Imaging optical near-fields of nanostructures

We present a method for imaging the optical near-fields of nanostructures, which is based on the local ablation of a smooth silicon substrate by means of a single, femtosecond laser pulse. At those locations, where the field enhancement due to a nanostructure is large, substrate material is removed. The resulting topography, imaged by scanning electron or atomic force microscopy, thus reflects the intensity distribution caused by the nanostructure at the substrate surface. With this method one avoids a possible distortion of the field distribution due to the presence of a probe tip, and reaches a resolution of a few nanometers. Several examples for the optical near-field patterns of dielectric and metallic nanostructures are given.