Note: Controlling the length of plasmonic tips obtained by pulsed electrochemical etching.

We present a method to control the length of plasmonic gold tips through pulsed electrochemical etching. This method uses a cut-off circuit to interrupt the etching when the desired length is achieved, paving the way to tune the plasmonic properties of these tips through their shape. The control of the tip length by monitoring the cell voltage is the result of a study of the etching dynamics. The resulting tips possess a low apex radius and a small opening angle, allowing for high spatial resolution both in topography and in near-field imaging. The plasmonic behavior was confirmed in tip-enhanced Raman spectroscopy.

High resolution scanning near field mapping of enhancement on SERS substrates: comparison with photoemission electron microscopy.

The need for a dedicated spectroscopic technique with nanoscale resolution to characterize SERS substrates pushed us to develop a proof of concept of a functionalized tip-surface enhanced Raman scattering (FTERS) technique. We have been able to map hot spots on semi-continuous gold films; in order to validate our approach we compare our results with photoemission electron microscopy (PEEM) data, the complementary electron microscopy tool to map hot spots on random metallic surfaces. Enhanced Raman intensity maps at high spatial resolution reveal the localisation of hotspots at gaps for many neighboring nanostructures. Finally, we compare our findings with theoretical simulations of the enhancement factor distribution, which confirms a dimer effect as the dominant origin of hot spots.

A near field optical image of a gold surface: a luminescence study.

This paper addresses recent experimental findings about luminescence of a gold tip in near-field interaction with a gold surface. Our electrochemically etched gold tips show a typical, intrinsic luminescence that we exploit to track the plasmon resonance modeled by a Lorentzian oscillator. Our experimental device is based on a spectrometer optically coupled to an atomic force microscope used in tuning fork mode. Our measurements provide evidence of a strong optical coupling between the tip and the surface. We demonstrate that this coupling strongly affects the luminescence (intensity, wavelength and FHWM) as a function of the tip position in 2D maps. The fluctuation of these parameters is directly related to the plasmonic properties of the gold surface and is used to qualify the optical near field enhancement (which subsequently plays the predominant role in surface enhanced spectroscopies) with a very high spatial resolution (typically around 20 nm). We compare these findings to the independently recorded near-field scattered elastic Rayleigh signal.