TST, Vol. 13, No. 3, PP. 73-89
(Invited paper) A review of high-efficiency Pancharatnam–Berry metasurfaces
Wang Zhuo, Shulin Sun, Qiong He *, and Lei Zhou *
State Key Laboratory of Surface Physics and Department of Physics,
Fudan University, Shanghai 200438, China
* Email: qionghe@fudan.edu.cn; phzhou@fudan.edu.cn
(Received April 12, 2020)
Abstract: Manipulating circularly polarized (CP) electromagnetic waves as desired is important for a wide range of applications ranging from chiral-molecule manipulations to optical communication, but conventional natural-materials-based devices suffer from bulky configuration and low efficiencies. Recently, Pancharatnam–Berry (PB) metasurfaces have demonstrated strong capabilities to control CP waves in different frequency domains. In this article, we present a concise review on PB metasurfaces for CP light manipulations, focusing mainly on the research works done by our own group. After briefly introducing the working principles of PB metasurfaces, we separately discuss how to construct high-efficiency PB metasurfaces in reflection and transmission geometries, and how to utilize them to control CP waves in different frequency domains, including meta-lensing, meta-hologram, and surface couplers. Finally, we conclude this review with our perspectives on future developments of PB metasurfaces.
Keywords: PB metasurfaces, Geometric phase, Circular polarized light, High-efficiency, Phase gradient
doi:
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TST, Vol. 13, No. 3, PP. 90-111
(Invited paper) The Development of broadband millimeter-wave
and terahertz gyro-TWA
W. He 1*, L. Zhang 2, C. R. Donaldson 2, H. Yin 2, K. Ronald 2, A. W. Cross 2 and A. D. R. Phelps 2
1 College of Electronics and Information Engineering, Shenzhen University, Shenzhen, China
2 SUPA, Department of Physics, University of Strathclyde, Glasgow, G4 0NG, Scotland, UK
*1 Email: wenlong.he@szu.edu.cn
(Received 02 March 2020)
Abstract: The gyrotron travelling wave tube amplifiers (gyro-TWAs) presented in this paper can operate with high efficiency (30%), huge powers and wide bandwidths at high frequencies that no other amplifier can provide. In principle, this is a technology that can be scaled to >1 THz and operate with 20% bandwidths. Resonant coupling of two dispersive waveguide modes in a helically corrugated interaction region (HCIR) can give rise to a non-dispersive eigenwave over a wide frequency band. The synchronism between the ideal wave and an electron cyclotron mode, either fundamental or harmonic, of a large orbit electron beam contributes to the broadband amplification. An electron beam of 55 keV, 1.5 A with a velocity pitch angle of ~1 generated by a thermionic cusp gun is used in our 100 GHz gyro-TWA experiment, which achieves an unsaturated output power of 3.4 kW and gain of 36–38 dB. The design and experimental results of the many components making the gyro-TWA will be presented individually and then the whole system will be introduced. The amplification of a swept signal by the W-band gyro-TWA is demonstrated showing its capabilities in the field of telecommunications. Furthermore, the design studies of a cusp electron gun in the triode configuration and the realization of a 3-fold HCIR operating at 372 GHz will also be displayed.
Keywords: Gyro-TWA, Corrugated waveguide, Helically corrugated interaction region, Broadband amplification, Gyro-devices.
doi:
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TST, Vol. 13, No. 3, PP. 112-118
(Invited paper) Terahertz photoconductive antenna with all-dielectric nanopillars
Kemeng Wang 1, Jianqiang Gu 1*, Wenqiao Shi 1, Youwen An 1 and Weili Zhang 2
1 Center for Terahertz Waves and College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300072, China
2 School of Electrical and Computer Engineering, Oklahoma State University, Stillwater, OK 74078
*1 Email: gjq@tju.edu.cn
(Received 28 September 2020)
Abstract: Photoconductive antennas (PCAs), as a popular terahertz (THz) radiation source, have been widely used in spectroscopy, material characterization, biological imaging and detection of hazardous materials. However, PCAs have a relatively low energy conversion efficiency from femtosecond laser pulses to THz radiation which often limits the signal-to-noise ratio and bandwidth of THz imaging and spectroscopy systems. To address these limitations, here we report a THz photoconductive antenna emitter with all-dielectric nanopillars integrated on top of the SI-GaAs substrate to increase the generated photocarriers, which achieves a broadband and frequency insensitive THz power enhancement factor around 1.25 at frequencies 0.05 - 1.6 THz. Our results reported here provide a new method for increasing the THz power of PCAs, which paves the way for the subsequent researches of next-generation PCAs.
Keywords: Terahertz, Nanopillars, Photoconductive antenna, All-dielectric metamaterials
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