Evanescent waves are entities made of electromagnetic energy that can't propagate or sustain a resonance.

They decade exponentially from the point of the boundary were are created. Near a transmission line discontinuity, infinite modes are created, only modes over cut-off can propagate.

Evanescent modes can also couple with propagating modes. In electromagnetic mode may exhibit propagation in one direction and evanescent behavior in another direction.

Moreover, in a dielectric discontinuity a mode over cut-off may be under cut off in the other medium, behaving as an evanescent wave.

I hope to be helpful.

Alberto

Think of an evanescent field as an intermediate state.  A wave hits an interface or device and excites an evanescent field.  The energy in the evanescent field eventually couples back to a scattered wave.

One could say that evanescent fields do not contribute to transporting power because they do not oscillate.  The only example I can think of where evanescent fields contribute to power flow is in the case of tunneling through a thin dielectric layer.  Some would argue that the evanescent field outside of a waveguide transports power.  I would argue that the field outside of a waveguide is not evanescent in the direction of propagation so the rule about not transporting power would still hold.

There are certainly waves that oscillate and decay very quickly.  This may be mistaken for evanescent fields, but they are not.  They are simply decaying waves.  Be careful.

Plasma media is equally true?

In fact they can be confused: Evanescent waves can be identified since their attenuation constant and phase constant are perpendicular. In a discontinuity, they have a purely real propagation constant in the direction normal to the discontinuity plane and a purely imaginary propagation constant in the direction tangential to the discontinuity plane. In order to propagate through the discontinuity, the propagation constant needs to be imaginary (no attenuation) or complex in the normal direction.

Does their intensity and wavelength of the propagating light decay at the interface? And what is the physical significance of critical angle for total internal reflection (TIR)? why do the rays below critical angle refract while rays above critical angle reflect? how does evanescent waves behave with respect to critical angle?

Dear Hariharan,

Probably I understood what you are looking for, then I suggest you to read some pages in the following books:

First, read in "Dynamics of Smart Structures" by Ranjan Vepa, paragraph 2.5.2, pages 55-61. It refers to optical fibers, but the described principles are general.

Other useful readings are in "Optical Systems and Processes" by Joseph Shamir. Read the paragraph 2.4 "reflection and transmission at the boundary" pages 14-17.

"Crystalline Rare Earth Doped Sesquioxides OLD-films on Α-alumina" by Sebastian Bär, paragraph 5.1.4, pages 58-62.

Regarding plasma: I suggest you to read in "Principles of Nano-Optics" by Lukas Novotny and Bert Hecht, chapter 12 "noble metals as plasmas". In particular, give attention to pages 374 to 387.

Alberto

I suggest you to read Milan Milosevic book and paper: On the Nature of the Evanescent Wave, APPLIED SPECTROSCOPY, Vol.67, Is.2, p.126 (2013)

Thanks Mikhail, the reference is very useful!