Abstract-Spatial-spectral Terahertz Networks

 

Zheng Lin, Lifeng Wang, Bo Tan, Xiang Li

https://arxiv.org/abs/2101.02868

This paper focuses on the spatial-spectral terahertz (THz) networks, where transmitters equipped with the leaky-wave antennas send information to their receivers at the THz frequency bands. As a directional and nearly planar antenna, leaky-wave antenna allows for information transmissions with narrow beams and high antenna gains. The conventional large antenna arrays are confronted with challenging issues such as scaling limits and path discovery in the THz frequencies. Therefore, this work exploits the potential of leaky-wave antennas in the dense THz networks, to establish low-complexity THz links. By addressing the propagation angle-frequency coupling effects, the transmission rate is analyzed. The results show that leaky-wave antenna is efficient for achieving high-speed transmission rate and the co-channel interference management is not necessary when the THz transmitters with large subchannel bandwiths are not extremely dense. A simple subchannel allocation solution is proposed, which enhances the transmission rate compared with the same number of subchannels with equal allocation of frequency band. After subchannel allocation, a low-complexity power allocation method is proposed to improve the energy efficiency.

Abstract-Smaller antenna-gate gap for higher sensitivity of GaN/AlGaN HEMT terahertz detectors

Applied Physics Letters

Zhipeng Zhang, Xiang Li, Hua Qin, Jinfeng Zhang, Xinxing Li, Yang Shangguan, Lin Jin, Yunfei Sun, V. V. Popov,

(a) Schematic diagram of the GaN/AlGaN HEMT detector including the measurement circuit. (b) Zoom-in view of the central active region.

https://aip.scitation.org/doi/abs/10.1063/1.5142436

We report an attempt to improve the sensitivity of terahertz detection based on self-mixing in antenna-coupled field-effect transistors by enhancing the field-effect factor and the antenna factor with a reduced gate length and a reduced antenna-gate gap, respectively. An optical noise equivalent power (NEP) of 3.7 pW/Hz⎯⎯⎯⎯⎯√$3.7\hspace{0.17em}\text{pW}/\sqrt{\text{Hz}}$ at 0.65 THz was achieved in a GaN/AlGaN high-electron-mobility transistor (HEMT) with a gate length of 300 nm and an antenna-gate gap of 200 nm at room temperature. It was found that the antenna factor was inversely proportional to the antenna-gate gap, and the responses upon coherent/incoherent terahertz irradiation were well described by the self-mixing model. To fill the NEP gap of 0.1−1 pW/Hz⎯⎯⎯⎯⎯√$0.1-1\hspace{0.17em}\text{pW}/\sqrt{\text{Hz}}$ between room-temperature and cryogenic detectors by HEMT-based detectors at room temperature, impedance match needs to be carefully considered.

The authors acknowledge support from the National Natural Science Foundation of China (Nos. 61771466, 61775231, and 61975227), the Youth Innovation Promotion Association CAS (No. 2017372), the Six Talent Peaks Project of Jiangsu Province, China (XXRJ-079), and the Russian Foundation for Basic Research (No. 17-52-53063). The work in the Kotelnikov IRE RAS was carried out within the framework of the state task.

Abstract-Passive terahertz imaging detectors based on antenna-coupled high-electron-mobility transistors

Jiandong Sun, Yifan Zhu, Wei Feng, Qingfeng Ding, Hua Qin, Yunfei Sun, Zhipeng Zhang, Xiang Li, Jinfeng Zhang, Xinxing Li, Yang Shangguan, and Lin Jin

 (a) Scanning-electron micrograph of the detector with schematic measurement circuitry. (b) Zoom-in view of the central active region including the gate and the field-effect channel. (c, d) Backside and front-side views of the silicon hyperhemispherical lens with a detector chip assembled on the planar surface in a liquid nitrogen dewar with a TPX window.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-4-4911

Aiming at the requirement of passive terahertz imaging, we report a high-sensitivity terahertz detector based on an antenna-coupled AlGaN/GaN high-electron-mobility transistor (HEMT) at 77 K without using low-noise terahertz amplifier. The measured optical noise-equivalent power and the noise-equivalent temperature difference of the detector were about 0.3pW/Hz−−−√$0.3\mathrm{p}\mathrm{W}/\sqrt{\mathrm{H}\mathrm{z}}$ and 370 mK in a 200 ms integration time over a bandwidth of 0.7 − 0.9 THz, respectively. By using this detector, we demonstrated passive terahertz imaging of room-temperature objects with signal-to-noise ratio up to 13 dB. Further improvement in the sensitivity may allow passive terahertz imaging using AlGaN/GaN-HEMT at room temperature.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Two-terminal terahertz detectors based on AlGaN/GaN high-electron-mobility transistors

Applied Physics Letters

Jiandong Sun, Zhipeng Zhang,  Xiang Li, Hua Qin, Yunfei Sun, Yong Cai, Guohao Yu, Zhili Zhang, Jinfeng Zhang, Yang Shangguan, Lin Jin, Xinxing Li, Baoshun Zhang,  V. V. Popov,

(a) A top view of the detector. (b) Central gate and fluorine ion implantation area of the detector. (c) A schematic cross section of the detector corresponding to the dotted red line in (b). (d) A color-scale 2D plot of the spatial distribution of the mixing factor from a FDTD simulation at 648 GHz.

https://aip.scitation.org/doi/abs/10.1063/1.5114682

We report an approach to make two-terminal antenna-coupled AlGaN/GaN high-electron-mobility-transistor self-mixing terahertz detectors. Fluorine ion implantation is used to increase the threshold voltage of the AlGaN/GaN two-dimensional electron gas. An optimal implantation dose can be reached so that the detector responsivity is maximized at zero gate voltage or with the gate floating. The relationship between the ion dosage and the threshold voltage, electron mobility, electron density, responsivity, and noise-equivalent power (NEP) is obtained. A minimum optical NEP of 47  W/Hz⎯⎯⎯⎯⎯√$47\hspace{0.17em}\hspace{0.17em}\mathrm{W}/\sqrt{\text{Hz}}$ is achieved from a two-terminal detector at 0.65 THz. The capability of two-terminal operation allows for the design of a large array of antenna-coupled high-electron-mobility transistor detectors without the demanding needs of routing negative gate voltage lines around the antenna array and minimizing the gate leakage current.

The authors acknowledge support from the National Key Research and Development Program of China (No. 2016YFF0100501), the China National Natural Science Foundation (Nos. 61771466 and 61775231), the Youth Innovation Promotion Association CAS (No. 2017372), the Six Talent Peaks Project of Jiangsu Province, China (No. XXRJ-079), and the Russian Foundation for Basic Research (No. 17-52-53063).

Abstract-Fast Three-Dimensional Image Reconstruction of a Standoff Screening System in the Terahertz Regime

Jingkun Gao   Zhenmao Cui ;  Binbin Cheng,   Yuliang Qin, Xianjin Deng,  Bin Deng,   Xiang Li,  Hongqiang Wang

https://ieeexplore.ieee.org/document/8115312/

Based on a novel prototype terahertz screening system, its ability of fast three-dimensional (3-D) image reconstruction is demonstrated. We first give a brief introduction of the system. Then detailed analyses on signal model, system calibration, the influences of approximations, and the imaging performances are conducted. At last, proof-of-concept experiments are performed and preliminary 2-D and 3-D imaging results are presented. We show that the imaging problem can be resorted to a problem of spectral estimation finally. At this stage on this prototype system, a 180 cm × 90 cm region at 4-m range costs less than 0.1 s for 2-D imaging, 1 s for 3-D imaging and an extra 2 s for mechanical scanning. Faster speeds are on the horizon as hardware and software updates are ongoing. These results show its potential of real-time 3-D imaging for standoff concealed threats detection.

Abstract-Efficient Terahertz Wide-Angle NUFFT-Based Inverse Synthetic Aperture Imaging Considering Spherical Wavefront

Jingkun Gao, Bin Deng, Yuliang Qin, Hongqiang Wang, Xiang Li

http://www.mdpi.com/1424-8220/16/12/2120

An efficient wide-angle inverse synthetic aperture imaging method considering the spherical wavefront effects and suitable for the terahertz band is presented. Firstly, the echo signal model under spherical wave assumption is established, and the detailed wavefront curvature compensation method accelerated by 1D fast Fourier transform (FFT) is discussed. Then, to speed up the reconstruction procedure, the fast Gaussian gridding (FGG)-based nonuniform FFT (NUFFT) is employed to focus the image. Finally, proof-of-principle experiments are carried out and the results are compared with the ones obtained by the convolution back-projection (CBP) algorithm. The results demonstrate the effectiveness and the efficiency of the presented method. This imaging method can be directly used in the field of nondestructive detection and can also be used to provide a solution for the calculation of the far-field RCSs (Radar Cross Section) of targets in the terahertz regime.

Abstract-Backward wave oscillator for high power generation at THz frequencies

Diana Gamzina,  Xiang Li,  Christian Hurd,  Ye Tang,  Xuejiao Huang,  Yuan Zheng,  Logan Himes, Michelle Gonzalez,  Hanyan Li,  Pan Pan,  Rosa Letizia,  Jinjun Feng, Neville C. Luhmann, Claudio Paoloni,


https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10383/1038303/Backward-wave-oscillator-for-high-power-generation-at-THz-frequencies/10.1117/12.2273256.short



The progress in microfabrication techniques and three dimensional electromagnetic simulations have enabled the fabrication of vacuum electron devices up to 1 THz. In particular, the backward wave oscillator is a compact and powerful THz vacuum source, based on the transfer of energy from an electron beam to an electromagnetic (EM) wave propagating in a slow wave structure. The paper reports the design and fabrication challenges to realize a near-THz Backward Wave Oscillator for plasma diagnostics in nuclear fusion. In particular, the beam optics and confinement as well as the slow wave structure are described. Manufacturing as well as device implementation and demonstration considerations are discussed. The double corrugated waveguide is used with a cylindrical electron beam producing an output power on the order of 1 W.

© (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Abstract-Detection of incoherent broadband terahertz light using antenna-coupled high-electron-mobility field-effect transistors

Hua Qin, Xiang Li, Jiandong Sun, Zhipeng Zhang, Yunfei Sun, Yao Yu, Xingxin Li, Muchang Luo

(Submitted on 9 Mar 2017)

http://search.arxiv.org:8081/paper.jsp?r=1703.03128&qid=1494538993967ler_nCnN_318394178&qs=terahertz

The sensitivity of direct terahertz detectors based on self-mixing of terahertz electromagnetic wave in field-effect transistors is being improved with noise-equivalent power close to that of Schottky-barrier-diode detectors. Here we report such detectors based on AlGaN/GaN two-dimensional electron gas at 77~K are able to sense broadband and incoherent terahertz radiation. The measured photocurrent as a function of the gate voltage agrees well with the self-mixing model and the spectral response is mainly determined by the antenna. A Fourier-transform spectrometer equipped with detectors designed for 340, 650 and 900~GHz bands allows for terahertz spectroscopy in a frequency range from 0.1 to 2.0~THz. The 900~GHz detector at 77~K offers an optical sensitivity about $1~\mathrm{pW/\sqrt{Hz}}$ being comparable to a commercial silicon bolometer at 4.2~K. By further improving the sensitivity, room-temperature detectors would find applications in active/passive terahertz imaging and terahertz spectroscopy.

Abstract-Room-temperature, low-impedance and high-sensitivity terahertz direct detector based on bilayer graphene field-effect transistor

• Hua Qin^1,, 
• Jiandong Sun¹, 
• Shixiong Liang², 
• Xiang Li^1, 3, 
• Xinxin Yang^1, †, 
• Zehao He², 
• Cui Yu²,
• Zhihong Feng^2, , 

• ¹ Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics (SINANO), Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou 215123, PR China
• ² National Key Laboratory of ASIC, Hebei Semiconductor Research Institute, Shijiazhuang, Hebei 050051, PR China
• ³ School of Nano Technology and Nano Bionics, University of Science and Technology of China, Suzhou 215123, PR China
• 
  

http://www.sciencedirect.com/science/article/pii/S0008622317301653

We report a room-temperature, low-impedance and high-sensitivity terahertz direct terahertz detector based on bilayer graphene field-effect transistor (GFET). Epitaxially grown on silicon carbide, the bilayer graphene had an as-grown carrier mobility of 3000 cm²/Vs. The source/drain contacts were formed on the freshly cleaned graphene sheet to minimize the contact resistance and served also as terahertz antennas. The gate and the dielectric layer underneath with a length of 145 nm and a gap of 203 nm to the source/drain antennas were formed in a self-alignment process. Although the carrier mobility in the GFET was reduced to about 405 cm²/Vs, the high-quality ohmic contacts and the short graphene channel delivered an overall source-drain resistance less than 203 Ω. A voltage responsivity of 30 V/W and a noise-equivalent power of 51 pW/Hz^1/2were estimated in direct detection at 0.33 THz. By using the GFET detector as a two-terminal detector, i.e., with the gate floating, a transmission-type terahertz imaging was demonstrated. Such a low-impedance GFET detector is ready to be matched to a commercial 50-Ω low-noise radio/microwave frequency amplifier allowing for high-speed homodyne detection and heterodyne detection in a quasi-optical configuration.

Abstract-Terahertz Wide-Angle Imaging and Analysis on Plane-wave Criteria Based on Inverse Synthetic Aperture Techniques

Jing Kun Gao, Yu Liang Qin, Bin Deng, Hong Qiang Wang, Jin Li, Xiang Li,

http://link.springer.com/article/10.1007/s10762-016-0249-x

This paper presents two parts of work around terahertz imaging applications. The first part aims at solving the problems occurred with the increasing of the rotation angle. To compensate for the nonlinearity of terahertz radar systems, a calibration signal acquired from a bright target is always used. Generally, this compensation inserts an extra linear phase term in the intermediate frequency (IF) echo signal which is not expected in large-rotation angle imaging applications. We carried out a detailed theoretical analysis on this problem, and a minimum entropy criterion was employed to estimate and compensate for the linear-phase errors. In the second part, the effects of spherical wave on terahertz inverse synthetic aperture imaging are analyzed. Analytic criteria of plane-wave approximation were derived in the cases of different rotation angles. Experimental results of corner reflectors and an aircraft model based on a 330-GHz linear frequency-modulated continuous wave (LFMCW) radar system validated the necessity and effectiveness of the proposed compensation. By comparing the experimental images obtained under plane-wave assumption and spherical-wave correction, it also showed to be highly consistent with the analytic criteria we derived.