Tight focusing of fractional-order topological charge vector beams by cascading metamaterials and metalens

This paper shows how to tightly focus terahertz vector beams carrying fractional and integer topological charges. It cascades 3D-printed all-dielectric metamaterials with a metalens. The metamaterial modules generate integer-order vortex beams, fractional-order vortex beams (l = 4.5 and 8.5) and conjugate-charge superposition beams (l = 5 and 9) at 0.1 THz. A metalens with a numerical aperture of 0.87 and a 16 mm focal length then focuses them, giving a measured focal spot of 0.73λ (FWHM). The modules were 3D printed in red wax resin and measured with a THz camera. Simulations and experiments agree well, with remaining differences attributed to fabrication tolerances, thermal deformation and camera resolution. A wire-grid polarizer in front of the source set the incident linear polarization for measuring the vector beam characteristics.