Before the 1960s, all anti-Stokes emissions, which were known to exist, involved emission energies in excess of excitation energies by only a few kT. They were linked to thermal population of energy states above excitation states by such an energy amount. It was the well-known case of anti-Stokes emission for the so-called thermal bands or in the Raman effect for the well-known anti-Stokes sidebands. Thermoluminescence, where traps are emptied by excitation energies of the order of kT, also constituted a field of anti-Stokes emission of its own. Superexcitation, i.e., raising an already excited electron to an even higher level by excited-state absorption (ESA), was also known but with very weak emissions. These types of well-known anti-Stokes processes have been reviewed in classical textbooks on luminescence.1 All fluorescence light emitters usually follow the well-known principle of the Stokes law which simply states that excitation photons are at a higher energy than emitted ones or, in other words, that output photon energy is weaker than input photon energy. This, in a sense, is an indirect statement that efficiency cannot be larger than 1. This principle is

Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Interpretation of europium(III) spectra

Phosphor-converted white light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the next generation lighting industry and display systems due to their unique properties including but not limited to energy savings, environment-friendliness, small volume, and long persistence. Until now, major challenges in pc-WLEDs have been to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. A comprehensive understanding of the nature and limitations of phosphors and the factors dominating the general trends in pc-WLEDs is of fundamental importance for advancing technological applications. This report aims to provide the most recent advances in the synthesis and application of phosphors for pc-WLEDs with emphasis specifically on: (a) principles to tune the excitation and emission spectra of phosphors: prediction according to crystal field theory, and structural chemistry characteristics (e.g. covalence of chemical bonds, electronegativity, and polarization effects of element); (b) pc-WLEDs with phosphors excited by blue-LED chips: phosphor characteristics, structure, and activated ions (i.e. Ce 3+ and Eu 2+ ), including YAG:Ce, other garnets, non-garnets, sulfides, and (oxy)nitrides; (c) pc-WLEDs with phosphors excited by near ultraviolet LED chips: single-phased white-emitting phosphors (e.g. Eu 2+ –Mn 2+ activated phosphors), red-green-blue phosphors, energy transfer, and mechanisms involved; and (d) new clues for designing novel high-performance phosphors for pc-WLEDs based on available LED chips. Emphasis shall also be placed on the relationships among crystal structure, luminescence properties, and device performances. In addition, applications, challenges and future advances of pc-WLEDs will be discussed.

Phosphors in phosphor-converted white light-emitting diodes: Recent advances in materials, techniques and properties

Yttrium aluminum garnet (YAG) doped with Ce3+ is the phosphor of choice for the conversion of blue to yellow light in the rapidly expanding market of white light LEDs, but it is generally thought to suffer from a low luminescence quenching temperature The luminescence quenching temperature is an important parameter, especially in high-power LEDs, but surprisingly no systematic research has been done to measure and understand the temperature quenching of the yellow Ce luminescence in YAG:Ce Here we report on the luminescence temperature quenching in YAG:Ce For a wide range of Ce concentrations (between 0033% and 33%) the temperature dependence of the emission intensity and the luminescence lifetimes are reported The intrinsic quenching temperature of the Ce luminescence is shown to be very high (>700 K) The lower quenching temperatures reported in the literature are explained by thermally activated concentration quenching (for highly doped systems) and the temperature dependence of the oscillator st

Temperature Quenching of Yellow Ce3+ Luminescence in YAG:Ce

Visible-light persistent phosphors are being widely used as self-sustained night-vision materials because of their sufficiently strong and long afterglow (>10 h) and their ability to be excited by sunlight as well as room light. In contrast, persistent phosphors for near-infrared (NIR) wavelengths are lacking. Here we report a series of Cr(3+)-doped zinc gallogermanate NIR persistent phosphors that exhibit strong emission at 650-1,000 nm, extending beyond the typical 690-750 nm, and with a super-long afterglow of more than 360 h. These new NIR persistent phosphors are all-weather materials that can be rapidly, effectively and repeatedly charged by natural sunlight in almost all kinds of outdoor environment. Seconds to minutes of sunlight activation can result in more than two weeks of persistent NIR light emission. This new series of NIR persistent materials have potential applications in night-vision surveillance, solar energy utilization and in vivo bio-imaging.

Sunlight-activated long-persistent luminescence in the near-infrared from Cr 3+ -doped zinc gallogermanates

The properties of green thermoluminescence (TL) and persistent luminescence of Lu2O3:Tb3+ and Lu2O3:Tb3+,Ca2+ materials sintered in vacuum at 1700 °C were investigated. The concentration of Tb vari...

Thermoluminescence and Kinetics of Persistent Luminescence of Vacuum-Sintered Tb3+-Doped and Tb3+,Ca2+-Codoped Lu2O3 Materials

Luminescence properties of Ce-activated YAG optical ceramic scintillator materials

We have measured the emission spectra and decay kinetics of Ce-doped YAG under both optical and gamma excitation, at temperatures ranging from 60 to 600 K. The decay traces under gamma excitation display an unusual pattern of temperature dependence. Beginning with the lowest temperatures, the characteristic time constant of the decay coincides with the intrinsic (radiative) decay time up to about 180 K, at which point it suddenly jumps by more than 30%. The decay time remains longer than radiative until well above room temperature, then gradually reverts to the intrinsic value. This behaviour can be explained in terms of an array of shallow traps, whose existence is confirmed by the presence of thermoluminescent glow peaks. Depending on temperature, one or more of these traps acts as a temporary way-station for the free carriers that carry the excitation to the Ce ion, delaying its emission beyond what would be required by radiative decay alone.

https://iopscience.iop.org/article/10.1088/0953-8984/12/8/336/pdf

Kinetics of cerium emission in a YAG:Ce single crystal: the role of traps

Widening the Temperature Range of Luminescent Thermometers through the Intra- and Interconfigurational Transitions of Pr3+

A series of Lu2O3 samples doped with Eu was prepared in the form of sintered tablets. The activator concentration was 0.2, 1, 3, 5, 7 and 10% with respect to Lu. Emission spectra excited with light of various energies were taken at liquid nitrogen and room temperature. Excitation spectra of characteristic Eu emission lines for two different symmetry sites in Lu2O3 (C2 and S6) were also recorded. It was shown that Eu ions located in the two different symmetry sites possess characteristic excitation spectra in the regions both of the 7F0→5D1 absorption and of the charge transfer transitions. In both cases the levels of the Eu(S6) ions were located above those of the Eu(C2). In both emission and excitation spectra an energy transfer from Eu ions occupying the centrosymmetric S6 site in Lu2O3 to Eu ions in the noncentrosymmetric C2 site was observed. Some indications were found that an opposite back-transfer (from Eu in C2 to Eu in the S6 site) also occurs. Lower temperature significantly reduces the rate of the Eu(S6)→ Eu(C2) transfer. Decay kinetics indicated that even at room temperature the Eu(S6) emission could be further reduced if the dopant content was higher than 10%. Both at room and liquid nitrogen temperatures the decay kinetics of the Eu(C2) emissions are characterized by the time constants of about 1.0 ms for all concentrations, indicating that up to 10% of the dopant content no concentration or thermal quenching takes place. The liquid nitrogen Eu(S6) luminescence is characterized by a time constant of about 7 ms for the 0.2% specimen. At room temperature this value decreases to 5.6 ms.

https://iopscience.iop.org/article/10.1088/0953-8984/14/22/315/pdf

Eugeniusz Zych

Papers

Thermoluminescence and Kinetics of Persistent Luminescence of Vacuum-Sintered Tb3+-Doped and Tb3+,Ca2+-Codoped Lu2O3 Materials

Luminescence properties of Ce-activated YAG optical ceramic scintillator materials

Kinetics of cerium emission in a YAG:Ce single crystal: the role of traps

Widening the Temperature Range of Luminescent Thermometers through the Intra- and Interconfigurational Transitions of Pr3+

Concentration dependence of energy transfer between Eu3+ ions occupying two symmetry sites in Lu2O3