Tip-enhanced Spectroscopy Techniques Research Articles

Sample induced intensity variations of localized surface plasmon resonance in tip-enhanced Raman spectroscopy.

Tip-enhanced spectroscopy techniques, in particular tip-enhanced Raman spectroscopy (TERS), rely on a localized surface plasmon resonance (LSPR). This LSPR depends on the near field antenna, its material and shape, and the surrounding medium with respect to its relative permittivity and the volume fraction of the optical near field occupied by the sample. Here, we investigate the effects of the surface composition and topography on the change of the LSPR intensity in tip-enhanced spectroscopy on SrTiO3 nanoislands by monitoring the LSPR enhanced luminescence of gold tips. Our experimental results and analytical estimates indicate that by affecting the effective permittivity of the dielectric environment at the tip apex, the material composition as well as topography of the studied sample induce a change in LSPR intensity. This result significantly helps the understanding of the evolution or origin of the LSPR intensity during a typical TERS measurement, which in turn leads to a more accurate assessment of the relative intensity of different Raman modes in TERS.

Directional surface plasmon-coupled emission of tilted-tip enhanced spectroscopy

Abstract Understanding the spatial radiation pattern in tip-enhanced spectroscopy (TES) is crucial for plasmon-enhanced spectroscopy, chemical analysis and biochemical sensing. Although the TES technique has many excellent advantages, there is still room for improvement in terms of detection sensitivity. In this paper, we theoretically demonstrate the tip-tilted TES configuration featuring high directivity by using side illumination-collection condition. Taking full advantage of the characteristic of high directional emission ascribed to the far-field interference between localized surface plasmon resonance (LSPR) and surface plasmon polariton (SPP) modes, the collection efficiency of TES signals can be largely improved, greatly boosting the detection sensitivity of TES technology. Our theoretical results not only provide a deep understanding of the underlying physical mechanism of the directional surface plasmon-coupled emission of TES, but also serves as a promising guide for the rational construction of a highly efficient TES platform at the single molecular level.

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

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.