Vortex surface plasmon polaritons on a cylindrical waveguide: generation, propagation, and diffraction

Abstract

In this paper, we experimentally demonstrated excitation of terahertz vortex surface plasmon polaritons by end-fire coupling of radially-polarized annular beams with orbital angular momentum (the wavelength was 141 µm, and the topological charges were ±3 and ±9) to a 70 mm long, simply connected axis-symmetric transmission line and their propagation to the end of the line and diffraction into a free wave possessing the same topological charge as the input beam. The diameter of the line exceeded greatly the radiation wavelength, and, in contrast to experiments with nanowires, no azimuthal electromagnetic modes existed. We observed that 18 plasmons, locally excited on the input face perimeter by a wave with topological charge of ±9, traveled rotating over the tapering cylindrical line and transformed into 18 lobes of the decoupled free wave. The evidence of the possibility of the transformation of a vortex beam into vortex plasmons and back into vortex beam with the conservation of the topological charge opens a way for the development of plasmonic communication lines with coding and decoding of information at the free-wave stage. Application of this technique to complex beams consisting of a combination of different vortex modes can be a technological base for the development of multiplex plasmonic communication lines in the terahertz range. Since wave characteristics are easier to record than plasmon characteristics, sorting of free waves by the topological charges can be used for demultiplexing combined plasmons in multiplex systems.

Terahertz Bessel Beams Formed by Binary and Holographic Axicons

“…It is important to note that information about the intensity distribution in the crosssection of perfect beams is crucial for planning and conducting experiments on the excitation of vortex plasmons on cylindrical samples [64] (see Section 1). Plasmons were excited on a brass cylinder coated with a ZnS dielectric layer using the end-fire coupling technique.…”

“…In [62,63], it was shown that the diffraction of a Bessel beam by a periodic two-dimensional grating in the Talbot planes results in the formation of periodic gratings of annular microbeams, which can be regarded as analogs of perfect beams. A very interesting application was demonstrated in [64], where beams with orbital angular momentum were first used to excite vortex surface plasmon polaritons propagating along a cylindrical conductor for a distance of up to 150 mm. The Bessel beam was first transformed by means of a lens into the so-called "perfect beam" of an annular configuration [65][66][67][68] (see detailed description in Section 3.3), which when diffracting at the front edge of the cylinder [69][70][71], excited vortex plasmons around the perimeter.…”

Terahertz Bessel and "perfect" vortex beams generated with a binary axicon and axicon with continuous relief

“…Интегральная интенсивность излучения (нормированная на максимальное значение) в пределах кадров, показанных на рис. 2, для бинарного (BA) и голографического (HA) аксиконовСовершенные пучки, сформированные описанными выше аксиконами, использовались для генерации вихревых поверхностных плазмон-поляритонов (ВППП) терагерцового диапазона на осесимметричных проводящих линиях[28], которая показала возможность реализации плазмонных мультиплексных передающих линий, предложенных в[29]. Плазмоны, несущие орбитальный угловой момент с |  |  9, возбуждались методом дифракции (end-fire coupling technique) совершенных пучков, показанных на рис.…”

“…3ж), оказалась благоприятным фактором для эксперимента. Она позволила возбудить на краю цилиндра 18 независимых параллельно движущихся поверхностных плазмон-поляритонов, что дало возможность продемонстрировать (детали см [28]…”