D. L. Dexter

Bio: D. L. Dexter is an academic researcher from University of Rochester. The author has contributed to research in topics: Excited state & Neon. The author has an hindex of 8, co-authored 8 publications receiving 1126 citations. Previous affiliations of D. L. Dexter include Argonne National Laboratory.

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

Abstract: 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.

Superconvergence and Sum Rules for the Optical Constants

Abstract: A systematic procedure is given for the derivation of sum rules for the optical constants of material media from dispersion relations, in analogy with superconvergence techniques of high-energy physics. In addition to the well-known $f$-sum rules, a number of new sum rules are obtained for the refractive index, the dielectric tensor, and its inverse. In particular, it is shown that the average value of the real refractive index over the whole frequency spectrum is equal to unity. The physical implications of the new results are discussed in connection with the dispersion of optical constants, with the effect of external perturbations, and with the theory of natural optical activity.

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

Abstract: 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 ${I}_{E}$.

Energy transfer and two-center optical transitions involving rare-earth and O H − impurities in condensed matter

Abstract: We present theoretical calculations for energy transfer and two-center optical processes involving simultaneous rare-earth electronic and O${\mathrm{H}}^{\ensuremath{-}}$ vibrational transitions in condensed matter. Along the lines sketched at the 1978 Paris Luminescence Conference [(J. Luminescence 1979)]. We use an electrostatic coupling model and perturbation theory to develop formal expressions for transition rates. Approximate expressions are derived and numerical results are presented for several specific processes and systems. Comparison with transition rates for rare-earth (RE) ion pairs indicates that the effects discussed can be observed experimentally. The importance of these effects as a local probe of the structure of the RE-ion's environment (not only in solids) as well as means for production of a high population of a specific RE excited state is noted.

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

Abstract: 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

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

Abstract: 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.

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

Abstract: 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.