Photon Momentum in a Surface Plasmon

It is shown that the photon momentum in a surface plasmon is hk, where k is the plasmon wave vector. In a significantly delayed plasmon on a thin metal film or filament, the photon momentum is several hundred times larger than the momentum of a photon of the same frequency in a plane wave in free space. The plasmon-plasmon and free-space plane wave-plasmon scattering by a free electron is considered. It is shown that, in contrast to the plane wave-plane wave scattering, the electron acquires a significant momentum, while the scattered plasmon frequency significantly changes. The former process is similar to the Compton effect: scattering of X-ray and. quanta (having a significant momentum) by an electron. In the latter process, the system composed on an incident photon and an electron at rest, which has a small total momentum, generates a photon and an electron, each having a large momentum. A device for detecting electrons from plasmons excited in nearby metal nanofilms and nanowires can be developed on the basis of this principle.