Abstract-Tunable THz generalized Weyl points

Zhiping Yin, Fujia Chen, Kai Guo, Fei Shen, Keya Zhou, Jun Gao, Shutian Liu, and Zhongyi Guo

Fig. 1 Realization of Weyl points in a synthetic space. (a) Photonic crystals (PCs) with different p, q values. p and q form a parameter space, which determines the geometric structure of PCs. The inset shows one unit cell of the PC, where the first and the third layers are made of 5CB LCs (cyan), and the second and the forth layers are made of PDMS (gray). The thickness of each layer is related to its position in the p-q parameter space. (b) The band dispersion of PCs with two layers in one unit cell (red dash line) and four layers in one unit cell (blue solid line). Crossing points appear inside four layers’ dispersion. Here, 𝑑𝑎=125𝑢𝑚,$d_a=125um,$ 𝑑𝑏=35𝑢𝑚,$d_b=35um,$ and 𝑝=𝑞=0.$p=q=0.$ (c) The dispersion of PCs in the p-q space with 𝑘=0.5𝑘0,$k=0.5k_0,$ and 𝑘0=𝜋/(𝑑𝑎+𝑑𝑏).$k_0=\pi /\left(d_a+d_b\right).$ Here, two bands form a conical intersection. Panels (b) and (c) together show that the band dispersions are linear in all directions around the degenerated point in synthetic space, so we call it generalized Weyl point. (d) The equal frequency contours around generalized Weyl point in p-q space and its charge “-1”.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-2-512

Weyl points, as linearly double degenerated point of band structures, have been extensively researched in electronic and classical wave systems. However, Weyl points’ realization is always accompanied with delicate “lattice structures”. In this work, frequency-tunable terahertz (THz) generalized Weyl points inside the parameter space have been investigated and displayed by a specially designed photonic crystal with polydimethylsiloxane (PDMS) immersed in 4-cyano’-pentylbipenyl (5CB) liquid crystals (LCs). The reflective phase vortices as a signature of the generalized Weyl points are observed through our numerically simulations. Besides, interface states between photonic crystals and any reflective substrates are fulfilled too. Meanwhile, we could also change the orientation of LC molecule by the external magnetic field so as to tune the frequency of the first two bands’ Weyl point from 0.27698THz to 0.30013THz. This band lies in the short-range wireless communication. Thus, our proposal may be beneficial to the investigation and application of Weyl points’ properties and strongly localized states.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Actively Tunable Terahertz Switches Based on Subwavelength Graphene Waveguide

Zhongyi Guo, Xiaoru Nie, Fei Shen, Hongping Zhou, Qingfeng Zhou, Jun Gao, Kai Guo,

http://www.mdpi.com/2079-4991/8/9/665

As a new field of optical communication technology, on-chip graphene devices are of great interest due to their active tunability and subwavelength scale. In this paper, we systematically investigate optical switches at frequency of 30 THz, including Y-branch (1 × 2), X-branch (2 × 2), single-input three-output (1 × 3), two-input three-output (2 × 3), and two-input four-output (2 × 4) switches. In these devices, a graphene monolayer is stacked on the top of a PMMA (poly methyl methacrylate methacrylic acid) dielectric layer. The optical response of graphene can be electrically manipulated; therefore, the state of each channel can be switched ON and OFF. Numerical simulations demonstrate that the transmission direction can be well manipulated in these devices. In addition, the proposed devices possess advantages of appropriate ON/OFF ratios, indicating the good performance of graphene in terahertz switching. These devices provide a new route toward terahertz optical switching

Abstract-Broadband tunable terahertz polarization converter based on graphene metamaterial

Shiwen Luo, Bin Li, Anlan Yu, Jun Gao, Xinbing Wang,  Duluo Zuo,

https://www.sciencedirect.com/science/article/pii/S0030401817311574

We design and numerically investigate a broadband tunable terahertz polarization converter based on graphene metamaterial. The converter presents a broad conversion band with high polarization conversion ratio (>0.95) in terahertz frequency over a bandwidth which is 25.8% of the central frequency. The converter can be dynamically tuned by varying the Fermi Energy of the graphene without changing the geometric structure. The converter shows high conversion ratio for a wide range of incident angles from 0 to 40°. By the scaling of the proposed structure, the broadband properties of the converter can be easily spread to other frequency. The proposed metamaterial offers an approach in the manipulation of the light polarization and has potential applications in imaging, sensing and communications.

Abstract- Broadband diffusion metasurface based on a single anisotropic element and optimized by the Simulated Annealing algorithm

• Yi Zhao
• , Xiangyu Cao
• , Jun Gao
• , Yu Sun
• , Huanhuan Yang
• , Xiao Liu
• , Yulong Zhou
• , Tong Han
•  & Wei Chen

• http://www.nature.com/articles/srep23896

We propose a new strategy to design broadband and wide angle diffusion metasurfaces. An anisotropic structure which has opposite phases under x- and y-polarized incidence is employed as the “0” and “1” elements base on the concept of coding metamaterial. To obtain a uniform backward scattering under normal incidence, Simulated Annealing algorithm is utilized in this paper to calculate the optimal layout. The proposed method provides an efficient way to design diffusion metasurface with a simple structure, which has been proved by both simulations and measurements.