Abstract-Subdiffraction focusing of total electric fields of terahertz wave

Mengyu Yang, Desheng Ruan, Lianghui Du, Chunyan Qin, Zeyu Li, Cuiping Lin, Gang Chen, Zhong Quan Wen,



https://www.sciencedirect.com/science/article/abs/pii/S0030401819309307

Terahertz lens is an essential component in terahertz application. We propose a focusing of total electric fields planar lens based on super-oscillation. This planar lens is designed for a wavelength ($\lambda$) of 118.$8\mathrm{\mu }\mathrm{m}$ with a radius of 160$\lambda$, a focal length of 210 $\lambda$ and a numerical aperture of 0.606. Our experiment demonstrates a subdiffraction and subwavelength focusing of total electric fields. The full width at half-maximum of the focal spot is 0.67 $\lambda$, which is smaller than the diffraction limit of 0.825$\lambda$. The results shows that it has powerful application for terahertz imaging, especially in the fields of biomedical science.

Abstract-Distinctive Performance of Terahertz Photodetection Driven by Charge‐Density‐Wave Order in CVD‐Grown Tantalum Diselenide

Lin Wang,  Jin Wang,  Changlong Liu,   Huang Xu,  Ang Li,  Dacheng Wei,   Yunqi Liu,  Gang Chen,  Xiaoshuang Chen,  Wei Lu,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201905057?af=R

The quantum behavior of carriers in solid is the foundation of modern electronic and optoelectronic technology, but it is still facing huge challenges within inherited single‐particle quantum processes working at the millimeter wave/terahertz (THz) band. Here, a straightforward strategy for the direct detection of millimeter wave/THz photons in a sub‐wavelength metal‐TaSe2‐metal structure under strong interaction with a localized field of surface plasmon is proposed. By breaking the inversion symmetry under the perturbations of electric field and atomic reconstruction from van der Waals integration, the nonequilibrium electronic states under a radiant field can be manipulated in a collective fashion, leading to a large photocurrent responsivity over 40 A W−1 and noise equivalent power less than 1 pW Hz−1/2 even at room temperature. A more than 40‐fold enhancement in responsivity is achieved when transitioning from the normal phase to the CDW phase. The findings shed fresh light on the understanding of the delicate balance in the charge‐ordered phase, and facilitate the exploitation of a correlated electron system for optoelectronic applications in fields of security, remote sensing, and imaging.

Abstract-The Fabrication of Large-Area, Uniform Graphene Nanomeshes for High-Speed, Room-Temperature Direct Terahertz Detection

Weiqing Yuan, Min Li, Zhongquan Wen, Yanling Sun, Desheng Ruan, Zhihai Zhang, Gang Chen, Yang Gao, 

• https://link.springer.com/article/10.1186/s11671-018-2602-6

In recent years, graphene nanomesh (GNM), a material with high flexibility and tunable electronic properties, has attracted considerable attention from researchers due to its wide applications in the fields of nanoscience and nanotechnology. Herein, we have processed large-area, uniform arrays of rectangular graphene nanomesh (r-GNM) and circular graphene nanomesh (c-GNM) with different neck widths by electron beam lithography (EBL). The electronic properties of those high-quality GNM samples have been characterized systematically. Electrical measurements illustrated that top-gated field effect transistors with different neck widths of the GNM possessed different Ion/Ioff ratios. In particular, the devices based on r-GNM with a neck width of 30 nm were found to possess the largest Ion/Ioff ratio of ~ 100, and the band gap of the r-GNM was estimated to be 0.23 eV, which, to the best of authors’ knowledge, is the highest value for graphene ribbons or a GNM with a neck width under 30 nm. Furthermore, the terahertz response of large-area r-GNM devices based on the photoconductive effect was estimated to be 10 mA/W at room temperature. We also explored the practical application of terahertz imaging, showing that the devices can be used in a feasible setting with a response time < 20 ms; this enables accurate and fast imaging of macroscopic samples.

Abstract-All-dielectric metalens for terahertz wave imaging

Xue Jiang, Hao Chen, Zeyu Li, Hongkuan Yuan, Luyao Cao, Zhenfei Luo, Kun Zhang, Zhihai Zhang, Zhongquan Wen, Li-guo Zhu, Xun Zhou, Gaofeng Liang, Desheng Ruan, Lianghui Du, Lingfang Wang, Gang Chen,

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-11-14132

Terahertz wave imaging offers promising properties for non-destructive testing applications in the areas of homeland security, medicine, and industrial inspection. However, conventional optical lenses are heavy and bulky and difficult to integrate. An all-dielectric metasurface provides an attractive way to realize a planar lens of light weight that is ultrathin and offers ease of integration. Terahertz lenses based on various metasurfaces have been studied, especially for the application of wave focusing, while there are few experimental demonstrations of terahertz wave imaging lenses based on an all-dielectric metasurface. In the present work, we propose a metalens based on an all-dielectric metasurface with a sub-wavelength unit size of 0.39λ for terahertz wave imaging and experimentally demonstrate its performance in focusing and imaging. A large numerical aperture metalens was fabricated with a focal length of 300λ, radius of 300λ, and numerical aperture of 0.707. The experimental results show that the lens can focus THz waves with an incident angle up to 48°. More importantly, clear terahertz wave images of different objects were obtained for both different cases of forward- and inverse-incident directions, which demonstrate the reversibility of the metalens for imaging. Such a metalens provides a way for realization of all-planar-lens THz imaging system, and might find application in terahertz wave imaging, information processing, microscopy, and others.

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

Abstract-Towards sensitive terahertz detection via thermoelectric manipulation using graphene transistors

Changlong Liu, Lei Du, Weiwei Tang, Dacheng Wei, Jinhua Li, Lin Wang, Gang Chen, Xiaoshuang Chen,  Wei Lu

https://www.nature.com/articles/s41427-018-0032-7

Graphene has been highly sought after as a potential candidate for hot-electron terahertz (THz) detection benefiting from its strong photon absorption, fast carrier relaxation, and weak electron-phonon coupling. Nevertheless, to date, graphene-based thermoelectric THz photodetection is hindered by low responsivity owing to relatively low photoelectric efficiency. In this work, we provide a straightforward strategy for enhanced THz detection based on antenna-coupled CVD graphene transistors with the introduction of symmetric paired fingers. This design enables switchable photodetection modes by controlling the interaction between the THz field and free hot carriers in the graphene-channel through different contacting configurations. Hence a novel “bias-field effect” can be activated, which leads to a drastic enhancement in THz detection ability with maximum responsivity of up to 280 V/W at 0.12 THz relative to the antenna area and a Johnson-noise limited minimum noise-equivalent power (NEP) of 100 pW/Hz0.5at room temperature. The mechanism responsible for the enhancement in the photoelectric gain is attributed to thermophotovoltaic instead of plasma self-mixing effects. Our results offer a promising alternative route toward scalable, wafer-level production of high-performance graphene detectors.

Abstract-Photo-excited charge carriers suppress sub-terahertz phonon mode in silicon at room temperature

• Bolin Liao
• , A. A. Maznev
• , Keith A. Nelson
•  & Gang Chen

http://www.nature.com/articles/ncomms13174

There is a growing interest in the mode-by-mode understanding of electron and phonon transport for improving energy conversion technologies, such as thermoelectrics and photovoltaics. Whereas remarkable progress has been made in probing phonon–phonon interactions, it has been a challenge to directly measure electron–phonon interactions at the single-mode level, especially their effect on phonon transport above cryogenic temperatures. Here we use three-pulse photoacoustic spectroscopy to investigate the damping of a single sub-terahertz coherent phonon mode by free charge carriers in silicon at room temperature. Building on conventional pump–probe photoacoustic spectroscopy, we introduce an additional laser pulse to optically generate charge carriers, and carefully design temporal sequence of the three pulses to unambiguously quantify the scattering rate of a single-phonon mode due to the electron–phonon interaction. Our results confirm predictions from first-principles simulations and indicate the importance of the often-neglected effect of electron–phonon interaction on phonon transport in doped semiconductors.

Terahertz Imaging System Uses BWO As Source

http://mwrf.com/test-amp-measurement/terahertz-imaging-system-uses-bwo-source

An imaging system based on transmission and reflection modes in the terahertz region has been developed by using a backward-wave oscillator (BWO) as its source, a Golay-Cell as the detector, and an oscilloscope as a data acquisition unit.

Much research has shown that terahertz waves can penetrate a number of materials while generating images with high spatial resolution. A number of these terahertz imaging solutions rely on continuous-wave radiation sources like a backward-wave oscillator (BWO). BWOs offer high output power, good wave-front quality, working-wavelength tunability, and a high signal-to-noise ratio. At China’s Southeast University, a continuous-wave (CW) terahertz imaging system using a BWO as source, a Golay-Cell as a detector, and an oscilloscope as a data-acquisition unit has been developed by Gang Chen, Jie Pei, Fei Yang, Xiao Yang Zhou, Z.L. Sun, and Tie Jun Cui.

The system’s software, which is based on the oscilloscope, is designed to control object movement as well as the capture and display of continuous terahertz-wave image data. To show the system’s validity at room temperature, the team tested the imaging of different objects at 450 and 890 GHz. The system was affected by humidity, thickness, and material properties. In addition, imaging resolution was discovered to be better as incident frequency increased. The translation step also impacted imaging, showing that the appropriate frequency and translation step must be chosen to meet practical imaging requirements. See “Terahertz-Wave Imaging System Based On Backward Wave Oscillator,” IEEE Transactions On Terahertz Science And Technology, Sept. 2012, p. 504.