Direct observation of locally enhanced electromagnetic field

Surface enhanced Raman scattering and other nonlinear enhanced optical effects are well known to be induced by the surface of discontinuous or rough metal thin films. In the percolating range of concentration, theoretical calculations lead to locally enhanced field distributions at the surface of the films, due to huge fluctuations close to the phase transition threshold. Using a scanning near-field optical microscope (SNOM) of extremely high lateral resolution (10 nm), we have been able to record the held distribution close to the surface of discontinuous gold films in both transmission and reflection modes. We report here the direct observations, at a scale much shorter than the wavelength, of the giant field peaks, the so-called "hot spots". Their intensities and spatial distribution are found in good agreement with the theoretical predictions. (C) 2000 Elsevier Science B.V. All rights reserved.