High NA particle- and tip-enhanced nanoscale Raman spectroscopy with a parabolic-mirror microscope.

ABSTRACT

Detecting efficiently the plasmon-enhanced Raman signal of molecules created in the nanometre-sized gap between a metal nanoparticle or the apex of a sharp tip and a metal surface is the key problem in particle- or tip-enhanced local surface spectroscopy (Pettinger et al., 2004; Roth et al., 2006). The optical excitation field has to be polarized along the gap, and the field emerging from the gap has to be observed from the side. These geometrical restrictions usually limit the numerical aperture of the lens used for exciting the gap and collecting the scattered photons created in the gap. We present a novel method to overcome this problem. The solution is based on a confocal optical microscope with a high numerical aperture parabolic mirror for excitation and detection. Localized plasmons can be efficiently excited parallel to the surface normal by illuminating the parabolic mirror with a radially polarized doughnut mode and the field emerging sidewise from the gap can be efficiently collected by the rim of the parabolic mirror and directed to the detection system. First results on particle- and tip-enhanced Raman spectroscopic measurements of benzotriazole molecules adsorbed on gold films are presented.