Analysis of the effects arising from the near-field optical microscopy of homogeneous dielectric slabs

The term "near-field optical microscopy" refers to a range of techniques ca pable of producing images of surfaces at a resolution far exceeding the Ray leigh criterion for conventional microscopes. In the illumination mode scan ning nearfield optical microscope, the sample is placed in the near field o f an optical source formed from the aperture at the end of a fibre-optic ta per. In normal operation, the aperture is raster scanned over the sample an d, by recording the resultant far-field intensity transmitted through the s ample as a function of the source coordinates, an image is assembled whose appearance derives from a non-trivial combination of the optical and topolo gical composition of the sample. In this paper a numerical analysis of the near-field interaction of the source and sample is performed using a three- dimensional finite-difference time-domain method. A propagation technique i s applied to the near-field values to form a far-field signal that is repre sentative of the signal generated by a real instrument. In this way, the mi croscope output for a variety of very simple samples is simulated. It is sh own that the magnitude of the transmitted signal can be increased both by d ecreasing the separation between source and sample and, in some circumstanc es, by increasing the sample permittivity. Simulated microscope output from samples containing topological variation is compared with that produced by samples containing a variation in refractive index and it is demonstrated that contrast inversion may occur at different scan heights in the latter c ase, leading to the possibility of distinguishing between the different con trast mechanisms experimentally. (C) 2001 Elsevier Science B.V. All rights reserved.