Excitation and focusing of terahertz surface plasmons using a grating coupler with elliptically curved grooves

Using a diffraction grating with elliptically curved grooves engraved at the top of an aluminum slab, surface plasmons in the terahertz frequency range are efficiently excited with an incoming parallel beam and focused over the metal surface down to the diffraction limit. At 0.5 THz, the lateral confinement of the surface plasmon is 1 mm while its extension in air is 0.74 mm, much smaller than the calculated value for a semi-infinite substrate having the conductivity of bulk aluminum.