Some questions about surface plasmon polaritons

[TheCatsMeow]

Hi there,

To be clear, this is not a homework question, I am a graduate student reading about the use of plasmonics for biosensing. I felt I should post here instead of in the "general physics" forum since I do have questions, but they are more qualitative, nonetheless I will try to follow the template.

Homework Statement

I'm mostly reading about the Kretschmann configuration shown here:

The gist is that under total internal reflection conditions, the light produces an evanescent wave in the dielectric medium which excites surface plasmons. At the angle when this happens, the reflected light loses energy due to exciting the plasmons and there is a dip in the reflectance at this particular angle, which is sensitive to the dielectric medium's index of refraction.
I'm okay with this, but I'm wondering - why do we only regard this excitation at the dielectric-metal interface, and not the prism-metal interface?

The Attempt at a Solution

My initial thoughts are as follows -
It may be because the evanescent wave occurs in the dielectric under TIR, and there is no effect in the metal (i.e., the metal thickness must be much less than the penetration depth of the wave?) My understanding, though, is that the evanescent wave would be enhanced in the metal, and therefore exist even at the metal-prism interface (where there are also SPPs, I assume).

Likewise, it's possible that this effect does occur at the other interface and ignored because it is not dependent on the dielectric (which is to be sensed)

I look forward to hearing your thoughts.

 

[Cryo]

This diagram i not clear enough. Where is metal? Dark blue line? How thick is the metal? What is the light-blue medium between the prism and dark blue line? Glass slide? Does prism even come into close proximity with the metal? Or is there a glass slide between them?