Massive surface-plasmon polaritons

It is well-known that a quantum of light (photon) has a zero mass in vacuum. Entering into a medium the photon creates a quasiparticle (polariton, plasmon, surface-phonon, surface-plasmon polariton, etc.) whose rest mass is generally not zero. In this letter, devoted to the memory of Mark Stockman, we evaluate the rest mass of light-induced surface-plasmon polaritons (SPPs) and discuss an idea that collisions of two massive SPP quasiparticles can result in changes of their frequencies according to the energy and momentum conservation laws