Focused Excitation of Surface Plasmon Polaritons Based on Gap-Mode in Tip-Enhanced Spectroscopy

摘要

We propose a novel method to improve field enhancement factor in tip-enhanced spectroscopy techniques using gap mode configuration. In gap-mode, a silver coated tip is placed just above the focus at silver thin film deposited on a dielectric substrate. The enormous electric field enhancement generated at the focus is due to coupling of surface plasmon polaritons at the gap between the 48 nm silver film and silver coated tip. The proposed technique was demonstrated in both radially polarized and linearly polarized beam. Radial polarization was found to be better than linear polarization in gap mode because only p-polarized component can contribute to the excitation of surface plasmon of the silver film. The electric field distributions at the gap below tip are calculated using finite difference time domain method and experimentally visualized by scanning probe microscope. Numerical results are in good agreement with the empirical data.