Focusing far-field nanoscale optical needles by planar nanostructured metasurfaces

Far-field nanoscale optical needles are obtained using water-immersed planar nanostructured meta surfaces 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 mu m-diameter metasurface with a linearly polarized beam (x-polarized), an optical needle with 12.4 lambda(0) length has been produced at a mid-focal distance of 14.5 mu m. 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. (C) 2016 Published by Elsevier B.V.