Plasmonic nanoconfined light source

The extraordinary properties of surface plasmon polaritons (SPP) can be used for delivering, controlling and confining light on the nanoscale. We demonstrated the realization of a novel nanoscopic light source by grating-coupled excitation of SPP onto the shaft of conical nanoscopic metallic tips which upon propagation on the tip surface converge in a highly localized light spot at the ultrasharp apex region.

Linear gratings are written more than 10 µm away from the apex by focused ion beam milling. Broad-band laser excitation of the grating results in resonant excitation of SPP with spectral characteristics corresponding to the illumination conditions (incident angle, polarization) and grating period:

Spectrally resolved polarization anisotropy of the nonlocally excited light scattered by the tip apex. Upon broad-band laser (Ti:S, 10 fs@800 nm, FWHM=120 nm) grating illumination the observed spectra are centered around 735 nm and have a FWHM of about 20 nm, as expected from the excitation geometry and grating period. The two-fold symmetry presents a maximum intensity corresponding to a illumination polarized orthogonal to the grating groves (p-in), where plasmon excitation is possible.

In the context of using this intense nanscopic light source for near-field optical experiments, we characterized the field distribution around the apex region by studying the optical coupling of the tip-scattered light to smooth Au surfaces. Strong increase in the optical response is observed for incident polarization orthogonal to the grooves (p-in) on a lenghtscale of about 10 nm, comparable to the tip apex radius:

Near-field localization and enhancement of the tip-scattered light for grating illumination upon approaching a flat Au sample. While for the case of s-in polarization of the incident light a only weak and distance independent signal is observed, the p-in case shows a strong enhancement of the response confined on a 10 nm lenght scale.

This adiabatic nanofocusing on nano-designed tips provides a spectrally tunable nanoscopic light source suitable for background-free apertureless near-field microscopy and spectroscopy. By taking advantage of the ultrafast laser pulses, this may offer temporal resolution down to the 10 fs level, together with the nanometer spatial resolution.