Amplitude and phase of surface plasmon polaritons excited at a step edge

A combined experimental and theoretical study on the laser-induced excitation of surface plasmon polaritons (SPP) at well-defined step edges of a gold–vacuum interface is presented. As a relevant parameter determining the coupling efficiency between laser field and SPP, we identify the ratio between step height h and excitation wavelength λ. For specific values of h/λ, an almost complete suppression of SPP excitation is observed, which corresponds to a condition of constructive interference between the waves reflected at the top and bottom of the step. Experiment and theory show, furthermore, that next to the amplitude also the phase of the SPP is affected by the superposition.