Abstract-Terahertz mode selector based on multimode resonances in corrugated waveguides

 

Lan-Lan Xu, Ting Liu, Ya-Xian Fan, Huan Liu, and Zhi-Yong Tao

https://www.osapublishing.org/josab/abstract.cfm?uri=josab-38-8-2323

We propose a magnetically switchable terahertz (THz) mode selector based on four transverse mode resonances in corrugated waveguides. The theoretical and numerical results show that two passbands can be obtained in the transmission spectra around the resonance points. The passband of 0.9611–1.0006 THz outputs a single second-order transverse mode with the highest ratio of 99.03% while the passband of 0.8712–0.9111 THz contains the multiple transverse mode output with the first two modes. A tunable THz mode selector with frequency band of 0.5572–0.5926 THz is realized by filling the waveguide core with a liquid crystal (E7). The widest working frequency range of the mode selector reaches 28.4 GHz when the molecular steering angle 𝜃$\theta$ lies between 71.22° and 90°.

© 2021 Optical Society of America

Abstract-An efficient terahertz metamaterial linear polarization converter

Meng-Qiang Zou,  Huan Liu,  Yi Gong,  Ya-Xian Fan,  Zhi-Yong Tao,

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11196/111960L/An-efficient-terahertz-metamaterial-linear-polarization-converter/10.1117/12.2538629.short

An efficient terahertz (THz) metamaterial linear polarization converter is proposed. The structure consists of three layers of metal-dielectric-metal, including an anisotropic super-surface based on bimetallic arc structure as the top layer and a dielectric layer and a metal plate as middle layer and bottom layer, separately. The numerical analysis results show that broadband polarization conversion of a linearly polarized wave in the terahertz band is realized using the structure proposed. The polarization conversion ratio of the linearly polarized wave is more than 90% across a wide frequency range from 1.21 THz to 2.82 THz. The ratio of bandwidth to central frequency can reach 80%. The mechanism of high polarization conversion rate and broadband is analyzed. At the same time, the dependence of the polarization conversion rate of the structure on the incident angle is studied. The results show that the structure has good polarization conversion performance in the incident angle range of 0-60°. Compared with previous designs, the polarization converter has not only simple structure, but also wide bandwidth. It has potential application value in polarization modulation of terahertz wave.

© (2019) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Abstract-A High-Resolution Terahertz Electric Field Sensor Using a Corrugated Liquid Crystal Waveguide

Lan-Lan Xu, Yi Gong, Ya-Xian Fan,  Zhi-Yong Tao

https://www.mdpi.com/2073-4352/9/6/302

Liquid crystals (LCs) can always reflect variable optical properties in a broad terahertz (THz) band under external electric or magnetic fields. Based on the measurements of these varying properties, we can realize electric and magnetic field sensing with very high sensitivity. Here, we theoretically and numerically demonstrate a type of electric field sensor in the THz frequency range based on the defect mode arising in a periodically corrugated waveguide with liquid crystals. The Bragg defect structure consisting of periodically corrugated metallic walls and a defect in the middle can provide a narrow transmitted peak with controllable bandwidth, which can be used for external field sensing when it is filled with LCs. The molecular orientation of nematic LCs (E7) is not only very sensitive to the applied DC electric field but also very crucial to the effective refractive index of E7. Changing the effective index can efficiently shift the frequency of the transmitted peak in the THz spectrum. The simulated results show that the sensitivity can reach as high as 9.164 MHz/(V/m) and the smallest resolution is 0.1115 V/m. The proposed sensor and its significant performance could benefit electric field sensing and extend the applications of THz technology

Abstract-Vanadium dioxide-assisted broadband tunable terahertz metamaterial absorber

Huan Liu, Zhi-Hang Wang, Lin Li, Ya-Xian Fan,  Zhi-Yong Tao

https://www.nature.com/articles/s41598-019-42293-9

Tunable terahertz (THz) functional devices have exhibited superior performances due to the use of active materials, such as liquid crystals, graphene, and semiconductors. However, the tunable range of constitutive parameters of materials is still limited, which leads to the low modulation depth of THz devices. Here, we demonstrate a broadband tunable THz absorber based on hybrid vanadium dioxide (VO2) metamaterials. Unlike other phase change materials, VO2 exhibits an insulator-to-metal transition characteristic and the conductivity can be increased by 4–5 orders of magnitude under external stimulus including electric fields, optical, and thermal pumps. Based on the unique transition character of VO2, the maximum tunable range of the proposed absorber can be realized from 5% to 100% by an external thermal excitation. Meanwhile, an absorption greater than 80% in a continuous range with a bandwidth about 2.0  THz can be obtained when VO2 is in its metal phase at high temperature. Furthermore, the absorber is insensitive to the incident angle up to 50° and such a broadband THz absorber can be used in applications including imaging, modulating, cloaking, and so on.

Abstract-Self-adaptive terahertz spectroscopy from atmospheric vapor based on Hilbert-Huang transform

Huan Liu, Ya-Xian Fan, Lin Li, Hong-Ge Chen, Peng-Fei Wang, and Zhi-Yong Tao

           Fig. 1 Schematic diagram of the optical fiber integrated THz-TDS in transmission mode.
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-21-27279

Absorption lines of atmospheric vapor commonly appear in terahertz (THz) spectra measured in a humid air environment. However, these effects are generally undesirable because they may mask critical spectroscopic information. Here, a self-adaptive method is demonstrated for effectively identifying and eliminating atmospheric vapor noise from THz spectra of an all-fiber THz system with the Hilbert-Huang transform. The THz signal was decomposed into eight components in different time scales called the intrinsic mode functions and the interference of atmospheric vapor was accurately isolated. A series of experiments confirmed the effectiveness and strong self-adaptiveness of the proposed system in vapor noise elimination.

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

Abstract-Wide Band Terahertz Switch of Undulated Waveguide with VO2 Film Coated Inner Wall

Kun Zhang, Lu Zhang, Di Duan, Ya-Xian Fan, Zhi-Yong Tao

https://ieeexplore.ieee.org/document/8417923/


We propose a wide band terahertz (THz) switch of undulated waveguide with vanadium dioxide ( 
VO2 ) film coated inner wall. The forbidden bands formed by periodically undulated walls effectively restrain THz transmission when VO2 is in the insulator phase. An unexpected wide band appears in the transmission spectrum, connecting two forbidden bands, where metal phase VO2 covers the undulations and eliminates band gaps. Simulations confirm that the switch could be realized by varying the temperature in the frequency range of 0.71-1.07 THz, and hence such a switch could be applied in THz systems.

Abstract-A magnetically tunable non-Bragg defect mode in a corrugated waveguide filled with liquid crystals

Lu Zhang, Ya-Xian Fan,  Huan Liua, Xu Han, Wen-Qiang Luc, Zhi-Yong Tao,

https://www.sciencedirect.com/science/article/pii/S0375960118301737

A magnetically tunable, non-Bragg defect mode (NBDM) was created in the terahertz frequency range by inserting a defect in the middle of a periodically corrugated waveguide filled with liquid crystals (LCs). In the periodic waveguide, non-Bragg gaps beyond the Bragg ones, which appear in the transmission spectra, are created by different transverse mode resonances. The transmission spectra of the waveguide containing a defect showed that a defect mode was present inside the non-Bragg gap. The NBDM has quite different features compared to the Bragg defect mode, which includes more complex, high-order guided wave modes. In our study, we filled the corrugated waveguide with LCs to realize the tunability of the NBDM. The simulated results showed that the NBDM in a corrugated waveguide filled with LCs can be used in filters, sensors, switches, and other terahertz integrated devices.

Abstract-Hypersensitive and tunable terahertz wave switch based on non-Bragg structures filled with liquid crystals

Lu Zhang,  Ya-Xian Fan, Huan Liu,  Lan-Lan Xu,  Jiu-Ling Xue,  Zhi-Yong Tao

http://ieeexplore.ieee.org/document/7933180/

We investigated a hypersensitive and tunable terahertz (THz) wave switch based on liquid-crystal-filled non-Bragg structures. Non-Bragg structures, which consist of periodically corrugated metallic tube walls, provide spectra with very sharp rising edges, making them usable for sensitive switching. Tunability can be achieved by dynamically shifting the rising edge of a THz spectrum by using an externally applied magnetic field to change the orientations of the nematic liquid crystal (E7) molecules. The simulated results revealed that the switch effects are hypersensitive and tunable in the THz frequency range and that such switches could be applicable in future THz systems.

Abstract-Thermally Tunable Narrow Band Filter Achieved by Connecting Two Opaque Terahertz Waveguides

Lan-Lan Xu,  Zhi-Yong Tao,  Tang-Qing Sang , Dan Xu,  Peng-Fei Wang, Ya-Xian Fan,

http://ieeexplore.ieee.org/document/7896525/


We propose a thermally tunable terahertz narrow band filter by combining two different periodic waveguides. When the terahertz tube wall is corrugated periodically, the transparent spectrum presents a very complex structure as various pass and stopbands emerging. The terahertz wave cannot propagate through the waveguide when its frequency falls into the stopbands. When we connected two tubes with different types of stopbands, it is very intriguing that we have found an unexpected ultra-narrow-band transparency in the frequency gap. Our findings provide a more feasible and effective terahertz waveguide filter beyond these existing ones. By using the finite-element method, we have obtained the filtering characteristics of the proposed structures. The filter bandwidth could be narrowed from 2.3 GHz to 0.29 MHz by increasing the number of waveguide segments and the highest Q -factor achieved is about 3.5×106 . Moreover, the tunability on temperature is obtained in a waveguide device of Au based on the combination of opaque structures.