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We characterized the longitudinal field formed at a tightly focused spot by a high numerical aperture objective lens using a tip-enhanced near-field microscope. The longitudinal field efficiently excites the localized surface plasmon polaritons at the metallic tip apex resulting in an electric field enhancement. Radially polarized light generated by a combination of four half-waveplates successfully increases the longitudinal field resulting in higher sensitivity for tip-enhanced Raman spectroscopy of adenine nanocrystals.
Hayazawa et al. (Wed,) studied this question.