As surface plasmon polaritons (SPP) continue to develop potential for controlling light on the nanoscale, the ability to excite and control these optical modes becomes increasingly important.   A new lens design strategy for SPPs is presented by Wintz, Capasso et al. of Harvard University.  The lens consists of a metasurface (nanostructured surface) composed of nanoslits that can steer the SPPs between foci on the surface based on the incident wavelength.

Using a  Nanonics MultiView 4000 system equipped with Nanonics NSOM probes, the authors map the surface plasmon polaritons through NSOM experiments at multiple wavelengths and polarizations demonstrating switchable focusing and steering of the SPPs as the wavelength or polarization is changed.  Their experimental setup, shown here, includes an NSOM probe mounted on to a tuning fork with incident laser radiation from below that passes through a polarizer in order to set the polarization.  Some of the laser light is transmitted through the metasurface, and depending on the relative intensities of the SPPs and transmitted light can produce an interference pattern. The NSOM tip is used to collect the light by interacting with the evanescent field of the SPP, converting energy from the SPP Mode into a propagating waveguide mode in the optical fiber. 
Additionally, they conduct spectrally resolved NSOM measurements by using the same experimental setup as above but now with the optical fiber connected to a spectrometer.  Shown on the left is an NSOM image collected at 580-700nm, where each pixel records the total counts in that wavelength range.  Individual spectra at different points from the image are shown on the right.
This strategy can be used to overcome coupling and focus issues currently present for SPPs as well as providing both wavelength and polarization tunability of the direction of SPP beam propagation.

Published: Nano Lett. 2015, 15, 3585−3589