Sorry if this question doesn't make sense; this is way out of my field of study. From what I've read about SPP's, they seem to be oscillations in electron densities moving over the surface of a metal/dielectric interface in one direction. However, I was recently reading about their potential applications in nano rectennas (rectifying antennas) and I don't understand why a seemingly (to me) pulsating DC current would need to be rectified. Thanks for your help.
Surface polaritons-plasmons (SPPs) are coupled oscillations of charge density and electromagnetic fields that can propagate as waves along the surface of two materials which have opposite signs of the real part their permittivity. Usually, in the considered frequency range, metals due to their electron plasma have a negative real part of permittivity and can be used together with a dielectric (air, etc.) to propagate these waves. These waves can be excited by light waves by appropriate couplers and usually have shorter wavelengths at a given frequency than light waves in vacuum. These waves can be considered to be sinusoidal waves where the electric field, magnetic field, charge density and current (in the metal) oscillate sinusoidally. To convert light waves into usable electrical power, the induced electric current oscillations induced by the ac field of the light in minuscule antennas, together with e.g. rectifying metal-insulator-metal diodes, can be utilized to produce a dc current/voltage to generate usable electric power.