Direct measurement of the Gouy phase shift for surface plasmon-polaritons

We directly measure the Gouy phase shift of surface plasmon-polaritons as they evolve through the focus using terahertz (THz) time-domain spectroscopy. This is accomplished by using a semi-circular groove inscribed in a metal foil to couple broadband freely propagating THz radiation to a converging propagating surface wave. Since the spatial properties of these waves, also referred to as THz surface plasmons, are not Gaussian, we perform numerical simulations to determine the electric field distribution on the metal surface. The associated Gouy phase shift can be obtained from the transverse spatial distribution of the converging wave. We find excellent agreement between our measurements and expectations based on the numerical simulations.