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Saturday, January 12, 2019
Abstract-Theoretical study on 2.52 terahertz Gaussian beam shaping based on an off-axis transmissive all-dielectric metasurface
Zewen Wang and Qi Li
Fig. 1. Schematic diagram of beam propagation process.
An off-axis transmissive all-dielectric square metasurface with a side length of 55 mm is designed and studied in this paper. It can shape the 2.52 terahertz Gaussian beam into a rigorous plane wave (we define the required field-quality of the rigorous plane wave region: peak-to-peak amplitude ripple less than 1 dB and phase ripple less than 10°). The metasurface is composed of silicon cylindrical resonators whose phase modulation can almost span over the entire 2π range and the poly (4-methyl-1-pentene) substrate. The diameters of the resonators are from 11 to 35 μm and the heights of them are all 60 μm. The working range of the obtained plane wave is calculated by the physical optics integral formula when the off-axis angle is 22°. The results show that when the off-axis propagation distance is 1050 mm, the area satisfying the plane wave condition is 17×20 mm2, and the diffraction efficiency (we define the diffraction efficiency: the ratio of the total light field energy on the target plane to the total energy of light field that is just after the metasurface along the propagation of incident light) of the rigorous plane wave region cross-section is 23.4%. In addition, a random silicon cylinder diameter fabrication error within ± 4.5 μm is taken into account for the actual fabrication condition. The error satisfies the normal distribution with a mean of 0 μm and a standard deviation of 3 μm. The influence of the fabrication error on the shaping result is studied. We anticipate that the whole study will have certain pragmatic applications in some fields, such as enlarging the capture area of terahertz optical tweezers.