Near-field nanofocusing through a combination of plasmonic Bragg reflector and converging lens

We report the near-field nanofocusing through a plasmonic lens containing a Bragg reflector and a converging lens, which consist of semitransparent annular grooves milled into a gold film with different periods along the radial direction. By illuminating the structure with a linearly polarized light, two tightly focal spots were detected by scanning near-field optical microscope. This plasmonic lens has considerably reduced direct light transmission, making the focal spots obvious. By raising the radius of half of every groove, one single spot was obtained. Furthermore, theoretical simulations prove that the light intensity of the focal spots can be doubled through adding the Bragg reflector surrounding the converging lens.