W.M. Yen

Green phosphorescence of CaAl2O4:Tb3+,Ce3+ through persistence energy transfer

Single-crystal CaAl2O4 fibers doped with Ce3+ or Tb3+ or double doped with Tb3+ and Ce3+ were prepared by the laser-heated pedestal-growth method. Ce3+-doped CaAl2O4 was found to have persistent phosphorescence in the blue violet lasting more than 10 h, while Tb3+-doped CaAl2O4 showed only a weak afterglow. However, long persistent green phosphorescence from Tb3+ was observed in the doubly doped fiber, CaAl2O4:Tb3+,Ce3+. It was found that the enhanced green phosphorescence from Tb3+ is due to persistence energy transfer from the thermally released electrons to Ce3+ followed by energy transfer to Tb3+. The finding suggests a promising approach for controlling the color of persistent phosphors.

Phosphorescent dynamics in SrAl2O4: Eu2 +, Dy3 + single crystal fibers

Abstract We report for the first time the growth of activated alkaline earth aluminate single crystals with long persistent phosphorescence. Measurements of the phosphorescence and decay dynamics on samples of SrAl2O4 crystals doped with Eu2 + only and with Eu2 + and Dy3 + indicate that the trapping rate of Dy ions is very fast; the efficiency of trapping is estimated to be about 40% at room temperature. The decay curve of the after-glow is nonexponential, and cannot be explained simply by thermally activated detrapping processes. Hole transport may be an important factor in persistent phosphorescence.

Optical and photoconductive properties of cerium-doped crystalline solids

Abstract The importance of donor- and acceptor-like charge transfer processes in the luminescence properties of rare earth doped crystalline solids have become increasingly evident in recent years. As an example, we present optical studies, including photoconductive spectroscopy, of cerium-doped hosts that exhibit drastically different luminescence efficiencies. The differences are demonstrated to be due to the location of the defect electronic levels with respect to the host bands. Small, but crucial, differences in the relative position of the lowest cerium 5-d level can result either in complete quenching of the emission or in luminescence with quantum efficiencies close to unity.

Saturation of 1.064 μm absorption in Cr,Ca:Y3Al5O12 crystals

Saturation absorption experiments are conducted on the title compound at 1.064 μm with laser light propagating along both the [001] and [111] crystallographic axes. Rotation of the crystals about these axes during the experiments reveals that the transmission of the polarized 1.064 μm radiation is highly anisotropic in the saturation regime. The details of the anisotropy and saturation curves are explained using a simple model of subsets of optical centers along with a relevant set of rate equations. The ground and excited‐state absorption cross sections at 1.064 μm are calculated to be σgs=5.7(±2)×10−18 cm2 and σes=8(±2)×10−19 cm2, respectively, and the optimum crystallographic orientation for utilization in optical devices is discussed.

Trapping processes in CaS:Eu2+,Tm3+

CaS:Eu2+,Tm3+ is a persistent red phosphor. Thermoluminescence was measured under different excitation and thermal treatment conditions. The results reveal that the charge defects, created by substituting Tm3+ for Ca2+, serve as hole traps for the afterglow at room temperature. Tm3+ plays the role of deep electron trapping centers, capturing electrons either through the conduction band or directly from the excited Eu2+ ions. These two processes, in which two different sites of Tm3+ are involved, correspond to two traps with different depths.

LUMINESCENCE EFFICIENCY OF CERIUM DOPED INSULATORS : THE ROLE OF ELECTRON TRANSFER PROCESSES

Insulating host materials doped with trivalent cerium show quantum efficiencies of the Ce3+ emission ranging from zero to unity. Comparing optical and photoelectrical properties of a very efficient scintillator material (Lu2(SiO4)O:Ce) to those of cerium doped oxides with quenched emission, the radical differences for these materials are demonstrated to originate from the location of the cerium energy levels with respect to the host conduction band. Photoionization and subsequent nonradiative relaxation processes responsible for the luminescence quenching are discussed in a donor–acceptor model for the impurity ion and a rule for luminescence efficiency is derived, applicable to a variety of phosphor and scintillator materials.

Site dependent thermoluminescence of long persistent phosphorescence of BaAl2O4:Ce3+

A very long persistent phosphor BaAl2O4 : Ce 3þ was prepared and studied. The Ce 3þ 5d-4f emissions from two Ba 2þ sites in the BaAl2O4 were observed at 450 and 402 nm. The lowest 4f-5d excitation peaks were recorded at 357 and 335 nm, respectively. The persistence times of the long afterglow emissions from Ce 3þ at the two sites were found to be longer than 10 h. Site-selective thermoluminescence spectra of the sample were measured. Two sets of thermoluminescence peaks were detected at )43, )26 and 27 C, and )53, )36 and 30 C (heating rate 13.3 C/min) corresponding to the two sites at 450 nm (site-1) and 402 nm (site-2), respectively. Dy 3þ ,C e 3þ co-doped samples were also prepared, which introduce new defect-related traps at 18, 50, and 82 C (heating rate 10 C/min). These defect-related traps due to Dy 3þ co-doping also contribute to the Ce 3þ afterglow at the two sites. 2002 Elsevier Science B.V. All rights reserved.

Effect of the matrix on the radiative lifetimes of rare earth doped nanoparticles embedded in matrices

The radiative lifetimes, τR, of the excited states of rare earth (RE) ions contained in nanocrystalline insulators are different compared to their values in crystallographically equivalent bulk crystals. Their lifetimes depend on the effective index of refraction of the media consisting of nanoparticles and the substance filling the space between them. Here the radiative rates were studied as a function of particle size and the amorphous matrices containing the nanoparticles. Effects due to the effective index of refraction and site distortions associated with the increased inhomogeneous broadening were observed.

Persistent energy transfer in CaAl2O4 :Tb3+, Ce3+

Ceramic samples of Tb3+ and Ce3+ singly doped and co-doped CaAl2O4 with 0.1 and 1 at. % doping concentrations have been prepared. The emission of the Ce3+ singly doped sample is at 413 nm. The persistence time of its afterglow is longer than 10 h. The strongest emission of the Tb3+ singly doped sample is at 543 nm and its afterglow persists for about 1 h. In the co-doped sample, the Ce3+ excitation is transferred to Tb3+ ion with an energy transfer rate of 109–1010 s−1 in the high concentration samples. The Ce3+ energy transfer occurs during its afterglow decay, so that the Tb3+ afterglow emission duration is also extended to about 10 h. Photoconductivity and thermoluminescence measurements have also been performed in these systems; the persistent sensitization of Tb3+ is also confirmed by these measurements.

Temperature dependent spectroscopic studies of the electron delocalization dynamics of excited Ce ions in the wide band gap insulator, Lu2SiO5

Electron delocalization processes of optically excited states of Ce3+ impurities in Lu2SiO5 were investigated by means of a temperature and spectrally resolved photoconductivity study. By monitoring separately the strength of the photocurrent resulting from excitation into each of the Ce3+ 5d absorption bands, over a broad temperature region, three different delocalization processes, namely direct photoionization, thermal ionization, and tunneling, have been identified. The relative probabilities and temperature dependencies of each of these processes are discussed. The observed exponential temperature increase in the photocurrent, which spans six orders of magnitude, allows for the exact placement of the lowest energy 5d levels of the Ce3+ ions within the band gap. For Lu2SiO5:Ce3+, the lowest 5d state is determined to be 0.45 eV below the conduction band edge.