Guiding, Focusing, and Sensing on the Subwavelength Scale Using Metallic Wire Arrays

Abstract

We show that two-dimensional arrays of thin metallic wires can guide transverse electromagnetic (TEM) waves and focus them to the spatial dimensions much smaller that the vacuum wavelength. This guiding property is retained for the tapered wire bundles which can be used as multichannel TEM endoscopes: they capture a detailed electromagnetic field profile created by deeply sub-wavelength features of the studied sample and magnify it for observation. The resulting imaging method is superior to the conventional scanning microscopy because of the parallel nature of the image acquisition by multiple metal wires. Possible applications include terahertz and mid-infrared endoscopy with nanoscale resolution. PACS numbers:Diffraction of light is the major obstacle to a variety of applications requiring concentrating optical energy in a small spatial volume: light cannot be confined to dimensions much smaller than half of its wavelength λ/2. Applications that would benefit from overcoming the diffraction limit include nonlinear spectroscopy and harmonics generation [1,2,3,4], sub-wavelength optical waveguiding [5,6,7], and nanofabrication [8]. Utilizing plasmonic materials with a negative dielectric permittivity circumvents diffraction limit because interfaces between polaritonic (ǫ < 0) and dielectric (ǫ > 0) materials support surface plasmons that can be confined to sub-λ dimensions. Examples of diffraction-beating devices based on plasmonics include superlenses [9,10,11,12], coupled-sphere waveguides [13], and sharp focusing tips [14].High losses associated with surface plasmonics are hampering many of these applications.Another challenge yet to be met is designing practical imaging modalities based on sub-λ plasmons that convert near-field electromagnetic (EM) perturbations into the far field where they can be easily observed. In this Letter we propose a solution to these two problems: a tapered multi-wire array supporting sub-wavelength transverse electromagnetic (TEM) waves. Examples of the multi-wire endoscopes based on such arrays (un-tapered and tapered) are shown in Fig. 1. We have demonstrated that the tapered endoscope can accomplish two tasks: (i) creating near the base of an endoscope a magnified image of deeply sub-wavelength objects (metal spheres, in our case) placed at the endoscope's tip [see Fig. 3(a)], and (ii) creating near the tip of an endoscope a de-magnified image of a mask placed at the endoscope's base [see Fig. 3(b)]. Accomplishing the first task is necessary for making a sub-λ sensor while accomplishing the second one -for making a sub-λ lithographic tool.Single metallic wires and coaxial wire cones have recently attracted considerable attention as low-loss waveguides [15,16] of TEM-like modes of THz and far-infrared radiation. Using a single wire waveguide has its limitations: for example, if a wire is used as a high spatial resolution sensor, then only a single bit of information can be collected without scanning the wire. We demonstrate that a bundle of closely spaced wires can act as a mul...

Hyperlenses and metalenses for far-field super-resolution imaging

“…Practical configurations of the hyperlens were proposed by different groups, including a cylindrical geometry 15,25,26 , tapered metallic wire arrays [27][28][29][30] or uniquely designed material dispersions [31][32][33][34] . To circumvent the problem of absorption in real materials and thereby avoid image distortions in the far field, proper configurational design must ensure that all rays originating from the object plane travel equal path lengths through the metamaterial.…”

“…Another super-resolution scheme based on metallic wire arrays was proposed to attain a wide range of working wavelengths over a long transport distance 27 . Experimental demonstrations, however, were limited to subwavelength imaging in the near field using metallic wires grown in dielectric templates 45,46 .…”

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“…The real superlens has already been fabricated experimentally by some groups [111][112][113][114][115][116][117][118][119]. Imaging through a superlens essentially requires the object as well as the image to be within the range of near-field.…”

Bandwidth and resolution of super-resolution imaging with perforated solids

“…27 Its origin comes from the electromagnetic hyperlens which provides subwavelength resolution through a flat dispersion curve accompanying with image magnification and conversion of near-field to far-field information. [28][29][30][31][32][33] Similarly, an acoustic superlens has its origin coming from the electromagnetic counterpart. [34][35][36][37] It is a planar slab which transports the near-field information for a distance to achieve sub-wavelength resolution in imaging.…”