Plasmonic Double-hole Bull's Eye Nano-antenna for Far-field Polarization Control

Plasmonic polarization conversion offers significant advantages over conventional methods, including a smaller device footprint and easier integration into photonic circuits. In this work, we numerically and experimentally investigate the polarization conversion properties of a plasmonic double-hole structure surrounded by circular nanograting, i.e., a bull's eye antenna. Using a combination of polarimetric imaging via back focal plane (BFP) microscopy and the Stokes parameter analysis, we demonstrate the functionality of our structure as a miniature on chip polarization converter. Our results show that this nanostructure enables complex polarization transformations, including converting linear to circular polarization and vice versa. The polarization conversion efficiency is found to be dependent on the periodicity of the circular gratings and is particularly pronounced in the central region of the Fourier space. Moreover, the strong asymmetric scattering leads to distinctive patterns in the Stokes parameters across various incident polarization states. This work provides insights into the plasmonic manipulation of light polarization at the nanoscale, with potential applications in miniature on-chip polarization convertors, polarization-controlled emitters, and advanced sensing technologies.