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-High-efficiency terahertz polarization devices based on the dielectric metasurface

Jian Zhou, JingJing Wang, Kai Guo, Fei Shen,  Qingfeng Zhou,  Zhiping Yin,  Zhongyi Guo,



https://www.sciencedirect.com/science/article/pii/S0749603617327180

Metasurfaces are composed of the subwavelength structures, which can be used to manipulate the amplitude, phase, and polarization of incident electromagnetic waves efficiently. Here, we propose a novel type of dielectric metasurface based on crystal Si for realizing to manipulate the terahertz wave, in which by varying the geometric sizes of the Si micro-bricks, the transmitting phase of the terahertz wave can almost span over the entire 2π range for both of the x-polarization and y-polarization simultaneously, while keeping the similarly high-transmission amplitudes (over 90%). At the frequency of 1.0 THz, we have successfully designed a series of controllable THz devices, such as the polarization-dependent beam splitter, polarization-independent beam deflector and the focusing lenses based on the designed metasurfaces. Our designs are easy to fabricate and can be promising in developing high-efficiency THz functional devices.

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-High-efficiency terahertz dual-function devices based on the dielectric metasurface

JingJing Wang, Jian Zhou, Kai Guo, Fei Shen, Qingfeng Zhou, Zhipingyin, Zhongyi Guo,




https://www.sciencedirect.com/science/article/pii/S0749603618308784

The multifunctional optical device has attracted great interests due to the wide range of modern optical applications. However, conventional optical devices are single function and bulky structure, thereby hindering the practicality and miniaturization of the optical systems. Here, we present the basic theory, simulated demonstration, and in-depth analysis of a novel type of dielectric metasurface based on crystal Si to simultaneously convert and focus an incident linear polarization into other linear polarization or circular polarization with high efficiency and high numerical aperture (NA). Additionally, we have also achieved a quart wave-plate (QWP) based deflector that enables both arbitrary anomalous beam deflection and the polarization conversion. Simulation results show that the invertible QWP based deflector, including the linear-to-circular (LTC) and circular-to-linear (CTL) conversions, demonstrate around 80% deflection efficiency at 1.0 THz. With similar concept, a half wave-plate (HWP) combined with beam steering has also been achieved. Taking into account the considerable manipulation efficiency and cost-efficient sample fabrication technique, our designs show the great potentials of metasurfaces as a versatile platform for designing practical functional devices in the THz range.

Abstract-High-efficiency terahertz polarization devices based on the dielectric metasurface

Jian Zhou,  JingJing Wang, Kai Guo, Fei Shen, Qingfeng Zhou, Zhiping Yin, Zhongyi Guo,

https://www.sciencedirect.com/science/article/pii/S0749603617327180

Metasurfaces are composed of the subwavelength structures, which can be used to manipulate the amplitude, phase, and polarization of incident electromagnetic waves efficiently. Here, we propose a novel type of dielectric metasurface based on crystal Si for realizing to manipulate the terahertz wave, in which by varying the geometric sizes of the Si micro-bricks, the transmitting phase of the terahertz wave can almost span over the entire 2π range for both of the x-polarization and y-polarization simultaneously, while keeping the similarly high-transmission amplitudes (over 90%). At the frequency of 1.0 THz, we have successfully designed a series of controllable THz devices, such as the polarization-dependent beam splitter, polarization-independent beam deflector and the focusing lenses based on the designed metasurfaces. Our designs are easy to fabricate and can be promising in developing high-efficiency THz functional devices.

Abstract-Actively controllable terahertz switches with graphene-based nongroove gratings

Linbao Luo, Kuiyuan Wang, Caiwang Ge, Kai Guo, Fei Shen, Zhiping Yin,  Zhongyi Guo

https://www.osapublishing.org/prj/abstract.cfm?uri=prj-5-6-604

We systematically investigated the tunable dynamic characteristics of a broadband surface plasmon polariton (SPP) wave on a silicon-graded grating structure in the range of 10–40 THz with the aid of single-layer graphene. The theoretical and numerical simulated results demonstrate that the SPPs at different frequencies within a broadband range can be trapped at different positions on the graphene surface, which can be used as a broadband spectrometer and optical switch. Meanwhile, the group velocity of the SPPs can be modulated to be several hundred times smaller than light velocity in vacuum. Based on the theoretical analyses, we have predicted the trapping positions and corresponding group velocities of the SPP waves with different frequencies. By appropriately tuning the gate voltages, the trapped SPP waves can be released to propagate along the surface of graphene or out of the graded grating zone. Thus, we have also investigated the switching characteristics of the slow light system, where the optical switching can be controlled as an “off” or “on” mode by actively adjusting the gate voltage. The slow light system offers advantages, including broadband operation, ultracompact footprint, and tunable ability simultaneously, which holds great promise for applications in optical switches.

© 2017 Chinese Laser Press