With the aid of surface plasmon polaritons (SPPs), optical researchers could potentially open up greater levels of light control on the subwavelength scale. But it's not easy to efficiently convert travelling SPP waves into highly localized 'hot spots' of light. Claus Ropers and colleagues in Germany and the USA have now made headway on this front (Nano Lett. doi: 10.1021/nl071340m; 2007).
Researchers have already toyed with the idea of launching surface plasmons onto sharp, tapered metallic structures to concentrate the optical energy into a tiny, intense spot for further use. A number of apertureless near-field imaging and spectroscopy techniques could benefit from having a very localized light source at the end of such a tip, as it would allow a significant reduction in the amount of far-field background light that illuminates the sample.
In their latest work, Ropers et al. focus SPPs onto sharp conical gold tapers with a tip radius of just a few tens of nanometres and an opening angle of about 15°. A one-dimensional grating (with 750-nm periodicity) is electrochemically etched onto the shaft of the cone, several micrometres from the tip end. By illuminating the grating with femtosecond laser light, SPPs are excited, which then travel along the shaft to the tip apex where they are radiated into the far field. A microscope objective and video camera or spectrometer are used to collect the scattered light and study it.
At the tip, the SPPs converge to form an intense radiative local light spot. The field is enhanced by about a factor of ten at the tip, and the excitation spot size reduces to a few tens of nanometres from a few micrometres in and near the grating. Moreover, the excitation is very efficient: for optimized coupling conditions, the total power of the light scattered from the tip is only about 0.1% to 1% of the incident light, that is, about 1 μW to 10 μW for an incident power of 1 mW.
Scattering losses are minimized by using tips with minimal surface roughness between the grating and the tip apex. By varying the angle of incidence of the incoming laser beam, the tip emission can be spectrally tuned. These tiny tips could serve as bright light sources for use in nanospectroscopy and near-field imaging, where modulation techniques are usually needed to extract the near-field signal from far-field background light.