Researchers from the University of Maryland and the NIST Center for Nanoscale Science and Technology have experimentally demonstrated superfocusing of light using an optical fiber-based surface plasmonic (SP) lens. Optical fibers have been used as low-loss light waveguides for decades. However light strongly diverges at the fiber end, limiting the utility of fiber optics for applications that require a high numerical aperture as well as a sub-diffraction-limit focal size. As described in a recent publication in Optics Express,* this limitation can be overcome by fabricating an SP lens directly on the end of an optical fiber. The fiber-based SP lens is composed of a set of concentric annular slits with varied nanoscale widths cut through a uniform gold coating at the end face of a single-mode optical fiber. The slits were fabricated using the focused ion beam system in the CNST NanoFab. When the light in the fiber impinges on the SP lens, surface plasmon polaritons (SPPs) at the slit edges are excited. Because the slit widths are smaller than the light wavelength, the light passes through the slits only as SPP modes that are coupled back to optical waves at the exit of the slits. Moreover, the SPP propagation delay can be controlled by the width of the slits in order to induce a curved wavefront of exiting optical waves and thereby achieve a tight focus at the desired distance. The researcher team has experimentally demonstrated a transverse spot size of 450 nm ± 60 nm, which is below the diffraction limit for the 808 nm light. Their fiber-based SP lens design can be readily integrated into many existing systems to bridge nanophotonics and conventional optics. Possible applications include laser nanofabrication, optical trapping, high-density optical storage, and high-resolution fluorescence sensing. The research team is currently exploring applications for fiber based SP lenses for both high-resolution imaging and biological sensing.
*Far-field superfocusing with an optical fiber based surface plasmonic lens made of nanoscale concentric annular slits, Y. Liu, H. Xu, F. Stief, N. Zhitenev, and M. Yu, Optics Express 19, 20233-20243 (2011).
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