Nanoscale observation of enhanced electromagnetic field

The surface of nanosized discontinuous or rough metal thin films is able to induce Raman scattering enhanced by several orders of magnitude. This effe ct has been theoretically attributed to the local field distribution at the surface of the films. As for the relevant parameter in phase transitions, the fields experience here huge fluctuations, leading to localized giant pe aks so called "hot spots". Using a Scanning Near-Field Optical Microscope ( SNOM) of extremely high lateral resolution (10 nm), we have been able to re cord the field distribution close to the surface of gold films. We report h ere the first direct observation of the hot spots with such lateral resolut ion. Their intensities and spatial distribution are found in good agreement with the theoretical predictions. We also have performed local spectroscop y, which shows up sharp variations at nanometric scale (much smaller than t he wavelength).