The hexagonal lattice of graphene is among the interesting aspects that enable it to exhibit autonomous exotic physical phenomena such as anomalous quantum Hall effect and high charge carrier mobility at room temperature. With such promising features, graphene has emerged as a top material of interest among physicists, engineers and materials scientists as it can be applied in optoelectronics and quantum computers. To this regard, many graphene-based polarization devices with diverse tunable functionalities have been developed where it has been used as a substitute to the convectional metal meta-surfaces. Still, such devices are usually backed with metallic ground plane thereby operating in the reflective mode in which the metallic ground plane and meta-surface form a Farby-Perot-like cavity. Unfortunately, the device performance may be deteriorated due to the effect of the Fabry-Perot resonance. This in-turn presents a problem in that the tunable polarization converters backed with metallic ground plane ends up with a narrow tunable bandwidth.
Researchers led by professor Haiou Li at Guilin University of Electronic Technology in China, proposed a study to broaden the bandwidth of a graphene-based circular polarization converter by phase compensation. The researchers purposed to present a reflective circular polarization converter consisting of a single graphene sheet patterned with butterfly-shaped holes, a dielectric spacer, and a 7-layer graphene ground plane. They hoped to present the frequency range at which the proposed circular polarization converter could be tuned. Their work is now published in the journal, Optics Express.
The research team commenced their empirical procedure by configuring the circular polarization converter where a complementarily graphene-based meta-surface, a dielectric spacer and a 7-layer graphene were co-joined. They then selected silicon of specific thickness for use as the dielectric. The team then used the 7-layer graphene as a reflective plane like the metallic ground in the conventional reflective polarization converter. Eventually, they investigated the performance of the proposed polarization converter after analyzing the conductivity of the single and multi-layer graphene.
The authors of this paper observed that the 7-layer graphene could reflect electromagnetic wave with high reflectivity and could also provide an additional phase for its reflected wave, over a broad bandwidth. The team also noted that by controlling the Femi energy of the 7-layer graphene, the additional phase could be modulated to be an appropriate value so as to compensate the phase shift produced by the frequency tuning of device. This was noted to keep the constructive interference condition at the surface of meta-surface, thereby, resulting in good device performance in a broad bandwidth.
The Haiou Li et al study has presented a theoretical investigation of the electric properties of a 7-layer graphene molded by randomly stacking mono-layer graphene. It has been seen that from the simulations performed, the tunable relative bandwidth of the proposed device has been significantly increased and is much wider than the polarized device backed with metallic ground. This work therefore presents a revolutionary step towards the development of active polarization device with high performance and broad tunable bandwidth.
Xi Gao, Wanli Yang, Weiping Cao, Ming Chen, Yannan Jiang, Xinhua Yu, Haiou Li. Bandwidth broadening of a graphene-based circular polarization converter by phase compensation. Optics Express, Vol. 25 2017
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