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

Abstract

The term “near-field optical microscopy” refers to a range of techniques capable of producing images of surfaces at a resolution far exceeding the Rayleigh criterion for conventional microscopes. In the illumination mode scanning near-field optical microscope, the sample is placed in the near field of an optical source formed from the aperture at the end of a fibre-optic taper. In normal operation, the aperture is raster scanned over the sample and, by recording the resultant far-field intensity transmitted through the sample as a function of the source coordinates, an image is assembled whose appearance derives from a non-trivial combination of the optical and topological 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 is applied to the near-field values to form a far-field signal that is representative of the signal generated by a real instrument. In this way, the microscope output for a variety of very simple samples is simulated. It is shown that the magnitude of the transmitted signal can be increased both by decreasing the separation between source and sample and, in some circumstances, 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 case, leading to the possibility of distinguishing between the different contrast mechanisms experimentally.