Evanescent wave coupling between two prisms

1. Jan 18, 2012

sfm29

Hello all,
I have recently encountered a problem in my understanding of how evanescent waves work. The problem will refer to the diagram found at http://www.popularscience.co.uk/features/nimtz2.jpg (while the evanescent wave isn't drawn this is the only example i could find).

So physically light enters the prism and is reflected back due to the incident angle being greater than the critical angle. Due to the boundary conditions put forward by Maxwell the parallel component of the electric field at the interface must be zero. This gives rise to a decaying electric field.

First question : what is this decaying field physically? surface vibrations of electrons?

This decaying field is then intercepted by a second prism and then light is seen to be emitted from the second prism.

Second question : What physically causes these photons? the evanescent field interacting with electrons at the second interface causing emission of light?

I understand that my confusion comes from a lack of understanding of an evanescent field probably. Though I would be happy with that if I hadn't already learned quantum tunneling as an undergraduate!! (an analogy to this problem)

Thanks for any help you can give in interpreting equations into a physical meaning my brain can comprehend!

2. Jan 18, 2012

Antiphon

The evanescencent field is one that can't propagate freely because of the way it is being formed.

In the case of the prism, the field just outside the prism is oscillating at the same frequency as the light in the glass. But instead of propagating in space at some angle away from the glass, it just hugs the glass and moves along it at the reduced speed of light in the glass. As a function of distance perpendicular to the prism it decays exponentially. This means the light can be coupled into the second prism with varying intensity by changing the size of the gap.

Evanescent propagation is best visualized using a rectangular waveguide. The lowest mode is the lowest frequency that can propagate down the guide. If you reduce the frequency a small amount below that, the wave will still go into the guide but it will decay exponentially.

The wave trying to leave the prism past the TIR angle is in the same situation as a cutoff waveguide.

3. Jan 19, 2012

Andy Resnick

Just to add, frustrated TIR can also been seen by grasping a glass full of water and looking down into the water: where the ridges of your fingerprints contact the glass you will see skin and in the valleys you will see what looks like a mirrored surface.

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