Focusing far-field nanoscale optical needles by planar nanostructured metasurfaces

Far-field nanoscale optical needles are obtained using water-immersed planar nanostructured metasurfaces illuminated with a 193 nm deep ultra-violet laser. The method is based on the vectorial angular spectrum theory and an established nonlinear optimization model. For a 50 μm-diameter metasurface with a linearly polarized beam (x-polarized), an optical needle with 12.4λ0 length has been produced at a mid-focal distance of 14.5 μm. The transverse beam sizes are as small as 129 nm and 59.4 nm in the x and y directions, respectively. The design results are agreed well with the rigorous electromagnetic calculations using three-dimensional finite-difference time-domain (FDTD) method with a suggested 25 nm-thick aluminum coating film for the metasurface. These far-field nanoscale optical needles are potentially applied in the fields of nanolithography, nanoprinting, and nanoscopy.