Spectroscopic study of the image formation in near-field microscopy, near an evanescent-homogeneous switching

Near-field optical microscopes provide highly resolved images of various sa mples. However, images are difficult to interpret owing to their sensitivit y to illumination conditions. Moreover, by contrast with classical microsco py, the near-field signal combines the contributions of evanescent and prop agative modes. In this study, we present results of a spectroscopic study i n near-field. Our purpose is to explain how a switching of one diffracted m ode from homogeneous to evanescent can modify image formation. The main poi nt is to establish a relation between the evanescence of one diffracted mod e and the fringes that are often observed in near-field experimental images . Moreover, on a metallic sample, the possible occurrence of plasmon resona nce contributes to image distortion in a mainly different way. We use a Fou rier series Rayleigh 3D method to explain image formation.