Apertureless near-field optical microscopy: A study of the local tip field enhancement using photosensitive azobenzene-containing films

The local optical field enhancement which can occur at the end of a nanometer-size metallic tip has given rise to both increasing interest and numerous theoretical works on near-field optical microscopy. In this article we report direct experimental observation of this effect and present an extensive study of the parameters involved. Our approach consists in making a "snapshot" of the spatial distribution of the optical intensity in the vicinity of the probe end using photosensitive azobenzene-containing films. This distribution is coded by optically induced surface topography which is characterized in situ by atomic force microscopy using the same probe. We perform an extensive analysis of the influence of several experimental parameters. The results are analyzed as a function of the illumination parameters (features of the incident laser beam, exposure time, illumination geometry) as well as the average tip-to-sample distance and tip geometry. The results obtained provide substantial information about the tip's field. In particular, they unambiguously demonstrate both the nanometric spatial confinement of the tip field and the evanescent nature of the nanosource excited at the tip's end. Most of the experimental results are illustrated by numerical calculations based on the finite element method and commented using the literature on the subject. Additionally, we discuss the origin of the optically induced topography on a nanometer scale and present some preliminary results of the apertureless near-field optical lithography based on local field enhancement. Our approach constitutes a useful tool to investigate the near-field of apertureless probes and should enable the optimization of the nanosource for any experiment requiring local optical excitation of the matter. (C) 2003 American Institute of Physics.