Abstract
Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated. By matching the cylindrical symmetry of the metal structure with the radially polarized terahertz field, surface plasmons are excited along its entire circumference. Constructive interference in the disc center produces a sharp frequency-dependent focal spot well described by a zero-order Bessel function. We map the field distributions on the disc by terahertz (THz) near-field microscopy and compare our results with numerical simulations. For comparison, the behavior of the plasmonic lens under linearly polarized THz illumination is also characterized. The remarkable focusing capabilities of such a plasmonic lens together with its simple structure offer considerable potential for THz sensing and imaging applications.
Highlights
Optimal focusing of surface plasmon polaritons in the center of a metal disc illuminated by radially polarized terahertz pulses is demonstrated
Different schemes for generating THz radiation with radial polarization have been demonstrated, either by passively converting linear into radial polarization [18], or by active generation of radially polarized THz transients via velocity-mismatched optical rectification [19] or emitted from photo-conductive antennas with radial electrode geometry [20, 21]. We adopt the latter approach based on generating single-cycle THz pulses with radial polarization by photoexcitation of a photo-conductive emitter consisting of concentric ring electrodes
Under radial polarization illumination the electric energy density is sharply focused at the center, in contrast to illumination by linearly polarized light, where the energy density distribution is dominated by a two-lobe pattern with a zero along the axis of symmetry
Summary
In many cases controlling the polarization state of light is of great importance. In particular, beams with axial symmetry such as radially polarized beams are of interest due to their unique properties, in particular enabling enhanced focusing [16, 17]. Different schemes for generating THz radiation with radial polarization have been demonstrated, either by passively converting linear into radial polarization [18], or by active generation of radially polarized THz transients via velocity-mismatched optical rectification [19] or emitted from photo-conductive antennas with radial electrode geometry [20, 21]. We adopt the latter approach based on generating single-cycle THz pulses with radial polarization by photoexcitation of a photo-conductive emitter consisting of concentric ring electrodes
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