Because fossil fuels are becoming scarce and because of the expected climate change, our standard of living can only be maintained by a significant increase in energy efficiency. Large amounts of energy are consumed for lighting and during operation of displays. Thus, the targets are the development of economical light sources like white-light-emitting diodes and display panels with enhanced efficiency. Solar energy is converted into electricity by solar cells, and their efficiency must be improved considerably. A possible contribution might be delivered by phosphors which allow the conversion of thermal radiation into electrical energy. Although the target of energy efficiency is very important, we must not overlook that medical imaging diagnostic methods require efficient and sensitive detectors. For the solution of these central questions, inorganic solid-state materials doped with rare-earth ions are very promising and are therefore in the focus of current research activities.

Recent Developments in the Field of Inorganic Phosphors

The 5d level positions of the trivalent lanthanides in inorganic compounds

Nonlinear Optical Crystals: A Complete Survey

In this review, the major achievements and research and development (R&D) trends from the last decade in the field of single crystal scintillator materials are described. Two material families are included, namely, those of halide and oxide compounds. In most cases, the host crystals are doped with Ce3+, Pr3+ or Eu2+ rare earth ions. Their spin- and parity-allowed 5d–4f transitions enable a rapid scintillation response, on the order of tens to hundreds of nanoseconds. Technological recipes, extended characterization by means of optical and magnetic spectroscopies, and theoretical studies are described. The latter provide further support to experimental results and provide a better understanding of the host electronic band structure, energy levels of specific defects, and the emission centers themselves. Applications in medical imaging and dosimetry, security measures, high-energy physics and the high-tech industry, in which X(γ)-rays or particle beams are used and monitored, are recognized as the main driving factor for R&D activities in this field.

Recent R&D Trends in Inorganic Single-Crystal Scintillator Materials for Radiation Detection

The Eu3+ charge transfer energy and the relation with the band gap of compounds

Large and good-optical-quality crystals of gadolinium and calcium oxoborate, Ca4GdO(BO3)3 (GdCOB) have been grown from a melt by the Czochralski pulling method. The crystal is absolutely insensitive to moisture. Linear- and quadratic nonlinear-optical properties of this new monoclinic biaxial borate crystal are reported. The crystal is transparent in the visible and the near IR (from 0.32 to 2.7 µm), with favorable phase matching conditions for second-harmonic generation. Experimental phase-matching results, measured with a femtosecond broadband pulse parametric generator source tunable from 0.8 to 2.100 µm, are compared with theoretical predictions. The effective nonlinear coefficients are determined, leading to deff=1 pm/V for type I crystals in the ZX plane. The damage threshold is as high as 1 GW/cm2 at 0.532 µm. The second-harmonic generation conversion efficiency of a Q-switched Nd:YAG laser with a 15-mm long crystal is greater than 50%. These values together with the possibility of growing large crystals make GdCOB an excellent candidate for the next generation of crystals for frequency conversion and parametric processes.

Linear- and nonlinear-optical properties of a new gadolinium calcium oxoborate crystal, Ca 4 GdO(BO 3 ) 3

The surge of interest in ytterbium-doped materials has led to their being proposed as substitutes for their widely used neodymium counterparts for infrared laser emission. Spectroscopic and laser investigations of ytterbium-doped Ca4GdO(BO3)3 (Yb:GdCOB), a new ytterbium-doped crystal, are reported. Laser performances under titanium:sapphire pumping suggest that this material is suitable for schemes that include laser-diode pumping. A maximum slope efficiency of 58.8% with a corresponding laser threshold of 40 mW has been demonstrated with a 1% output coupler. Laser oscillations were also observed for output-coupler transmissions of as much as 10%. Self-frequency doubling of infrared-laser emission has been observed for the first time to our knowledge in an ytterbium-doped medium. The recent demonstration of the interesting self-frequency-doubling properties of neodymium-doped Ca4GdO(BO3)3 suggests that Yb:GdCOB could also be efficient in visible laser emission because of its lack of reabsorption at the second-harmonic wavelength. All these results show that the calcium oxoborate family can be useful compact and efficient laser-diode-pumped visible-laser sources.

Ytterbium-doped Ca 4 GdO(BO 3 ) 3 : an efficient infrared laser and self-frequency doubling crystal

Cerium doped lutetium pyrosilicate Lu2Si2O7:Ce3+ (LPS), a new inorganic scintillator, displays particularly promising performance. This material can be readily pulled from the melt. A high light output (average value: 26 300 ph MeV−1), a relatively good energy resolution (9%) and a fast decay time (38 ns) without afterglow make this new scintillator very attractive, in particular for medical imaging. Optical characterizations and scintillation properties of LPS:Ce large single crystals are presented, including timing properties and study of the scintillation yields as a function of incident energy.

https://iopscience.iop.org/article/10.1088/0953-8984/15/12/326/pdf

Scintillation properties of Lu2Si2O7:Ce3+, a fast and efficient scintillator crystal

EPR study of Ce3+ ions in lutetium silicate scintillators Lu2Si2O7 and Lu2SiO5

The properties of Ce3+ in CaBPO5, LiCaPO4 and Li2CaSiO4 have been studied in the 100–400 nm wavelength region with time resolved spectroscopy at temperatures between 10 and 300 K. Emphasis is on the relationship between the energies of the five 5d levels of Ce3+ and the crystalline environment. Good agreement between predicted and observed average energy of the 5d configuration is demonstrated. In addition values for the bandgap of the host materials, Stokes shifts and luminescence decay time data are presented.

https://iopscience.iop.org/article/10.1088/0953-8984/15/3/315/pdf

A. Kahn-Harari

Papers

Linear- and nonlinear-optical properties of a new gadolinium calcium oxoborate crystal, Ca 4 GdO(BO 3 ) 3

Ytterbium-doped Ca 4 GdO(BO 3 ) 3 : an efficient infrared laser and self-frequency doubling crystal

Scintillation properties of Lu2Si2O7:Ce3+, a fast and efficient scintillator crystal

EPR study of Ce3+ ions in lutetium silicate scintillators Lu2Si2O7 and Lu2SiO5

4f?5d spectroscopy of Ce3+ in CaBPO5, LiCaPO4 and Li2CaSiO4