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

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) nearfield 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.

Gouy phase shift of a tightly focused, radially polarized beam

“…In the terahertz regime, radially polarized terahertz pulses have been produced either via velocity-mismatched optical rectification in (001)-oriented ZnTe [42], or by photoconductive emitters consisting of single-ring electrodes. The latter emitters have been used, for example, to achieve enhanced coupling to the plasmonic surface modes of metal wires [43][44][45], or for optimal plasmonic focusing on a metal disc [46]. The intensity of radially polarized pulses can be significantly enhanced using large-area emitters consisting of multiple interdigitated ring electrodes that are alternately shielded from the excitation beam [47].…”

Ripple formation on nickel irradiated with radially polarized femtosecond beams

“…Surface Plasmon Polariton (SPP) excitation by localized fields originating from CVB and propagation on metal surfaces have been extensively explored [7,[11][12][13][14]. While SPP excitation at a metal/air interface is reported to lead to periodic ripple patterns similar to those induced by linearly polarized beams [15][16][17][18][19][20][21], only a few studies have been performed to correlate the characteristics of the CVB with induced morphological changes [22,23].…”

Breaking Bullseye’s Symmetry for Axial Field Focusing

“…Such a configuration is important, for example, for the observation and excitation of intersubband transitions of multiple quantum wells embedded in the substrate, magnetic resonant mode in metallic elements such as the plannar split ring and 3D subwavelength resonant element which is available recently. This can be done by a radially polarized [14] (respectively azimuthal) free space excitation of a classic bullseye. However this kind of radiation might not be easy to obtain, and requires a careful alignment in order to obtain the full coupling efficiency.…”