Plasmonic scanning probe tips

In this project we investigate the fundamental processes which determine the optical response of individual metal tips used for tip-enhanced and -scattering spectroscopy. Simple analytical models allow for a description of the spectral response and underlying dephasing based on the Drude dielectric response:

Here the tip is modeled as a prolate hemispheroid in the quasistatic approximation. The dephasing time is shown as a function of the aspect ratio a/b (major / minor axes). With increasing aspect ratio the plasmon resonance red-shifts which is accompanied by a increase in the plasmon dephasing time.

Studies of the intrinsic tip luminescence provide the experimental insight into the tip plasmon as well as the plasmonic coupling between the tip and a metal substrate that leads to large field-enhancement factors. In the following figure, spectra are acquired with the tip at the indicated distance from the substrate. We can clearly see an increase in the overall emission intensity accompanied by a red-shift of the resonance peak as well as a narrowing of the spectral lineshape in accordance with theory.

The observed tip luminescence on approaching a Au substrate acquired at the distances indicated. The black lines are Lorentzian fits. On approaching the sample, a significant increase in the intensity is observed accompanied by a clear red-shift in the luminescence spectrum and a decreased linewidth.