Gap maps and intrinsic diffraction losses in one-dimensional photonic crystal slabs

A theoretical study of photonic bands for one-dimensional (1D) lattices embedded in planar waveguides with strong refractive index contrast is presented. The approach relies on expanding the electromagnetic field on the basis of guided modes of an effective waveguide, and on treating the coupling to radiative modes by perturbation theory. Photonic mode dispersion, gap maps, and intrinsic diffraction losses of quasi-guided modes are calculated for the case of self-standing membranes as well as for Silicon-on-Insulator structures. Photonic band gaps in a waveguide are found to depend strongly on the core thickness and on polarization, so that the gaps for transverse electric and transverse magnetic modes most often do not overlap. Radiative losses of quasi-guided modes above the light line depend in a nontrivial way on structure parameters, mode index and wavevector. The results of this study may be useful for the design of integrated 1D photonic structures with low radiative losses.Comment: 9 pages, 8 figures, submitted to Physical Review

Introduction:Who's at the bottom? Examining claims about racial hierarchy

Why do claims about racial hierarchy matter? The question whether some groups are worse off than others is highly pertinent at a time when there is growing recognition of multiple forms of racisms and racial oppression. It is widely accepted that racial hierarchies are still with us today, and this concept is peppered throughout writings on ‘‘race’’ and racisms, but, what, exactly, are racial hierarchies, how do racial hierarchies continue to matter, and in what ways do they operate? This special issue, which focuses on the USA and Britain, also addresses the following questions: Does the concept of racial hierarchy aid us in illuminating racial inequalities and the differential experiences of groups in Western multiethnic societies such as the USA and Britain? What sorts of criteria are used in arguments about the place of groups along racial hierarchies? What are the political implications of claims made about racial hierarchies