Search

High pressure studies of monoclinic yttrium aluminum oxide single crystals (Y4Al2O9 -YAM) doped with Ce3+ ions grown by micro-pulling-down method are reported. The results of absorption measurements in the mid-infrared prove the existence of four different sites in which Ce3+ ions substitute Y ions, in accordance with the crystallographic structure of YAM. The lowest 5d level of Ce3+ is located very close to the bottom of the conduction band for two of four Ce3+ related centers at ambient pressure, which results in strong temperature quenching of their luminescence. Two other Ce3+ centers do not emit at ambient pressure since their 5d levels are resonant with the conduction band. Application of high pressure above 11 GPa restores their luminescence. Consequences of such energy structure of various Ce3+ sites in YAM for the Dorenbos theory are discussed. The bandgap energy of YAM is found to be 5.93 eV, which is the smallest out of the three yttrium aluminum oxides. High-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements identify a phase transition occurring at pressures between 8 and 11 GPa, which has martensitic character. The second phase transition to another structure (likely with hexagonal symmetry) occurs at a pressure around 16 GPa.

We investigated the 3 P 0 and 1 D 2 luminescence of Y 4 Al 2 O 9 (YAM) crystals activated with Pr 3+ ions after excitation of the 1 D 2 level. Under cryogenic temperature four distinct types of emission and excitation spectra are observed, which are attributed to 4f–4f transitions of four different sets of Pr 3+ centers. The energy level structure of Pr 3+ ions occupying four distinct, low symmetry Y 3+ sites in YAM have been determined using site selective laser excitation and luminescence. To get better insight into local site occupancy, luminescence decay studies were also conducted. It was found that energy transfer between Pr 3+ ions doped into four different sites is absent. Anti-Stokes, upconversion luminescence originating from the 3 P 0 state while exiting the 3 H 4 → 1 D 2 transition was observed. The whole orange to blue upconversion was site selective at cryogenic temperatures. The dynamical behavior of the upconverted emission arising from four sites was studied and modelled via energy transfer process.

In this work we analyse spectroscopic features and energy transfer mechanisms in the YAM: Ce 3+ +Pr 3+ system. Absorption spectra of the YAM: Ce 3+ samples were measured, revealing absorption bands in the range of 300–400 nm allowing for relatively efficient absorption of the solar radiation. Measured excitation and emission spectra indicate the existence of energy transfer from Ce 3+ to Pr 3+ . Cerium photoluminescence decay times of 5d emission were measured and energy transfer efficiency reached 16.86% for the best sample containing 0.5% Ce 3+ and 1% Pr 3+ . Spectral properties were investigated to establish nature of this transfer, particularly whether it occurs to the 3 P 0 or 1 D 2 level or both. While we proved the existence of transfer to 3 P 0 level emission from 1 D 2 was also observed. Energy transfer from Ce 3+ 5d bands to the 3 P J multiplet is necessary to enable further transfer to two Yb 3+ ions in down-conversion process in prospective Ce 3+ +Pr 3+ +Yb 3+ system.

Eu 3+ doped Y 4 Al 2 O 9 (YAM) crystals were prepared by the micro-pulling down method. Optical-absorption and laser-selective-excitation techniques along with the luminescence decays have been used to reveal that Eu 3+ ions in YAM occupy three distinct sites, which were characterized and discussed. The Stark energy levels of Eu 3+ at three different sites in YAM were assigned from selectively excited emission spectra at 10 K. The intensity ratio of forced electric dipole ( 5 D 0 → 7 F 2 ) and magnetic dipole ( 5 D 0 → 7 F 1 ) transitions was discussed in order to obtain information about the degree of asymmetry of the luminescent centers. These results were confirmed by the luminescence lifetime measurements. The temperature dependent photo-luminescence spectra indicated that there is no energy transfer between different sites in the 10–300 K range.

Sm 3+ doped Y 4 Al 2 O 9 (YAM) crystals were prepared by the micro-pulling down method. Optical-absorption and laser-selective-excitation techniques along with luminescence decays have been used to reveal that Sm 3+ ions in YAM normally occupy four sites, which were characterized and discussed. The comprehensive sets of Stark energy levels of the four Sm 3+ centers in YAM were established. The intensity ratio of forced electric dipole ( 4 G 5/2 → 6 H 9/2 ) and magnetic dipole ( 4 G 5/2 → 6 H 5/2 ) transitions has been used to estimate the degree of asymmetry of Sm 3+ sites. The lifetime of Sm 3+ ions and the cross-relaxation rates were experimentally determined as a function of concentration for two distinct crystallographic Sm 3+ centers. The dependencies of the nearest surroundings on the relaxation of the 4 G 5/2 excited state was discussed.

A series of Yb3+-doped yttrium aluminum monoclinic Y4Al2O9 (Yb:YAM) single crystals with different Yb3+ ion concentrations of 0.1, 1, 5 and 10 at.% have been grown by means of micro pulling down method. Low temperature absorption measurements revealed that Yb3+ incorporates at several well defined centers. Site selective excitation and emission experiments allowed to locate the energy levels of the ground 2F7/2 and 2F5/2 manifolds for the major centers detected in the system. The temperature dependence of the transition energies and linewidths of the resonant transitions was measured from 10 to 300 K and was well described by one phonon near resonant processes. The effect of Yb3+ concentration on the fluorescence spectra structure of Yb:YAM has been also investigated. It was observed that with the increasing of Yb3+ concentration luminescence from the low energy site dominates the emission spectrum. The fluorescence dynamics recorded at temperature range from 10 to 300 K under selective excitation of each site was analyzed.

Spectroscopic properties of trivalent dysprosium (Dy 3+ ) activated Y 4 Al 2 O 9 (abbreviated YAM) crystals have been investigated. The Judd–Ofelt formalism has been applied to the analysis of the room temperature absorption spectrum. The calculated radiative lifetime of the 4 F 9/2 state is in a very good agreement with the measured one. Branching ratios were obtained from emission spectrum and used in modified Judd–Ofelt method calculations. Concentration dependent fluorescence dynamics profiles have been investigated in Dy 3+ :YAM to understand better processes responsible for concentration quenching of the dysprosium 4 F 9/2 emission. The cross-relaxation transfer rates were experimentally determined as a function of Dy 3+ concentration. Decays were modelled using Inokuti–Hirayama approach to obtain an information on the energy transfer mechanism. The lasing properties of Dy 3+ :YAM have been discussed.

Spectroscopic properties of trivalent terbium (Tb3+) activated Y4Al2O9 (abbreviated YAM) crystals were studied. Concentration and temperature dependent emission spectra and fluorescence dynamics profiles have been investigated in YAM:Tb3+ in order to understand better processes responsible for quenching of the terbium 5D3 and 5D4 emissions. Decays were modelled using Inokuti–Hirayama approach to obtain information on the energy transfer mechanism. The cross-relaxation transfer rates were experimentally determined as a function of temperature and Tb3+ concentration. The investigation revealed strong influence of cross-relaxation process on 5D3 emission quenching. The two different processes responsible for the increase of fluorescence quenching with growing temperature were observed, both related to thermal activation energy. For temperatures above 700 K, the temperature dependence of the emission intensity ratio (5D3/5D4) becomes linear and the decay times are rapidly decreasing monotonously with increasing temperature, what is confirming the potential of Y4Al2O9:Tb3+ material in high temperature luminescence thermometry.

An investigation of spectroscopic properties of (SrTiO 3 –TiO 2 ):Pr 3+ eutectic and, for comparison, of bulk SrTiO 3 :Pr 3+ and TiO 2 :Pr 3+ crystals is presented. Luminescence spectra have been measured under both 450 nm and 350 nm excitation wavelength. For UV excitation they are characterized by a dominant red luminescence corresponding to transition from the 1 D 2 level of Pr 3+ ions. The mechanism responsible for quenching of blue (from 3 P 0 state) and intensification of red luminescence is proposed to be thermally-induced radiationless relaxation involving a low-lying Pr 3+ –Ti 4+ intervalence charge transfer state. Measured decay constants of 1 D 2 excited state of Pr 3+ are compared with values obtained for other praseodymium doped titanate hosts.

Concentration dependant emission spectra and fluorescence dynamic profiles have been investigated in PrxLa1−xAlO3 single crystals in order to better understand processes responsible for concentration quenching of the praseodymium 3 P 0 and 1 D 2 emissions. The cross-relaxation transfer rates were experimentally determined as a function of Pr3+ concentration. Decays were modeled and nearest-neighbor trapping rates were calculated.

Crystals of PrxLa1−xAlO3 solid solution have been grown by the Czochralski method for the first time, to the best of our knowledge. Crystals with high praseodymium concentration tend to grow spirally. The color of the crystals changes with chemical composition. X-ray diffraction shows an increase of the lattice constants with an increase in lanthanum ion concentration. The thermal conductivity has been investigated in the temperature range from 5 to 300 K. Completely different behavior of thermal conductivity is observed at low temperatures for the investigated crystals, whereas it remains roughly constant at high temperatures. The values of magnetic susceptibility lie within the expected range that could be expected for a paramagnetic material.

An investigation of spectroscopic properties of Tb 3+ ions in terbium-scandium-aluminum garnet - terbium-scandium perovskite Tb 3 Sc 2 Al 3 O 12 –TbScO 3 (TSAG-TSP) eutectic and, for comparison, in bulk Tb 3 Sc 2 Al 3 O 12 (TSAG) and TbScO 3 (TSP) crystals is presented. The emission and fluorescence decays from the 5 D 4 level of Tb 3+ ion after pulsed, one-photon excitation are characterized as a temperature function from 10 to 300 K. Spectrally selected groups of Tb 3+ ions located in particular phases of eutectic were characterized using a time–resolved spectroscopy. The cross-relaxation process leading to the quenching of luminescence from the 5 D 4 excited state of Tb 3+ ions in all investigated materials was observed and analyzed. The presented eutectic materials have a high potential for photonic applications.

The praseodymium aluminum perovskite−praseodymium hexaluminate (PrAlO3−PrAl11O18) eutectic has been studied for the first time. It has been grown by the micro-pulling down method from a composition...

The praseodymium-aluminum perovskite−praseodymium oxide, PrAlO3−Pr2O3, eutectic has been studied. The growth of the eutectic by the micro-pulling down method is presented. The PrAlO3−Pr2O3 is not air-stable. The cause of the eutectic instability is the instability of the Pr2O3 phase. When exposed to air, the Pr2O3 phase changes into a new compound having a hexagonal unit cell with the lattice parameters a = b = 6.455(1) A, c = 11.322(4) A. The PrAlO3−Pr2O3 eutectic grows with a regular complex microstructure. The luminescence spectra are presented for both the eutectic and the new phase generated by exposing the eutectic to air.

European Doctoral Program on metamaterials has been initiated by the European Union (EU) Network of Excellence METAMORPHOSE. So far, twenty European academic institutions have established a consortium that operates a geographically distributed doctoral school in the emerging and multidisciplinary field of metamaterials