Abstract
While freely propagating photons cannot be focused below their diffraction limit, surface-plasmon polaritons follow the metallic surface to which they are bound, and can lead to extremely sub-wavelength energy volumes. These properties are lost at long mid-infrared and THz wavelengths where metals behave as quasi-perfect conductors, but can in principle be recovered by artificially tailoring the surface-plasmon dispersion. We demonstrate - in the important mid-infrared range of the electromagnetic spectrum - the generation onto a semiconductor chip of plasmonic excitations which can travel along long distances, on bent paths, to be finally focused into a sub-wavelength volume. The demonstration of these advanced functionalities is supported by full near-field characterizations of the electromagnetic field distribution on the surface of the active plasmonic device.
Highlights
IntroductionGuiding, focusing and confining long-wavelength radiation with metals requires artificially tailoring the dispersion of surface-plasmon polaritons (SPPs), which at mid-infrared (mid-IR, 10-100 THz) and THz (1-10 THz) frequencies are barely distinguishable from free photons when they propagate on flat metallic surfaces
While freely propagating photons cannot be focused below their diffraction limit, surface-plasmon polaritons follow the metallic surface to which they are bound, and can lead to extremely sub-wavelength energy volumes
These properties are lost at long mid-infrared and THz wavelengths where metals behave as quasi-perfect conductors, but can in principle be recovered by artificially tailoring the surface-plasmon dispersion
Summary
Guiding, focusing and confining long-wavelength radiation with metals requires artificially tailoring the dispersion of surface-plasmon polaritons (SPPs), which at mid-infrared (mid-IR, 10-100 THz) and THz (1-10 THz) frequencies are barely distinguishable from free photons when they propagate on flat metallic surfaces. Such dispersion engineering can be achieved with a properly designed sub-wavelength metallic patterning [1,2,3]: the new, artificial dispersion relation leads to SPPs with larger wavevectors than SPPs on planar surfaces (Fig. 1(a)), and with reduced decay-length in the direction orthogonal to the metal surface. This is rarely seen at long IR wavelengths [16, 17], but it is crucial to prove that the EM field is confined within sub-wavelength proximity of the artificially patterned metallic surface in all three dimensions of space
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