Nanofocusing of electromagnetic radiation

Journal name:
Nature Photonics
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Published online


Nanofocusing of electromagnetic radiation, that is, reducing the cross sections of propagating optical modes far beyond the diffraction limit in dielectric media, can be achieved in tapered metal–dielectric waveguides that support surface plasmon–polariton modes. Although the main principles of nanofocusing were formulated over a decade ago, a deep theoretical understanding and conclusive experimental verification were achieved only a few years ago. These advances have spawned a variety of new important technological possibilities for the efficient delivery, control and manipulation of optical radiation on the nanoscale. Here, we present the underlying physical principles of radiation nanofocusing in metallic nanostructures, overview recent progress and major developments, and consider future directions and potential applications of this subfield of nano-optics.

At a glance


  1. Insulator-metal-insulator and metal-insulator-insulator nanofocusing configurations.
    Figure 1: Insulator–metal–insulator and metal–insulator–insulator nanofocusing configurations.

    Insulator–metal–insulator (IMI) structures: a, conical tapered metal rod with a rounded tip16, 17, 19, 20; b, metal wedge surrounded by dielectric media15, 28, 29, 30; c, tapered metal strip with varying width and constant thickness31, 32; d, cylindrical crescent with an electromagnetic singularity41; e, optical fibre with hemispherical termination coated with a tapered metal film whose smallest thickness is at the top of the hemisphere33. Metal–insulator–insulator (MII) structure: f, tapered high-permittivity dielectric wedge on a metal substrate with SPP focusing in the opposite direction to the taper39, 40. Figure d reproduced with permission from ref. 41, © 2010 ACS.

  2. Metal-insulator-metal and hybrid nanofocusing configurations.
    Figure 2: Metal–insulator–metal and hybrid nanofocusing configurations.

    Metal–insulator–metal (MIM) structures: a, one-dimensionally tapered gap between two metal media15, 18, 21, 22, 23, 24, 27; b, two-dimensionally tapered gap between two metal films26; c, tapered V-shaped groove with gradually reducing taper angle and depth34; d, two 'kissing' cylinders or touching spherical particles with an electromagnetic singularity at the contact point (which is also the focal point of the structure)41, 42, 43. Hybrid IMI/MIM structure: e, tapered chain of coupled nanoparticles (combines the features of nanoantennas and nanofocusing structures with IMI and MIM configurations)47. Figure reproduced with permission from: b, ref. 26, © 2012 NPG; c, ref. 1, © 2010 NPG; d, ref. 39, © 2010 ACS.

  3. Typical field distributions in nanofocusing structures.
    Figure 3: Typical field distributions in nanofocusing structures.

    a, A typical distribution of |E|2 along a tapered section of a metal film31, 32 or a tapered metal ridge55. The top inset shows a typical cross-sectional distribution of the field near the tip of a metal film on a dielectric substrate31, 32. The bottom inset shows an expanded view of the area near the structural tip. b, Distribution of the z-component of the electric field near the tip of a glass sphere of 500-nm radius coated with a tapered gold film of minimum thickness of 5 nm at the tip at λvac = 632.8 nm (the z-axis corresponds to the axis of the structure)33. c, Distribution of |E| in a tapered cylindrical gold rod in air with γ = 30°, tip radius R = 5 nm and λvac = 632.8 nm (ref. 11). d, Field distribution in a tapered air gap in gold with the incident bulk radiation coupled to the gap SPP72. e, Distribution of the electric field intensity in a nanofocusing silicon wedge with γ = 1.43° between a silver surface and air for λvac = 577 nm (ref. 39). Figure reproduced with permission from: a, ref. 31, © 2008 OSA; b, ref. 33, © 2012 OSA; d, ref. 72, © 2010 ACS; e, ref. 39, © 2010 ACS.

  4. Plasmon nanofocusing configurations with tapered nanowires.
    Figure 4: Plasmon nanofocusing configurations with tapered nanowires.

    a, Scanning electron microscope (SEM) image of an electrochemically etched gold tip with a grating coupler. Superimposed on this SEM image is an optical image of grating excitation of SPPs, which shows their subsequent propagation, nanofocusing and re-radiation at the tip apex for a wavelength of ~800 nm (ref. 63). b, A tapered section of a gold film on a sapphire substrate; the fundamental mode in the strip is nanofocused near the taper tip31. c,d, Experimental realization of nanofocusing in gold-film tapers similar to that shown in b and connected by a 2-m-long nanowire32, showing an SEM image of the connected taper structure (c) and a near-field amplitude of forward-propagating waves at a wavelength of 1,550 nm (d). e,f, Near-field Raman imaging with a tapered nanowire61. An SEM image (e) of a photonic-crystal cavity fabricated on a silicon nitride membrane with a tapered silver nanowire having a conical shape and a radius of curvature at the apex of 5 nm. A Raman intensity map (f) (in kilocounts per second, kcps) with a scanning step size of 7 nm across a submicrometre silicon nanocrystal/SiOx trench. Variations of the Raman signal along the dashed red line in f indicate the spatial resolution of ~7 nm. Figure reproduced with permission from: a, ref. 63 © 2010 ACS; b, ref. 1 © 2010 NPG; c,d, ref. 32 © 2009 APS; e,f, ref. 61 © 2010 NPG.

  5. Plasmonic nanofocusing configurations with tapered gaps.
    Figure 5: Plasmonic nanofocusing configurations with tapered gaps.

    a, A gold V-shaped groove structure supported by a silicon wafer for nanofocusing at 1.5 μm (ref. 71). b, A cross section of a 350-nm-period array of ultrasharp, 450-nm-deep convex grooves milled in gold (scale bar, 300 nm) that absorbs on average >90% of light polarized perpendicular to the groove direction in the range 450–800 nm (ref. 52). The inset shows the 2D groove array discussed in the text. c,d, Experimental realization of 3D nanofocusing with the two-dimensionally tapered-gap nanofocusing configuration26 shown in Fig. 2b, showing a tapered silica-filled gap sandwiched between two 50-nm-thick gold layers (c), and a map of the two-photon-induced photoluminescence signal obtained by illuminating the back aperture of the structure using a focused laser beam (wavelength, 830 nm) polarized perpendicular to the propagation direction in the tapered gap waveguide (d). Figure reproduced with permission from: a, ref. 1 © 2010 NPG; b, ref. 52 © 2012 NPG; c,d ref. 26 © NPG.


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  1. Nanophotonics Pty. Ltd., GPO Box 786, Albany Creek, Queensland 4035, Australia

    • Dmitri K. Gramotnev
  2. Institute of Technology and Innovation, University of Southern Denmark, Niels Bohrs Allé 1, DK-5230 Odense M, Denmark

    • Sergey I. Bozhevolnyi

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