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

Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

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Multiferroics: a magnetic twist for ferroelectricity

“…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Experiments on simple magnetic model systems

CRYSTALLINE MATERIALS Introduction Physical Properties Optical Properties Mechanical Properties Thermal Properties Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties GLASSES Introduction Commercial Optical Glasses Specialty Optical Glasses Fused Silica Fluoride Glasses Chalcogenide Glasses Magnetooptic Properties Electrooptic Properties Elastooptic Properties Nonlinear Optical Properties Special Glasses POLYMERIC MATERIALS Optical Plastics Index of Refraction Nonlinear Optical Properties Thermal Properties Engineering Data METALS Physical Properties of Selected Metals Optical Properties Mechanical Properties Thermal Properties Mirror Substrate Materials LIQUIDS Introduction Water Physical Properties of Selected Liquids Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Commercial Optical Liquids GASES Introduction Physical Properties of Selected Gases Index of Refraction Nonlinear Optical Properties Magnetooptic Properties Atomic Resonance Filters APPENDICES Safe Handling of Optical Materials Abbreviations, Acronyms, and Mineralogical or Common Names for Optical Materials Abbreviations for Methods of Preparing Optical Materials and Thin Films Fundamental Physical Constants Units and Conversion Factors

/pdf/handbook-of-optical-materials-3e2ukwmx2l.pdf

Handbook of Optical Materials

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Multiferroicity: the coupling between magnetic and polarization orders

Energy transfer studies employing the sodium rare earth tungstates (scheelite structure) indicate a number of modes of, and requirements for, nonradiative interaction. Direct dipole‐dipole interactions are prevalent. However, dipole‐quadrupole interactions are observed for the states of the larger rare earth ions that lie high in energy. The relation of lifetime to intensity changes with the mode of transfer. Phonon assisted transfer is enhanced by exchange coupling between rare earth ions in nearest or next nearest neighbor positions. Excitation may migrate between Tb ions in such positions when thermally activated and provided a suitable quenching center (e.g., Nd, Eu) is present. Such migration appears to be directional in comparison to transfer by radiation reabsorption. The latter is essentially random. Methods for analyzing lifetime data for transfer interactions are also discussed.

https://iopscience.iop.org/article/10.1149/1.2424184/pdf

Characterization of Energy Transfer Interactions between Rare Earth Ions

An empirical relation fitting the position in energy of the lower d-band edge for Eu2+ or Ce3+ in various compounds

Fluorescence quenching effects caused by multipolar energy‐transfer interactions between rare‐earth ions are examined. Transfer between remote ions by way of transitions that are matched in energy appears to be essentially dependent upon concentration alone (e.g., Sm3+ self‐quenching) while transfer by non‐resonant mechanisms can show a strong dependence upon crystal structure (e.g., Eu3+ self‐quenching). Examples of thermally dependent multipolar transfer interactions are also given.

Energy Transfer Between Rare‐Earth Ions

The stimulated-emission cross sections and fluorescence branching ratios for the principal fluorescence transitions of YAG:Nd emanating from the $^{4}F_{\frac{3}{2}}$ state have been measured at room temperature. The peak cross section of the well-known 1064.2-nm laser line is here found to have a value of 4.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$ ${\mathrm{cm}}^{2}$. Since the cross sections reported here are inconsistent with the generally accepted value of near unity for the radiative quantum efficiency $\ensuremath{\eta}$ of the $^{4}F_{\frac{3}{2}}$ state the quantity $\ensuremath{\eta}$ has been measured directly. A value of 0.56\ifmmode\pm\else\textpm\fi{}0.11 is found for $\ensuremath{\eta}$ which is consistent with the observed fluorescence lifetime of the $^{4}F_{\frac{3}{2}}$ state and the measured cross sections.

Stimulated-emission cross section and fluorescent quantum efficiency of Nd 3+ in yttrium aluminum garnet at room temperature

The dependence of the resistivities of nickel and nickel zinc ferrites prepared from high purity reagents upon composition and firing temperature has been studied.It has been determined that iron deficient nickel zinc ferrites are of much higher resistivity than iron deficient nickel or zinc ferrites.The iron deficient ferrites prepared, which were fired in oxygen below 1300°C and have Ni:Zn ratios greater than 3:7, show a positive thermoelectric voltage. Other compositions or materials fired above 1300°C have a negative thermoelectric voltage.The effect of excess iron upon resistivity is dependent upon the presence or absence of zinc at temperatures below 1300°C.

L. G. Van Uitert

Papers

Characterization of Energy Transfer Interactions between Rare Earth Ions

An empirical relation fitting the position in energy of the lower d-band edge for Eu2+ or Ce3+ in various compounds

Energy Transfer Between Rare‐Earth Ions

Stimulated-emission cross section and fluorescent quantum efficiency of Nd 3+ in yttrium aluminum garnet at room temperature

dc Resistivity in the Nickel and Nickel Zinc Ferrite System