Imaging theory of vectorial optical near field based on reciprocity of electromagnetism.
Near-field microscopy is widely used for characterizing electromagnetic field at the nanoscale. In scanning near-field optical microscopy (SNOM), a nanoprobe offers the opportunity to extract optical field quantities, such as amplitude, phase, polarization and chirality. However, owing to the complexity of the probe-field interaction, a general and intuitive theory is still highly needed to interpret and analyze the vectorial field response of the near-field probes. Here, we develop the near-field imaging theory and models based on the reciprocity of electromagnetism in near-field optics and multipole expansion, for both the aperture-type and apertureless SNOM. Our models are in close analogy with the multipolar Hamiltonian for the light-matter interaction energy and highlight the coupling mechanism between the probe and the near-field. Applying the proposed models, we systematically investigate two typical vectorial near-field imaging processes: the optical magnetism detection with a split-ring probe and the probing of transverse spin angular momentum with a high-index nanoparticle. The theoretical results are in good agreement with the reported experiments. Our work may provide a powerful tool for interpreting and analyzing the vectorial near-field imaging processes and paves the way for designing novel functional near-field probes and detecting various physical quantities of the optical near field.
Publisher URL: http://arxiv.org/abs/1801.07849
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