# Evanescent wave coupling

In my optics course we have observed that in several applications, such as prism coupling of modes in an optical waveguide, coupling between two waveguides, attenuated total internal reflection (or frustrated internal reflection) in spectroscopy, part of the intensity of the incident wave goes from one medium to another one through a surface which has total internal reflection. This is because outside this surface we have an evanescent wave whose electric field goes to zero as an exponential function. In spite of that, in another part of the course we demonstrated (as was done, for instance, in Pedrotti et al., "Introduction to Optics") that this exponential electric field is perpendicular to the interface, therefore the Poynting vector is parallel to the surface. This means that there cannot be transferred energy from the first medium to the second one, because there is no flux of the Poynting vector in that direction. Therefore, how can we have the coupling effects described before? I think there is something incorrect (or at least not always valid) in the deduction about the evanescent field, but I can not come up with what could it be.

EDIT (23-dec-2016):

Looking on the web, I found this page: https://en.m.wikipedia.org/wiki/Surface_plasmon, in which we have this application: https://en.m.wikipedia.org/wiki/Surface_plasmon_resonance. I don't think it is exactly my case, but since I haven't studied them yet in my course I can't fully comprehend them. Can this be a possible explanation to my problem? Why?

EDIT (26-dec-2016):

Trying to solve my question, I found a Wikipedia page about Near Field and Far Field in electromagnetic waves and in the "Near Field characteristics" section there is a description of some effects in the near field region. Are these effects the same as mine? Is there a way to investigate them more mathematically than as it is done there? Can somebody suggest some reference? Is there a way to explain them in Classical Physics or the only way to do that (as it seems on that page, if it is my case) is in term of virtual photons in Quantum Field Theory?

• The "coupling" part in your question is not perfectly clear, could you pick a precise example? The evanescent wave Poynting vector may not be perfectly along the surface when the medium is not semi-infinite. If it is just a thin slab, it can have a perpendicular component and transmit energy to the other side. Commented Dec 8, 2016 at 22:10
• The main application I am thinking about is coupling between two planar, dielectric waveguides. As presented in class, we used the approximation of an infinite slab in two directions. Commented Dec 8, 2016 at 22:15
• @fffred the coupling is known as frustrated total internal reflection. Commented Dec 9, 2016 at 11:24