The push to improve data capacity in wireless networks has led to an increase in carrier frequency in the terahertz band. This has enabled the use of direct beams transmission due to the shorter wavelength. One method of further densification of transmitted communication channels is through the use of vortex Bessel beams, which allow for signal combination in a single channel. In addition, these beams offer improved beam stability through their unique features of non-diffraction and self-healing, which is particularly important in wireless transmission where atmospheric turbulence is a factor. Vortex Bessel beams are characterized by a topological charge, l, which cannot be determined from their intensity profile alone. Therefore, spatial filtering is necessary at the receiver stage to identify the desired mode carried by a distorted or combined beam. A diffractive optical element with a specially designed complex transmission can serve as a filter matched to the desired mode. This study presents the experimental results of Bessel mode identification using beams formed by binary spiral binary phase axicons that transform the Gaussian mode of high-power terahertz radiation from the Novosibirsk Free Electron Laser. The specified mode is passed through a filter (similar spiral binary axicon with |l| = 1, 2, 3, or 4, and a lens) and then detected by a pyroelectric camera in the lens focal plane. A positive response, indicated by a narrow peak, confirms the compatibility of the specified and filtering modes. Experiments were conducted on single-mode and combined beams (l = -1 and -2), and mode identification was also demonstrated for beams passed through an inhomogeneous medium.
Currently, in photonic integrated circuits, the circuit size is limited by diffraction, which acts as a barrier to further development in the field of optical communications. Plasmonics could potentially address this size mismatch between electronic and photonic components. Photonics and plasmonics can complement each other, given the appropriate conditions, allowing optical signals to be converted into surface plasmon-polaritons (SPPs) and vice versa. Therefore, plasmonic integrated planar circuits for wireless communication devices are actively being developed. Transitioning to the terahertz (THz) frequency range will enhance data transmission and processing speed. In the development of plasmonic integrated circuits, understanding the optical properties of surfaces is crucial. Current methods for determining the optical properties of surfaces in the THz frequency range lack sufficient sensitivity to transitional surface layers of metal and films that are much thinner than the optical wavelength. This work will demonstrate experimental methods for measuring the penetration depth of the SPP field into the air. Through these methods, and utilizing other plasmonic refractometric characteristics, it is possible to reconstruct the effective surface dielectric permittivity of the metal.
Surface plasmon interferometry has found wide application in optical sensor devices in the visible range. In the THz region, where the surface plasmon polaritons (SPPs) propagation length extends to tens of centimeters, surface plasmon interferometers can be effectively used to control quality of the metal surface, to determine the effective permittivity of thin metal and dielectric coatings used in THz plasmonic integrated circuits, and for various sensor applications. The first THz SPP interferometer design and experimental results obtained with them in 130 to 358μm wavelength region for testing of metal surfaces and thin dielectric coatings deposited on it will be presented in this paper. The SPPs were generated by the Novosibirsk free electron laser coherent radiation on flat surfaces with gold sputtering, coated with ZnS layers of thickness from 0 to 3μm. Based on the measurement results, the value of the effective permittivity of the deposited gold was found; it turned out to be an order of magnitude smaller than that of bulk crystalline gold, which show the practical relevance of this method.
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