Spatiotemporal Analysis of an Efficient Fresnel Grating Coupler for Focusing Surface Plasmon Polaritons

Time-resolved photoemission microscopy is used to investigate the spatiotemporal properties of surface plasmon polaritons launched at a Fresnel-type grating coupler. By milling a multiline structure of segmented grooves with dimensions derived from Fresnel zones into a plasmonic material, efficient focusing of surface plasmon polaritons can be accomplished. We demonstrate the presence of pulse broadening at the focus associated with propagation delays in the device. Moreover, our experimental data implies an enhancement of the plasmonic field energy density at the focus in excess of a factor of 10, with a focal spot size at the Abbe limit for our lens.