Efficient unidirectional polarization-controlled excitation of surface plasmon polaritons

Efficient excitation of surface plasmon polaritons (SPPs) remains one of the most challenging issues in areas of plasmonics related to information communication technologies. In particular, combining high SPP excitation efficiency and acceptance of any polarization of incident light appeared to be impossible to attain due to the polarized nature of SPPs. Here we demonstrate plasmonic couplers that represent arrays of gap SPP resonators producing upon reflection two orthogonal phase gradients in respective linear polarizations of incident radiation. These couplers are thereby capable of efficiently converting incident radiation with arbitrary polarization into SPPs that propagate in orthogonal directions dictated by the phase gradients. Fabricated couplers operate at telecom wavelengths and feature the coupling efficiency of ∼25% for either of two linear polarizations of incident radiation and directivity of SPP excitation exceeding 100. We further demonstrate that an individual wavelength-sized unit cell, representing a meta-scatterer, can also be used for efficient and polarization sensitive SPP excitation in compact plasmonics circuits. Devices capable of converting light of any polarization into surface plasmon polaritons could aid the development of on-chip optical circuits. Anders Pors and co-workers from the University of Southern Denmark and the University of Burgundy in France have now fabricated such a ‘coupler’ from gradient metasurfaces — arrays of miniature gold patches on a thin dielectric layer covering a metal-coated glass substrate. Their device is compatible with wavelengths of light in the telecommunications window (around 1,500 nm) and does not require a specific polarization in order to excite plasmons. Furthermore, it allows the direction of plasmon propagation to be controlled by changing the polarization of the incident beam. Calculations indicate that the coupling efficiency could be as high as 40%, with a directivity of around 50:1.