Abstract-Optimal plasmonic focusing on a metal disc under radially polarized terahertz illumination

                                   Figure. When a THz wave illuminates a metallic disc surface plasmon waves
                                   are launched around its circumference propagating towards its center where
                                   they interfere at a common focus. For illumination by a linearly polarized wave
                                   a two-lobe structure is formed with a zero at the center (left panel). Using 
                                   radial polarization instead, results in optimal focusing of the surface waves 
                                   forming a strong and symmetric focal spot (right panel).

                                 
Stefan Waselikowski^1,2, Christian Fischer², Jan Wallauer^1,2 and Markus Walther^1,2,3

http://iopscience.iop.org/1367-2630/15/7/075005
markus.walther@physik.uni-freiburg.de                            
¹ Freiburg Materials Research Center, University of Freiburg, Stefan-Meier-Strasse 21, D-79104 Freiburg, Germany
² Department of Molecular and Optical Physics, University of Freiburg, Stefan-Meier-Strasse 19, D-79104 Freiburg, Germany
³ Author to whom any correspondence should be addressed

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.


GENERAL SCIENTIFIC SUMMARY
Introduction and background. Spectrocopy and imaging with terahertz (THz) radiation holds great promise for potential applications, e.g. in (bio-)molecular sensing, security screening or quality control. In order to increase spectroscopic sensitivity for THz sensing, or spatial resolution for THz imaging, it is of great importance to be able to confine the radiation to small volumes or focal spots. Unfortunately, THz field confinement is usually limited to millimetres due to the large wavelength of THz radiation (hundreds of micrometers to millimetres). Focusing THz surface-plasmon-polaritons (bounded electromagnetic waves propagating along conducting surfaces) instead of free-space radiation allows to confine fields to a small spot close to a surface, thereby potentially breaking the diffraction-limit.
Main results. We demonstrate focusing of THz radiation in the centre of simple metal discs. By using radially polarized radiation instead of linear polarization, optimal plasmonic focusing is achieved, since the radially polarized waves perfectly match the symmetry of the circular focusing element.
Wider implications. The remarkable focusing capabilities of such a plasmonic lens together with its simple structure offer considerable potential for achieving high sensitivities in THz sensing, as well as high spatial resolution in THz imaging applications.