From my understanding, surface plasmons (SPs) are oscillations of electrons caused by incident EM waves. A photon from the EM wave can couple with a plasmon to create a surface plasmon polariton (SPP). Some books/websites use the words SPP and SP interchangeably, so I am confused as to whether they really are the same thing or not. I am not sure what this 'coupling' really is. My friend assures me that SPPs are actually EM waves since they are coupled with photons, but I think they are still just mechanical waves since they cannot propagate through vacuum and are longitudinal, at least to my understanding. Does anyone know what exactly is coupling, and if this really does cause something to act as an EM wave, if the coupling is done with a photon??