and a Nd 3+ :LaF 3 single crystal grown by the micro-pulling-down method modified for fluoride crystal growth is discussed. Emission resulting from excitation with 157 nm pulses of a F 2 laser and by 290 nm femtosecond pulses of a Ti:sapphire laser show that the luminescence spectral and temporal characteristics are similar for both excitation cases and that they have good prospects as a VUV laser material.