Local excitation of surface plasmon polaritons in random surface nanostructures

We investigate local excitation of surface plasmon polaritons (SPPs) at a 55-nm-thick gold layer covered with randomly located scatterers (density similar to75 mum(-2)) by using an uncoated fiber tip of a near-field optical microscope as a radiation source and detecting the radiation scattered in the direction of resonant (at flat regions) SPP excitation. In the wavelength range of 725-773 nm, we observe a considerable decrease in the detected signal from the random scattering structures as compared to that from the flat (corrugation-free) regions. We relate the obtained optical images to the electromagnetic local density of states corresponding to SPPs. Correspondence between these images (recorded in the illumination mode) and those obtained in the collection mode with similar structures is also discussed.