Superoscillatory Metalens for Polarization Conversion and Broadband Tight Focusing of Terahertz Waves

For terahertz wave applications, such as medical imaging, space exploration, security inspection etc., the lens is fundamental for use as the optical focusing element. Far-field subdiffraction focusing devices play a special role in such terahertz applications. Metasurfaces can provide complete control over the amplitude, the phase, and the polarization of the optical field. They also show excellent potential in connection to focusing and polarization conversion devices. In the paper, we propose a group of subwavelengh dielectric birefringent meta-atoms that operate in the terahertz range. Over a full coverage of 2 pi, they can produce a simultaneous integration of the polarization conversion and the phase modulation. Based on the concept of optical superoscillation, a metalens with a numerical aperture (NA = 0.975) is optimized for a wavelength, lambda, of 2500 mu m. Under the illumination of linearly polarized waves, the broadband polarization conversion, and the sub-wavelength tight focusing, is achievable between the wavelengths 1900 and 2700 mu m. Simulation results demonstrate that the spots have sizes over the range 0.418 lambda - 0.556 lambda, while the sidelobe ratio (SR) is kept at a low intensity level between 14.3% and 16.2%. Importantly, continuous adjustment of the focal spot size is feasible by changing the incident polarization from 0 degrees to 45 degrees with respect to the axis. For incident polarization angles between 75 degrees and 90 degrees, the inner FWHMs of the hollow dark spot are far smaller than the corresponding super-oscillation criterion. Due to these improved performance characteristics, such superoscillatory metalens have wide potential in many terahertz applications. This approach is shown to open a novel avenue for the polarization conversion and the broadband tight focusing of terahertz waves.