Carbon Nanotubes for the Optical Far-Field Readout of Processes That Are Mediated by Plasmonic Near-Fields br

As science progresses at the nanoscopic level, itbecomes more and more important to comprehend the interactionstaking place at the nanoscale, where optical near-fields play a key role.Their phenomenology differs significantly from the propagative lightwe experience at the macroscopic level. This is particularly importantin applications such as surface-enhanced spectroscopies for single-molecule detection, where often the optimization of the plasmonicstructures and surfaces relies on far-field characterizations. Theprocesses dominating in the far-field picture, though, are not the samedominating in the near-field. To highlight this, we resort to verysimple metallic systems: isolated gold nanorods in solution. We showhow single-walled nanotubes can be exploited to read out processesoccurring at the near-field level around metallic nanoparticles andmake the information accessible in the far-field region. This is implemented by monitoring the spectral profile of the enhancement ofthe photoluminescence and Raman signal of the nanotubes for several excitation wavelengths. Through this excitation-resolvedstudy, we show that the far-field optical readout detects the transversal and longitudinal dipolar plasmonic oscillations of goldnanorods, whereas the near-field readout through the nanotubes reveals other mechanisms to dominate. The spectral position of themaximum enhancement of the optical near-field mediated signals are located elsewhere than the far-field bands. This dichotomybetween near-field and far-field response should be taken into account when optimizing plasmonic nanostructures for applicationssuch as surface-enhanced spectroscopies.