In recent years, metasurfaces (MSs), as planar version of metamaterials, have demonstrated strong capabilities to control EM waves, generating many fascinating effects such as ultrathin lenses, planar holograms, photonic spin Hall effects as well as asymmetric transmission (AT) effects. AT for both circularly polarized waves and linearly polarized waves have been initially explored in either multilayer structures or single layer profiles. For instance, a two-layer L-shaped groove structure has been engineered to realize dualband linear polarization asymmetric transmission. The EM wave absorption property gives metasurface a wide range of applications.
In 2008, Smith first proposed the concept of a perfect metamaterial absorber (PMA) and realized EM wave absorption. Because the ideal absorption property of PMA is based on the resonance characteristics of the unit cell, the inherent narrow bandwidth performance limits its application. To realize broad bandwidth absorption, a variety of innovative technologies have been proposed, such as multilayer structure, single-layer multiresonant structure, lumped-element loading, and Salisbury screen.
We propose a new strategy to realize high efficiency and ultra-broadband asymmetric transmission in an ultra-thin profile by using the topologically coding optimization method. The meta-atom consists of two outer orthogonal gratings and a central lattice particle optimized by genetic algorithm. The optimized central lattice suppresses the transmission fluctuations by tuning the coupling among different metallic layers, resulting in very broad band and high transmissions. Experimental results show that our metasurface achieved perfect reflection over 95% and high cross-polarization transmission over 80% for y- and x-polarized incidence from 5.3 GHz to 16.7 GHz, respectively.
We also propose a novel absorption structure, which combines indium tin oxide film and metal resonator. The former realizes impedance matching with free space in a broad bandwidth at moderate frequency range while the latter shows the resonant property at low frequency. Based on this absorption structure, we design the zigzag-shaped structure to realize high-efficiency and ultra-broadband absorption. To demonstrate the feasibility of our method, we fabricate a sample and perform measurements. The measurement results show that our sample can achieve ultra-broadband absorption with high-efficiency of over 90% from 1 GHz to 18 GHz, which is in good agreement with simulation results.
Our findings pave a way to high performance and broadband polarization transformers and polarization-controlled devices working in different frequency domains and provide a valuable technique for broadband device design.
High-efficiency and ultra-broadband asymmetric transmission metasurface based on topologically coding optimization method
Wenye Ji, Tong Cai, Guangming Wang, Haipeng Li, Canyu Wang, Haisheng Hou, and Chiben Zhang, Optics Express 27.3.2844 (2019)
Wenye Ji, Tong Cai, Guangming Wang, Yong Sun, Haipeng Li, Canyu Wang, Chiben Zhang, and Qing Zhang, Optics Express 27.22.32835 (2019)