Near-Field Imaging of Surface Plasmon Polaritons Excited by Chains of Gold Nanodiscs

We present a study of the near-field pattern created by chains of gold nanodiscs situated on a gold thin film and illuminated at oblique incidence. Each disc generates surface plasmon polaritons that propagate on the gold surface. The created waves interfere between them and with the illuminating beam. We observed that when the discs are separated by a distance smaller than the half wavelength, the chain behaves like a continuous ridge. When the discs separation increases, a complex periodic pattern appears and extends up to several wavelengths from the chain. For some specific separation distances, a directional emission of surface plasmon is also observed. The experimental results are in good agreement with numerical simulations performed by considering each disk as an independent dipole-like surface plasmon source.