Importance of confined fields in near-field optical imaging of subwavelength objects

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Abstract
The detailed imaging process of subwavelength objects deposited on a planar surface is studied within the framework of a three-dimensional model of scanning near-field optical microscope. The model consists of a truncated pointed fiber approaching a planar surface on which a three-dimensional protrusion is deposited. For this geometry, Maxwell’s equations are solved exactly by applying the field-susceptibility method in the direct space. The technique provides precise evaluations of the physically relevant near and far fields. In order to refine the understanding of the imaging process of subwavelength objects, we present simulated images of low-symmetry protrusions for two different modes of polarization and as a function of the approach distance. These simulations show clearly that subwavelength surface defects induce confined optical near-field distributions that are directly related to the shapes of the objects. We conclude that the central problem of near-field optical microscopy is the optimal detection of the confined fields that are set up by the objects themselves.