Abstract
High efficiency, uniform power distribution, and broadband working of beam splitters are of crucial importance for large format heterodyne array receivers at terahertz frequencies. We present two types of quad-beam splitters (QBSs) based on coding metamaterials, designed to generate 1 × 4 and 2 × 2 beams respectively in the THz range. The designed metasurfaces are composed of a periodic array of metal-dielectric-metal (MDM) scattering unit cells, thus introducing a well-designed phase shift in the electromagnetic wave. For normally incident electromagnetic waves, the simulation results show that the 2 × 2 QBS has higher power efficiency and broader bandwidth in comparison with the 1 × 4 QBS. The 2 × 2 QBS works well in that the power efficiency is larger than 70% and the power distribution difference of the main beams is less than 2.8% in the range of 0.75–1.35 THz, which indicates an ultra-broad frequency range up to 57.1%. Meanwhile, under oblique incidence, the power distribution and the power efficiency among the 2 × 2 quad-beam nearly remain stable when the wave incident is within 40-degree. A prototype is fabricated and the 2 × 2 QBS behavior is experimentally demonstrated at 0.85 THz, where the power efficiency is as high as 70.4% and the power distribution of each beam approximately is uniform under the 20-degree incidence. To further improve the efficiency of astronomical observations, a 36-beam splitter is designed by using a simple two-dimensional addition method, and the power efficiency can reach 89.9% at 0.85 THz, and the working bandwidth is 57.1% from 0.75 to 1.35 THz with evenly distributed main beams. The simulation results show that the beam splitting function can also be well realized under the waves with 20-degree oblique incidence. Compared to conventional Fourier phase gratings, the metasurface-based beam splitters provide broadband performance and a simpler fabrication process, which is very expected in compact heterodyne array receiver systems.
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