Local optical field in the neighborhood of structured surfaces: phase singularities and Talbot effect B. Levine * , M. Caumont * , C. Amien ** , B. Chaudret ** , B. Dwir *** and W.S. Bacsa * * Laboratoire de Physique des Solides, Université Paul Sabatier, UMR-CNRS 118 route de Narbonne, Toulouse 31062, France, bacsa@lpst.ups-tlse.fr ** Laboratoire de Chemie de Coordination, UP-CNRS, 205, route de Narbonne, 31077 Toulouse, amiens@lcc-toulouse.fr *** Laboratory of Nanostructures, Swiss Federal Institute of Technology Lausanne, Lausanne 1015, Switzerland, benjamin.dwir@epfl.ch ABSTRACT We have probed the local optical field in the vicinity of structured surfaces to observe optical coherence effects. We have used a sub-wavelength sized optical fiber probe to map the local field of microgratings and nanometer sized gold particles. While probing the middle field region, the ratio of the aperture size to the object-probe distance is smaller (a/h =1) than in the nearfield region (a/h=10). In the middle field one probes mostly the transverse field component and probe induced effects are reduced. We have observed self-imaging (Talbot effect) of the micrograting structure in the direction of the incident beam and apparent phase singularities from the deposited gold particles in the direction of the reflected beam. Keywords : local optical field, structured surfaces, middle field, Talbot effect, phase singularities 1 INTRODUCTION Structured surfaces with dimensions comparable or smaller than the wavelength of light are increasingly used as parts of functional units in integrated devices. Lens based optical microscopes rely on ray optics and ignore the wave aspect and the lateral resolution is limited by diffraction. Scanning optical probe techniques with apertures smaller than the wavelength circumvent the diffraction limit of lens based systems. Nearfield optics (1) aims to use the enhanced longitudinal local field using an optical probe in the proximity of surfaces. The transmis sion of optical waves through the aperture of the probe is in general strongly reduced as the aperture size drops below the size of the wavelength. Apertures in the size range of /4 are typically used. To reach the nearfield the distance to the surface is often ten times smaller than the aperture size although the lateral resolution is given by the aperture size. The small probe-substrate distance makes it difficult to image surfaces with height variations in the size range of the probe aperture. We note that the illuminating beam which strikes the surface overlaps with that on the surface scattered field within its vicinity. The overlap of propagating beams gives rise to lateral and vertical standing waves with wave fronts perpendicular to the direction of counter-propagating waves (2-4). The optical probe is used to detect the local optical field of the standing waves or interferograms. The standing waves are clearly related to the surface structure and can give information about the surface at variable distances from the surface. To obtain high lateral resolution a smaller distance is still needed but the distance can be larger than in nearfield optics by one order of magnitude. We denote the distances ranging from 0.5 m to 100 m as the middle field. We report here local optical field measurements in the proximity of agglomerated nano-sized gold particles on a polished silicon wafer and micrometer sized optical gratings etched into GaAs. We also discuss the different types of observed standing waves displaced in the incident or reflected beam direction. Figure 1 shows standing wave patterns recorded at two different distances (100 m, 10 m) from the surface. The optical probe and surface have been illuminated by a laser beam (wavelength: 669nm, 10mW, s-polarized) and the local field is detected through a metal coated optical fiber probe (Nanonics Imaging Ltd., aperture size 100nm). The gold particles 2-3nm in size have been deposited on a polished Si wafer from a suspension and the solvent was evaporated off. Concentric diffraction fringes around the agglomerated gold particles are observed (Fig.1) as well as diagonal fringes. The incident and reflected beam of a flat surface form standing waves parallel to the surface. They become visible as parallel fringes when the image plane is tilted with respect to the surface. The diagonal fringes are due to the tilt of the image plane with respect to the surface and can be corrected by tilting the image plane with the piezo-electric scanner (CP-R Digital Instruments). The tilt angle is given by the fringe spacing (5). The fact that the diagonal fringes are not exactly straight lines indicates that the image plane is not only tilted but slightly deformed due to the movement of the pie zo-electric tube scanner. The substrate is not displaced in a perfectly parallel manner. The imaging of optical standing waves on flat surfaces in turn can be used to verify the symmetric scanner movement. We find that the concentric fringes depend on NSTI-Nanotech 2004, www.nsti.org, ISBN 0-9728422-9-2 Vol. 3, 2004 5