# Evanescent Wave Refraction

I recently learned that a beam of light that undergoes total internal reflection leads to the formation of an evanescent surface wave on the reflection surface. What happens if this wave encounters a material with a higher or lower refractive index (the edge of the prism for example)? Will it refract? Can they, themselves undergo total internal reflection?

Evanescent waves can indeed refract and undergo total internal reflection, but your depiction isn't accurate. The evanescent wave is only present in the areas where there is a bona fide propagating wave on the other side of the boundary, so if you have a pencil-like beam with a limited impact area, the evanescent wave will be localized to that area. Similarly, for the evanescent wave to refract along the blue path in your diagram, you would need the entire prism to be filled with light performing total internal reflection, and while doing so refracting at the other boundary as shown by the blue path.

The considerations in my answer to Does light reflect if incident at exactly the critical angle? may help understand this better.

• Thank you for your response. I see what you are saying about the evanescent waves only forming in the area above the beam. If this is the case, can you cover the surface with a metal and carry the wave via surface plasmons? Would they refract or is that more of an electron thing? – James Jul 12 '17 at 10:48
• If the metal is in direct contact with the initial dielectric, then you won't have the evanescent wave at all. If it is at a finite (but small) distance away from the boundary, then what you have is an evanescent-wave coupling like the one in microwave "tunnelling" experiments. This is normally done with a second dielectric, though - getting light to couple to metals beyond a localized effect is hard. – Emilio Pisanty Jul 12 '17 at 11:18
• According to the wikipedia article on surface plasmon resonance, it looks like you can couple it both ways (metal on surface or with a gap). Lets say you couple it with a gap (Otto configuration). The surface plasmon is now on the interface between the gap and the metal. What happens if you fill the gap down stream of the plasmon creation with a higher refractive index dielectric? Would that affect the plasmon's path? – James Jul 12 '17 at 13:58
• @James That's a good deal of a broader question that can be answered in a comment ─ you should ask it separately, and even then, make it rather more concrete and with more details on the specifics than you're providing here. – Emilio Pisanty Jul 12 '17 at 15:26

evanescent wave actually is not a wave! because it has not an EM oscillating nature. it is a decay of energy.

and in certain circumstances can change to oscillating wave and propagates in the medium.

most of the waveguide, ring resonator, and fibers couplers use evanescent to the oscillating wave mechanism.