Metagratings for tunable unidirectional steering and focusing of surface plasmons (Presentation Recording)

In this paper, we present new results on the controlled directional steering and focusing of surface plasmon polaritons (SPPs) via 1D and 2 D metagratings by changing the angle of incidence, the incident wavelength and polarization. These findings build on previous work of our group on polarization controlled steering of SPPS using fishbone meta gratings and greatly expand on the functionality of the latter using novel designs. First we show that by creating a running wave of polarization along a one dimensional metallic metagrating consisting of subwavelength spaced rotated apertures that propagates faster than the SPP phase velocity, we can generate surface plasmon wakes, which are the two-dimensional analogue of Cherenkov radiation. The running wave of polarization travels with a speed determined by the angle of incidence and the photon spin angular momentum. We utilize this running wave of polarization to demonstrate controlled steering of the wakes by changing both the angle of incidence and the polarization of light, which we measure through near-field scanning optical microscopy. Next we report a simple 2D metagrating design strategy that can be used for focusing, polarization beam splitting, waveguide coupling, and even phase control at the focus of an SPP beam. We experimentally verify our 2D metasurface by creating a four wavelength plasmonic demultiplexer, which also has polarization selectivity (on/off). The wavelength demultiplexer is designed such that each of the four wavelengths is focused to a different spot outside of the structure. Coupling of free space light to SPPs is achieved by milling subwavelength apertures into a thin gold film. This methodology can be easily extended to any wavelength where SPPs exist, for an arbitrary number of wavelengths, and with polarization selectivity and phase control at the focus as well.