Resonant and non-resonant generation and focusing of surface plasmons with circular gratings

We report the generation and focusing of surface plasmon polariton (SPP) waves from normally incident light on a planar circular grating milled into a silver film. The focusing mechanism is explained by using a simple coherent interference model of SPP generation on the circular grating by the incident field. Experimental results concur well with theoretical predictions and highlight the requirement for the phase matching of SPP sources in the grating to achieve the maximum enhancement of the SPP wave at the focal point. NSOM measurements show that the plasmonic lens achieves more than a 10-fold intensity enhancement over the intensity of a single ring of the in-plane field components at the focus when the grating design is tuned to the SPP wavelength. We discuss the technique's adaptability for surface enhanced nano-scale spectroscopy.