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-Reflective liquid crystal terahertz phase shifter with tuning range of over 360°

Jun Yang, Chenggang Cai,  Zhiping Yin, Tianyu Xia, Shuaicheng Jing,   Hongbo Lu, Guangsheng Deng

https://ieeexplore.ieee.org/document/8402281/

This study presents the design and experiment of a terahertz reflective phase shifter based on liquid crystal. The proposed voltage-tunable phase shifter takes advantage of the variable permittivity of the liquid crystal, adjusted by an external electric field, providing tunable reflective phase shift of terahertz (THz) waves. We analysed the performance of this phase shifter using a simple isotropic and homogeneous model considering the angle of incidence. The phase variation was achieved in a range greater than 360° and frequencies from 349 to 361 GHz. An array constituted of 900 patch elements was fabricated using the photolithographic process. It provided tunable phase range of 350° over the frequency range of 353.5-359.3 GHz at 40 V, where a maximum phase shift 362.6° was achieved at 357 GHz. The LC dielectric constant was obtained by fitting the computed reflectance spectrum to the measured result. We compared accurate model, considering both inhomogeneity and anisotropy of the liquid crystal, with the simplified model. In addition, the differences were analysed between the measurements, accurate and conventional model. The results show the liquid crystal phase shifter is great for beam scanning antenna array in THz.

Abstract-Antireflection self-reference method based on ultrathin metallic nanofilms for improving terahertz reflection spectroscopy

Weien Lai, Haibing Cao, Jun Yang, Guangsheng Deng, Zhiping Yin, Qian Zhang, Beatriz Pelaz, and Pablo del Pino

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-15-19470

We present the potential of an antireflection self-reference method based on ultra-thin tantalum nitride (TaN) nanofilms for improving terahertz (THz) reflection spectroscopy. The antireflection self-reference method is proposed to eliminate mutual interference caused by unwanted reflections, which significantly interferes with the important reflection from the actual sample in THz reflection measurement. The antireflection self-reference model was investigated using a wave-impedance matching approach, and the theoretical model was verified in experimental studies. We experimentally demonstrated this antireflection self-reference method can completely eliminate the effect of mutual interference, accurately recover the actual sample’s reflection and improve THz reflection spectroscopy. Our method paves the way to implement a straightforward, accurate and efficient approach to investigate THz properties of the liquids and biological samples.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-A Tunable Polarization-Dependent Terahertz Metamaterial Absorber Based on Liquid Crystal

Guangsheng Deng, Yujiao Lu, Zhiping Yin,  Weien Lai, Hongbo Lu,  Jun Yang, Aifeng Yang, Yang Ye, Dayong Liu, Baihong Chi,

http://www.mdpi.com/2079-9292/7/3/27

In this paper, a tunable polarization-dependent terahertz (THz) metamaterial absorber based on liquid crystal (LC) is presented. The measurement results show that absorption peak is at 239.5 GHz for a TE-polarized wave and 306.6 GHz for a TM-polarized wave, without exerting the bias voltage on the LC layer. An increase in bias voltage affects the orientation of LC molecules and causes redshifted resonant frequencies. By adjusting the bias voltage from 0 to 10 V, frequency tunabilities of 4.7% and 4.1% for TE- and TM-polarized waves, respectively, were experimentally demonstrated. Surface current and power loss distribution was analyzed to explain the physical mechanism of the absorber, while the absorption dependence on geometrical parameters and incident angles was also studied in detail. According to the obtained results, the proposed absorber is shown here to be capable of achieving tunable polarization-dependent absorption, and to have potential application in terahertz polarization imaging, terahertz sensing, and polarization multiplexing

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-Electrically tunable terahertz dual-band metamaterial absorber based on a liquid crystal

Zhiping Yin,  Yujiao Lu,  Tianyu Xia,  Weien Lai,  Jun Yang,  Hongbo Lua,  Guangsheng Deng,

http://pubs.rsc.org/en/content/articlelanding/2018/ra/c7ra13047c#!divAbstract

In this paper, a liquid crystal (LC) based tunable metamaterial absorber with dual-band absorption is presented. The proposed absorber is analysed both numerically and experimentally. The analysis shows that the two absorption peaks, originating from the new resonant structure, are experimentally detected at 269.8 GHz and 301.4 GHz when no bias voltage is applied to the LC layer. In order to understand the absorption mechanisms, simulation results for the surface current and power loss distributions are presented. Since liquid crystals are used as the dielectric layer to realize the electrically tunable absorber, a frequency tunability of 2.45% and 3.65% for the two absorption peaks is experimentally demonstrated by changing the bias voltage of the LC layer from 0 V to 12 V. Furthermore, the absorber is polarization independent and a high absorption for a wide range of oblique incidence is achieved. The designed absorber provides a way forward for the realization of tunable metamaterial devices that can be applied in multi-band detection and imaging.

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

Abstract-Graphene-based tunable polarization sensitive terahertz metamaterial absorber

• Guangsheng Deng, 
• Ping Chen, 
• Jun Yang, 
• Zhiping Yin, 
• Longzhen Qiu

• Academy of Photoelectric Technology, Hefei University of Technology, Hefei 230009, China

Received 2 February 2016, Revised 4 May 2016, Accepted 27 May 2016, Available online 9 June 2016

http://www.sciencedirect.com/science/article/pii/S0030401816304680

A tunable metamaterial absorber composed of a metal ground plane, a SiO₂ dielectric spacer, a graphene layer and a metal pattern layer, which is polarization sensitive is numerically proposed at terahertz (THz) frequencies. Our calculated results show that when the Fermi level of graphene is fixed at 0.7 eV, the absorptivities for x-polarized wave are about 0.985 at 6.42 THz, and 0.991 at 8.37 THz, respectively, while the absorptivity for y-polarized wave is about 0.99 at 7.22 THz. Moreover, the calculated electric field and surface current distributions, along with power loss distributions enable us to deeply understand the physical mechanism of resonance absorption. More importantly, absorption spectra at different Fermi levels of graphene and different incident angles are displayed and tuning functions are discussed in detail. This work may provide a further step in the development of potential applications based on metamaterial absorber, such as THz polarization imaging, THz sensing and polarization multiplexing.