Linearly Dichroic Plasmonic Lens and Hetero-Chiral Structures
It is well known that chiral structures, lacking in-plane mirror symmetry, exhibit circular dichroism. Namely, they can discriminate the handedness of circularly polarized light. The field of Plasmonic Lenses is concerned with structures on metal layers that can couple impinging light into surface plasmons-polaritons (SPPs) and combine them into a focal spot. With the main goal being the focusing of the out-of-plane SPP field components. Archimedes’ spiral Plasmonic Lenses (ASPLs), which are chiral by definition, are used as circularly dichroic lenses – focusing circular polarization with proper handedness and resulting in a dark focal spot for the opposite handedness.
In contrast, much lesser emphasis had been made on linearly dichroic devices – namely devices focusing electromagnetic waves with a specific linear polarization – while blocking (not focusing) the orthogonally polarized wave.
We demonstrate that a combination of two ASPLs with opposite handedness (chirality) and unity geometrical charge results in a Hetero-Chiral plasmonic lens (HC lens) with a linearly dichroic focal spot. In the process of their merging, the chiral symmetry of the constituents is broken, however the mirror symmetry is not completely restored implying that the combined structure should distinguish between horizontal and vertical excitations leading to linear dichroism. Fig. 1a. gives the schematics and Fig. 1d,e the experimental results of the actual HC lens.
A further simplification of the 2 engaged spirals can be gained by removing the central part of the structure (Fig. 1b) obtaining a structure resembling a distorted circle. Intriguingly, this slight deformation results in a rather strong linear dichroism of the focal spot (Fig. 1f,g). Different selective erasure procedure can be also applied to result in the Half-circles structure (Fig. 1c,h-i).
We study, experimentally verify and quantitatively compare several members of the HC family, deriving necessary conditions for linear dichroism and several comparative engineering parameters. The HC structures were fabricated by FIB in gold. The samples were illuminated by relevant polarizations and the transmitted near field pattern was collected using aperture-less NSOM exhibiting excellent match to the simulation results.
Powered by Eventact EMS