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I am currently reading about exciting surface plasmon polaritons (SPPs). One of the most common methods described in literature for exciting a SPP is using the Otto configuration (image from https://doi.org/10.1364/OE.24.019517):

I understand that it's impossible to excite a SPP by a light plane wave incident from free space because the in-plane momentum has to be conserved and the incoming momentum would be too small. I also understand that we use a high-refractive index prism in the Otto configuration, because the momentum of light in a prism with refractive index n will be n times greater than in vacuum.

I don't understand why there has to be an air gap, though. Can someone tell me if it is important and what I'm missing?

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I believe that in the perfect configuration for SPP you want to have total internal reflection of the incident beam which gives you an evanescent wave with the right phase matching (if you tune the air gap carefully).

You may want to read page 420 of the following notes: https://www.photonics.ethz.ch/fileadmin/user_upload/Courses/NanoOptics/plasmonss.pdf

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