Near-field optical microscopy (aperture-less type) enables optical analysis and imaging with the nanoscale spatial resolution owing to light field localized at a nanometric volume at a plasmonic tip apex. It has been recognized as a powerful analytical technique for a few decades since its invention, and has still shown tremendous progress, contributing to a wide variety of scientific fields. Its spatial resolution, for example, has been drastically improved in recent years, which now goes down to the single molecular level.
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