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Evanescent field interactions in the scattering from cylinders with applications in super-resolution Pawliuk, Peter Cornelius


The diffraction limit defines the maximum resolution of an imaging system that collects and focuses waves. This limited resolution arises from the finite length of the waves used to create the image. Therefore, the only way to increase the resolution is to use higher frequencies with shorter wavelengths. For situations in which increasing the frequency is not possible or not desirable, super-resolution imaging techniques can be applied to overcome the diffraction limit. Super-resolution is possible with the inclusion of evanescent waves, which exhibit unlimited spatial frequencies. Evanescent waves decay exponentially away from their surface of origin so they are difficult to recover. One way to recover evanescent wave information is to scatter the wave from a small object. This scattering converts part of the evanescent wave into radiation that can propagate into the far-field where it can be detected. In order to characterize this conversion, the two-dimensional scattering of evanescent fields from a single cylinder and from multiple cylinders is investigated. The scattering models are derived using an analytical approach where the electromagnetic fields are broken down into cylindrical waves so that the boundary conditions on the cylinders can be applied directly. The incident field can be formulated from a vector plane-wave spectrum, which allows for an arbitrary combination of radiative and evanescent waves. Multiple cylinders of various sizes can be used to approximate the scattering from many two-dimensional objects. For simulating the imaging of objects buried underneath a surface, or near a planar interface, the model is separated into two dielectric half-spaces. An example of a super-resolution application for these models is the simulation of apertureless near-field scanning optical microscopy (ANSOM). In ANSOM, a probe is placed in the extreme near-field of an object in order to scatter the evanescent fields that are formed by the illumination of the object. Images created by ANSOM are fundamentally different from traditional images and are difficult to interpret. The simulations provide insight into how the images are formed and what information they contain.

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