Experimental study of surface-plasmon scattering by individual surface defects

A direct-write ablation technique has been implemented in a photon scanning tunneling microscope setup. This combination allows us to study surface-plasmon (SP) scattering by in situ created individual surface defects, while the sizes and shapes of the defects are varied continuously. It is found that within a certain range of its size, a ``hill'' on an otherwise flat surface can be the source of a very narrow plasmon beam. This effect is explained using the Huygens-Fresnel principle. Shadowing and refraction of the SP field by smaller defects has also been observed. In order to explain these results we introduce an effective SP refractive index for two classes of surface defects: shallow topographical defects and areas covered with absorbed molecular layers. This concept allows us to achieve a qualitative understanding of plasmon scattering in many practical cases. Some simple optical elements for the control of SP propagation are suggested and demonstrated. Our observations suggest numerous practical applications in multichannel chemical sensing, biosensing, and integrated optics.