Dispersion (optics)

Abstract: The method of dispersion correction as an add-on to standard Kohn-Sham density functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coefficients and cutoff radii that are both computed from first principles. The coefficients for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination numbers (CN). They are used to interpolate between dispersion coefficients of atoms in different chemical environments. The method only requires adjustment of two global parameters for each density functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of atomic forces. Three-body nonadditivity terms are considered. The method has been assessed on standard benchmark sets for inter- and intramolecular noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean absolute deviations for the S22 benchmark set of noncovalent interactions for 11 standard density functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C(6) coefficients also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in molecules and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems.

Abstract: Theoretical calculations supported by numerical simulations show that utilization of the nonlinear dependence of the index of refraction on intensity makes possible the transmission of picosecond optical pulses without distortion in dielectric fiber waveguides with group velocity dispersion. In the case of anomalous dispersion (∂2ω/∂k2>0) discussed here [the case of normal dispersion (∂2ω/∂k2<0) will be discussed in a succeeding letter], the stationary pulse is a ``bright'' pulse, or envelope soliton. For a typical glass fiber guide, the balancing power required to produce a stationary 1‐ps pulse is approximately 1 W. Numerical simulations show that above a certain threshold power level such pulses are stable under the influence of small perturbations, large perturbations, white noise, or absorption.

Abstract: In this chapter, after presenting a brief review of the various types of optical waveguides, we outline the key principles and parameters which describe and define the operation of optical waveguides and fibres The ways in which propagation through optical fibres affects the properties of the guided waves are discussed, including dispersion and non linear effects Power transfer between propagating waves is essential to the operation of a number of components and the fundamentals of coupling theory are reviewed In summary, the theory given provides the foundation for understanding the detailed operation of a wide variety of optical components and systems based on optical fibre technology

Abstract: The physical mechanisms which can produce second-order dielectric polarization are discussed on the basis of a simple extension of the theory of dispersion in ionic crystals. Four distinct mechanisms are described, three of which are related to the anharmonicity, second-order moment, and Raman scattering of the lattice. These mechanisms are strongly frequency dependent, since they involve ionic motions with resonant frequencies lower than the light frequency. The other mechanism is related to electronic processes of higher frequency than the light, and, therefore, is essentially flat in the range of the frequencies of optical masers. Since this range lies an order of magnitude higher than the ionic resonances, the fourth mechanism may be the dominant one. On the other hand, a consideration of the linear electro-optic effect shows that the lattice is strongly involved in this effect, and, therefore, may be very much less linear than the electrons. It is shown that the question of the mechanism involved in the second harmonic generation of light from strong laser beams may be settled by experiments which test the symmetry of the effect. The electronic mechanism is subject to further symmetry requirements beyond those for piezoelectric coefficients. In many cases, this would greatly reduce the number of independent constants describing the effect. In particular, for quartz and KDP there would be a single constant.

Abstract: Wynne-Edwards has written this interesting and important book as a sequel to his earlier (1962) Animal Dispersion in Relation to Social Behaviour. Reviewing it has proven to be a valuable task for one who normally is only at the periphery of the group selection controversy. My comments will be organized into three sections: one regarding the factual content of the book, a second attempting to relate my own expectations and predictions based on soft selection with facts described by Wynne-Edwards, and a third criticizing the argument that has been advanced for group selection. A number of important studies have been summarized in this book. Foremost is the extensive work on red grouse with which Wynne-Edwards has been associated for more than 30years. A great deal of ecological, physiological, nutritional, and behavioural information regarding this bird has been reviewed in seven chapters (pp. 84-170) with additional comments liberally sprinkled elsewhere. Anyone interested in avian biology who has missed this important study must read this book. Also extensively reviewed is the work of Michael Wade on group selection in Tribolium, the flour beetle. Wade's data are presented in enough detail that his work can be understood by those who have not seen the original publications. [Twice (pp. 210 and 233) reference is made to a 40-fold difference between two of Wade's selected lines, whereas the figure on p. 209 suggests that the difference is nearer 7-fold. Figure 11-16 also contains an error.] A number of other studies have been reviewed in some, but not exhaustive, detail. Among these are Smithers and Terry's analysis of immunology in schistosomiasis, Birdsell's studies of the social structure of Australian aborigines and Sewell Wright's shifting balance theory of evolution. Numerous other observations are cited in the text, none in so great detail as those mentioned here, and many in such rapid succession as nearly to overwhelm the reader. Many of Wynne-Edwards' conclusions are based on densityand frequencydependent selection, territoriality and the migration of individuals between and within populations at various heirarchal levels (in-groups, demes, populations and higher categories). Still, there is no mention of authors such as Howard Levene or Wyatt Anderson. Consequently, I feel justified in presenting