Nouvelles conditions aux limites exactes pour les champs électromagnétiques à la surface des matériaux absorbants

Recent experiments using laser light by Langlois et al. have shown for the first time that actual diffraction patterns from edges do depend on the material and ridge shape of the scatterer, as well as on the polarization of the incident wave. These results have brought out the inadequacy of the scalar diffraction theory for diffraction angles larger than about 1°. Farther out one must use exact electromagnetic theory, with the attendant requirement that exact boundary conditions be known for the fields on the surface of the scattering object. These boundary conditions are well known for perfect conductors (Neumann's or Dirichlet's conditions) or even for good conductors (Leontovich's conditions). However, in the visible range one finds that copper is not a good conductor. Therefore, we develop in the present paper new exact boundary conditions, which generalize those of Leontovieh. In actual fact our new boundary conditions have enabled us to perform edge diffraction calculations for a considerably larger range of complex refraction indices covering the materials used. More generally one may claim that our boundary conditions do apply for all usual materials (copper, aluminium, ebonite … ), in as much as they are absorbing and the local ridge curvature of the scattering objects is far larger than the wavelength.

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“Perfect” Terahertz Vortex Beams Formed Using Diffractive Axicons and Prospects for Excitation of Vortex Surface Plasmon Polaritons

Applied Sciences ◽

10.3390/app11020717 ◽

2021 ◽

Vol 11 (2) ◽

pp. 717

Author(s):

Boris Knyazev ◽

Valery Cherkassky ◽

Oleg Kameshkov

Keyword(s):

Surface Plasmon ◽

Surface Plasmon Polaritons ◽

Bessel Beam ◽

Diffraction Theory ◽

Terahertz Range ◽

High Resistivity ◽

Visible Range ◽

Optical Elements ◽

Scalar Diffraction ◽

Plasmon Polaritons

Transformation of a Bessel beam by a lens results in the formation of a “perfect” vortex beam (PVB) in the focal plane of the lens. The PVB has a single-ring cross-section and carries an orbital angular momentum (OAM) equal to the OAM of the “parent” beam. PVBs have numerous applications based on the assumption of their ideal ring-type structure. For instance, we proposed using terahertz PVBs to excite vortex surface plasmon polaritons propagating along cylindrical conductors and the creation of plasmon multiplex communication lines in the future (Comput. Opt. 2019, 43, 992). Recently, we demonstrated the formation of PVBs in the terahertz range using a Bessel beam produced using a spiral binary silicon axicon (Phys. Rev. A 2017, 96, 023846). It was shown that, in that case, the PVB was not annular, but was split into nested spiral segments, which was obviously a consequence of the method of Bessel beam generation. The search for methods of producing perfect beams with characteristics approaching theoretically possible ones is a topical task. Since for the terahertz range, there are no devices like spatial modulators of light in the visible range, the main method for controlling the mode composition of beams is the use of diffractive optical elements. In this work, we investigated the characteristics of perfect beams, the parent beams being quasi-Bessel beams created by three types of diffractive phase axicons made of high-resistivity silicon: binary, kinoform, and “holographic”. The amplitude-phase distributions of the field in real perfect beams were calculated numerically in the approximation of the scalar diffraction theory. An analytical expression was obtained for the case of the binary axicon. It was shown that a distribution closest to an ideal vortex was obtained using a holographic axicon. The resulting distributions were compared with experimental and theoretical distributions of the evanescent field of a plasmon near the gold–zinc sulfide–air surface at different thicknesses of the dielectric layer, and recommendations for experiments were given.

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DIFFRACTION OF 3.2 CM. ELECTROMAGNETIC WAVES BY CYLINDRICAL OBJECTS

Canadian Journal of Physics ◽

10.1139/p54-039 ◽

1954 ◽

Vol 32 (6) ◽

pp. 372-380 ◽

Cited By ~ 4

Author(s):

A. B. McLay ◽

S. T. Wiles

Keyword(s):

Electromagnetic Waves ◽

Incident Beam ◽

Diffraction Theory ◽

Central Peak ◽

Beam Direction ◽

Cylinder Axis ◽

Scalar Diffraction ◽

Conducting Cylinder ◽

Scalar Diffraction Theory ◽

Diffraction Patterns

Diffraction patterns of a brass tube and a hard rubber rod, each a cylinder of 1 in. diameter, in a nearly plane beam of square-wave modulated 3 cm. waves with electric vector parallel to the cylinder axis, have been measured in several planes transverse to the incident beam direction. Experimental results for the conducting cylinder agree closely with calculations based on scalar diffraction theory. Patterns of the dielectric rod show a pronounced central peak immediately behind the rod and other intensity effects differing from the conducting cylinder patterns, particularly in the vicinity of the shadow.

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Quasi-Continuous Metasurface Beam Splitters Enabled by Vector Iterative Fourier Transform Algorithm

Materials ◽

10.3390/ma14041022 ◽

2021 ◽

Vol 14 (4) ◽

pp. 1022 ◽

Cited By ~ 1

Author(s):

Jinzhe Li ◽

Fei Zhang ◽

Mingbo Pu ◽

Yinghui Guo ◽

Xiong Li ◽

...

Keyword(s):

Fourier Transform ◽

Computational Cost ◽

Three Dimensional ◽

Diffraction Theory ◽

Optical Systems ◽

Subwavelength Structures ◽

Beam Splitters ◽

Scalar Diffraction ◽

Target Energy ◽

Diffraction Patterns

Quasi-continuous metasurfaces are widely used in various optical systems and their subwavelength structures invalidate traditional design methods based on scalar diffraction theory. Here, a novel vector iterative Fourier transform algorithm (IFTA) is proposed to realize the fast design of quasi-continuous metasurface beam splitters with subwavelength structures. Compared with traditional optimization algorithms that either require extensive numerical simulations or lack accuracy, this method has the advantages of accuracy and low computational cost. As proof-of-concept demonstrations, several beam splitters with custom-tailored diffraction patterns and a 7 × 7 beam splitter are numerically demonstrated, among which the maximal diffraction angle reaches 70° and the best uniformity error reaches 0.0195, showing good consistency with the target energy distribution and these results suggest that the proposed vector IFTA may find wide applications in three-dimensional imaging, lidar techniques, machine vision, and so forth.

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