CC BY
11THz-I-2
"Perfect" Vortex Beams in the THz Range: Generation
and Application
B. Knyazev12, Yu. Choporova12, V. Gerasimov12, O. Kameshkov12, A. Lemzyakov12, N. Osintseva12, V. Pavelyev13, K. Tukmakov3
1- Novosibirsk State University, Ul. Pirogova 1, Novosibirsk, Russia, 630090
2 - Budker Institute of Nuclear Physics of SB RAS, Pr. Lavrentyeva 11, Novosibirsk, Russia,
630090
3 - Samara National Research University, Moskovskoe Shosse 34, Samara, Russia, 443086
ba_knyazev@phys. nsu.ru
In modern optics, special attention is drawn to "perfect vortex beams" (PVBs), ring beams with orbital angular momentum. These beams can be obtained by focusing Bessel beams with a lens. In the case of ideal Bessel beams, a PVB is an ideal ring. Real Bessel beams (BBs) always bear traces of their origin to one degree or another. In the terahertz range, BBs can be created using three types of diffractive phase axicons (Fig. 1, the upper row). They form practically identical Bessel beams, at least near the optical axis, as seen from the middle row in Fig.1. However, after focusing with a lens (the bottom row in Fig. 1) the perfect beams obtained differ significantly from each other. It is seen from the results of numerical calculations [1] shown in Fig. 1.
Illuminating binary spiral axicons coupled to a sectioned half-wave plate and a lens with a focal distance f = 75 mm by Gaussian beam of the Novosibirsk free electron laser with a wavelength of 141 ^m, we obtained radially polarized perfect beams with topological charges t = 3 h C.=9. These beams, which had a diameter of 10 mm, were used to generate surface plasmon polaritons (SPPs) at the front end of an axisymmetric transmission line (ATL) by the end-fire coupling technique. The ATL was made of brass and covered with a 1- /m thick zinc sulfide layer. The decay length of the plasmon field in the air can be estimated to be about 300 ^m by analogy with gold-ZnS flat samples. It was found that the orbital angular momentum of the exciting wave was transferred to the generated SPPs. The rotating plasmons propagated along a line 70 mm long, which was smoothly narrowing to a diameter of 2 mm, and transferred the angular momentum to a free wave, which was emitted from the rear end of the ATL. It was convincingly shown in the experiments that plasmons arose at the intersections of a PB, split into spiral segments (see the lower-left frame in Fig. 1), with the edge of the cylinder and propagated to the end of the ATL along parallel disjoint trajectories, the number of which was equal to 2\t |. The experimentally measured rotation angle of the plasmon trajectory on the cylindrical surface of the transmission line and the dynamics of the rotation angle of the resulting free wave, which retained the number of lobes equal to 2\i|, were in good agreement with the values
of these quantities calculated under the assumption of conservation of angular momentum. The results obtained can be used, for example, to create plasmonic communication lines and sensors. This work was supported by the Russian Science Foundation, grant 19-12-00103. Experiments were carried out at the Novosibirsk free electron laser, which is a part of the Shared Research Facility "Siberian Synchrotron and Terahertz Radiation Center."
[1] B. Knyazev, V. Cherkassky, O. Kameshkov, Perfect terahertz vortex beams formed using diffractive axicons and prospects for excitation of vortex surface plasmon polaritons, Applied Sciences, vol. 11, 717, (2021).
Helical binary axicon Helical kinoform axicon Holographic axicon
Fig. 1. Three types of phase axicons (upper row) that form Bessel beams with a topological charge C = 9 and a transverse wave number k = 3.1 mm"1 (middle row). Bottom row: perfect beams obtained in the focal plane of the lens with f = 100 mm.
