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

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

Photovoltaic (PV) technologies for solar energy conversion represent promising routes to green and renewable energy generation. Despite relevant PV technologies being available for more than half a century, the production of solar energy remains costly, largely owing to low power conversion efficiencies of solar cells. The main difficulty in improving the efficiency of PV energy conversion lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of a semiconductor material. In recent years, luminescent materials, which are capable of converting a broad spectrum of light into photons of a particular wavelength, have been synthesized and used to minimize the losses in the solar-cell-based energy conversion process. In this review, we will survey recent progress in the development of spectral converters, with a particular emphasis on lanthanide-based upconversion, quantum-cutting and down-shifting materials, for PV applications. In addition, we will also present technical challenges that arise in developing cost-effective high-performance solar cells based on these luminescent materials.

Enhancing solar cell efficiency: the search for luminescent materials as spectral converters

Tellurite glass: a new candidate for fiber devices

Publisher Summary This chapter discusses statistical exchange-correlation in the self-consistent field. There are two sides to a self-consistent field calculation: the determination of the potential and the solution of Schrodinger's equation for the one-electron problem. The solution of Schrodinger's equation has fortunately advanced far enough through the application of the electronic digital computer so that it can be regarded for most purposes as being a standardized technique. The main point of the method of Gaspar, Kohn, and Sham is that they derive the approximate exchange correction from an approximate Hamiltonian for the system by varying the spin-orbitals to minimize the average value of this Hamiltonian for the ground state. The approximate Hamiltonian has many important and valuable features that are described in the chapter.

Statistical Exchange-Correlation in the Self-Consistent Field

A unified treatment is given in the adiabatic approximation for phonon sideband intensities, multiphonon relaxation transition probabilities, and phonon-assisted energy-transfer probabilities. The intensity distribution of phonon sidebands is determined by coupling constants of the vibrational modes with electrons or holes and a criterion for the appearance of discrete sidebands is given. Transition probabilities of multiphonon relaxation processes among various excited levels of an ion are shown to depend exponentially on the energy gap between these levels, in agreement with recent experimental results. A similar dependence is derived for the energy-transfer probabilities between two ions on the energy mismatch between excitation energies of these ions.

Phonon Sidebands, Multiphonon Relaxation of Excited States, and Phonon-Assisted Energy Transfer between Ions in Solids

The probability of cooperative energy transfer from two excited ions to a nearby ion is computed and compared with the probability of excitation by stepwise energy transfer. For ${\mathrm{Yb}}^{3+}$ sensitization of ${\mathrm{Er}}^{3+}$, ${\mathrm{Ho}}^{3+}$, and ${\mathrm{Tm}}^{3+}$ activated hosts, the cooperative transition rate from two excited ${\mathrm{Yb}}^{3+}$ ions is estimated to be in the range ${10}^{7}$-${10}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, depending on the degree of overlaps of absorption and emission bands. In the case of the ${\mathrm{Er}}^{3+}$+${\mathrm{Yb}}^{3+}$ and ${\mathrm{Ho}}^{3+}$+${\mathrm{Yb}}^{3+}$ systems, the dependence of output intensity on the exciting light intensity ${I}_{E}$ and the concentration of Yb cannot discriminate between cooperative and serial transfer, but estimates of the transfer probabilities show that the stepwise process dominates. It is suggested that the ${\mathrm{Tb}}^{3+}$+${\mathrm{Yb}}^{3+}$ system would be an excellent system in which to observe the cooperative effect unambiguously. In the ${\mathrm{Tm}}^{3+}$+${\mathrm{Yb}}^{3+}$ system also, we expect stepwise transfer to dominate except for unfeasibly low ${I}_{E}$.

Cooperative and Stepwise Excitation of Luminescence: Trivalent Rare-Earth Ions in Yb 3 + -Sensitized Crystals

The relation between excitons and “plasmons” in insulators is studied, and it is shown that both excitons and “plasmons” may be obtained in the singlet longitudinal states. When the energy of interband transition is nearly equal to the free electron plasmon energy $\hbar\omega_{p}$, the energy and the angular dispersion of “plasmon” loss peaks are closely related to the structure of the pair band. In this case the “plasmon” solution of the dispersion relation is expected on the higher energy side of M 3 -(or M 2 -) van Hove singularity provided the mean value of imaginary part of the dielectric constant exceeds some critical value below this energy and is small above this energy. Recent experimental data of energy loss spectra on alkali-halides are analyzed making use of the optical data and the results of band structure calculations.

Toru Miyakawa

Papers

Phonon Sidebands, Multiphonon Relaxation of Excited States, and Phonon-Assisted Energy Transfer between Ions in Solids

Cooperative and Stepwise Excitation of Luminescence: Trivalent Rare-Earth Ions in Yb 3 + -Sensitized Crystals

Excitons and Plasmons in Insulators