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A method of designing new Bessel beam generator
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Abstract
Catenary nanostructures enable continuous phase control. However, the ordinary catenary nanostructure has narrow width at both ends and is not easy to be fabricated. On the other side, it was difficult to build complex model directly in simulation software CST, and the simulation process was complicated in the past. The equal-width catenary slit is proposed to replace the normal catenary slit. And the equal-width catenary-type metasurface has been designed to generate Bessel beam, which provides a new idea for the design of two-dimensional optical devices. In the process of modeling and simulation, CST is combined with Matlab for co-simulation, and all operations, such as modeling, simulations, and parameter modification, are completed directly in Matlab. This method can be used to design complex structures, and more ideal simulation results can be obtained combined with the numerical optimization ability of Matlab.-
Keywords:
- metasurfaces /
- catenary /
- Bessel beam /
- CST /
- Matlab
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References
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Overview
Overview: The metasurface is an ultra-thin two-dimensional material arranged by metal or dielectric subwavelength structures. Phase-modulated metasurface is an important branch of metasurfaces, which mainly includes propagation, geometric, and circuit-type phase metasurfaces. The phase modulation of incident circular polarization can be realized in geometric phase metasurfaces by using the spatially varying subwavelength structure. Usually, the geometric phase metasurface is realized by discrete structures whose rotation angle has a correspondence with the geometric phase. However, discrete elements can only generate discrete phase distributions. Catenary is a kind of mechanical curves. The geometric phase metasurfaces composed of catenary structures can realize continuous geometric phase. The catenary metasurface is composed of catenary slits etched on the metal film. The slit is translated down by a catenary and the two ends are connected. Similar to discrete metasurfaces, catenary slits can be considered as a patchwork of discrete elements. The tangent angle of the catenary corresponds to the rotation angle of the coordinate system of the discrete element. As described above, catenary slit is composed of vertical-translated catenary, so both ends of the slit will be very narrow, which is not conducive to experimental processing. Therefore, it is necessary to design equal-width catenary metasurfaces. On the other hand, in the design and simulation of the metasurfaces, CST is usually used for modeling and simulations. When designing V-shaped antennas, rectangular apertures, and other subwavelength structures such as normal catenary slits, the modeling process can be performed directly in CST. However, it is more complex for equal-width catenary slits and other irregularly shaped structures. On the other hand, when it comes to the need to optimize the arrangement of subwavelength element array, it is necessary to repeatedly model in CST, but to analyze data and optimize the structure in Matlab, which will affect the work efficiency. In previous studies, researchers have proposed co-simulation with Matlab and CST. Because of the software version, the purpose of the work, and the CST's numerous underlying commands, however, it is difficult to reproduce. In this paper, when designing the catenary metasurfaces, Matlab R2016b is used to call CST Microwave Studio version 2016 for modeling and simulations. A two-dimensional Bessel beam generator composed of an array of equal-width catenary slits is obtained. This article also tells the details of the co-simulation, so that researchers can better apply it to their own work.
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Figure 1.
Diagrams of catenary slits. (a) Ordinary catenary slit; (b) Equal-width catenary slit
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Figure 2.
Equal-width catenary array structure. (a) Phase distribution of zero-order Bessel beam generator; (b) Zero-order Bessel beam generator constructed by equal-width catenary arrays
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Figure 3.
Flowchart of CST simulation called by Matlab
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Figure 4.
Equal-width catenary arrays in CST
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Figure 5.
Intensity distribution of Bessel beam on x-z plane obtained by CST
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Figure 6.
Intensity and phase distributions of Bessel beam on x-y plane. (a)~(d) Represent the phase distributions on x-y plane at certain distances from the structure; (e)~(h) Denote the intensity distributions corresponding to the phase distributions (a)~(d), respectively; (i)~(l) are the cross views of the intensity shown by the dashed lines in figures (e)~(h), respectively
