Capture of electromagnetic radiation in plasmas with diffuse edges

We aim at showing, firstly, that geometrical properties in a transparent medium with diffuse edges could define a region where electromagnetic radiation would be captured due to total internal reflection. Secondly, we are interested in finding a simple criterion, on the one hand, to realize that radiation capture is occurring in a given system, and, on the other hand, to apply it to technology. Assuming a spherically symmetric system, we apply the Fermat’s extremal principle in the framework of the geometrical optics to show that radiation generated in the system could be captured in a region where the refractive index, n(r, ν), varies with r, r being the distance from the center of the system, so that the product n(r, ν)r has a negative slope in the region where the radiation capture would occur.