Frustrated total internal reflection: Where does energy come from?

[greypilgrim]

Hi,

When we have total internal reflection of light at the boundary of an optically dense to an optically thinner medium, one can show that the component of the Poynting vector perpendicular to the boundary is zero, i.e. there is no energy flow into the thinner medium. However, the electrical field is not zero in the thinner medium but decays exponentially with a certain penetration depth.

If we now add an optically denser medium just behind the thin medium, we get frustrated total internal reflection, i.e. a propagating wave in the second dense medium with a nonzero Poynting vector. How is it possible that energy crosses the thin medium although the Poynting vector in the thin medium is zero?

 

[UltrafastPED]

The evanescent field is "stationary" in the first case - hence no energy is flowing out and the Poynting vector is zero.

With the second piece of material brought close to the evanescent field it will penetrate (no longer frustrated because now it is low to high index), so it leaks through. Once it leads through into the second piece of glass the field can propagate.

You should also look into the Goos-Hanchen effect: http://en.wikipedia.org/wiki/Goos–Hänchen_effect

The following article goes into more depth:
http://www.cleyet.org/Misc._Physics/Microwave-Optics/Evanescent-FTIR/FTIR review.pdf