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-Strong terahertz response in quantum well photodetector based on intradonor transition by magnetic field

AIP Advances

C. H. Yu, Lin Li, Teng Fei Xu,   Bo Zhang, X. D. Luo,  Wei. Lu,

Schematic of THz detection in GaAs/AlGaAs THz QWP based on IDTs in AlGaAs barriers under magnetic field. Step ①: ISBT in GaAs wells, ②: Landau quantization and shift of LL ε0,0 with magnetic field in GaAs wells, ③: electron transfer from LL ε0,0 in GaAs wells to the 1s donor ground state in AlGaAs barriers after magnetic field reaches its threshold Bt, ④: IDT in AlGaAs barriers, ⑤: thermal phonon absorptions. PTI is a two-step process including both step ④ and ⑤. εe means higher donor states in AlGaAs barriers, such as 2p+, (210) and (310) observed in measurement. The figure is not drawn to scale.

https://aip.scitation.org/doi/abs/10.1063/1.5051203

We report on spectral investigation of photocurrent of a nonconventional GaAs/AlGaAs quantum well photodetector (QWP) which realizes a response to terahertz (THz) radiation by intradonor transitions (IDTs) in AlGaAs barriers rather than typical intersubband transitions (ISBTs) in GaAs wells. The photodetector shows dramatically enhanced photocurrent intensity and THz response when under a perpendicular magnetic field. This magnetic field helps to improve the absorption quantum efficiency, remove the restriction of the polarization selection rule, and extract high density electrons from two-dimensional electron gas in GaAs wells after electrons transfer into AlGaAs barriers. The effect of the magnetic field on the peak intensity and the linewidth of the photocurrent peak responses is exploited to distinguish IDT and ISBT and to identify the crucial role of electron transfer in THz detection in QWP. This work is useful for exploring detection strategy and technology for high responsivity THz photodetector.

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.

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