Abstract-High power Continuously Frequency-tunable Terahertz Radiation Sources and Transmission Lines for DNP-enhanced NMR System

Diwei Liu, Tao Song,  Hao Shen,  Jie Huang, Ning Zhang, Chenghai Wang, Wei Wang, Shenggang Liu

https://ieeexplore.ieee.org/document/8510299

The effect of the electron beam quality of a 250GHz continuously frequency-tunable gyrotron used for Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance is tuned by changing the operating voltage Vo or the operating magnetic field Bo on the operating frequency and the beam-wave interaction efficiency is investigated. Meanwhile, an improved transmission and mirror system with a well-focused Gaussian-like output beam is designed to match the DNP-NMR sample.

Abstract-Electronic-spintronic terahertz emitter

Chengpeng Yu and Shenggang Liu

https://journals.aps.org/prapplied/accepted/8e076OfbK2412f3ea38e6805b1e14fbed85327339

We present a novel terahertz (THz) radiation source combining the principles of spintronics and vacuum electronics: Electronic-Spintronic THz Emitter (E-STE). Different from the traditional laser driven Spintronic THz Emitters, E-STE is driven by low energy electron beams. Working through four steps: the electron beam excitation of hot electrons, the electron super-diffusion, the Inverse Spin Hall Effect, and the diffraction radiation of the periodic gratings, E-STE is able to emit continuous, monochromatic THz radiation. Detailed theoretical modeling and numerical calculation show, E-STE can cover the whole THz frequency regime (0.1-10THz) by modifying the period of the gratings. The electron beam voltage and current are relatively low (voltage U=5V, 25V, peak current I~100mA), while the radiation power density and working efficiency are relatively high (The power density can reach 108W/cm2, the working efficiency can reach 25 high-power E-STE may contribute to the development of THz radiation source integrated on chip.

Abstract-Preliminary experimental study of graphene-based terahertz radiation excited by an electron beam

Xiaodong Feng, Min Hu, Jun Zhou, Seng Gong, Tao Zhao, Renbin Zhong, Shenggang Liu,

https://ieeexplore.ieee.org/document/8391577

There are several proposed graphene-based coherent terahertz radiation sources excited by an electron beam in the terahertz frequency band. The properties of graphene such as graphene conductivity are of great importance to achieve these graphene-based terahertz radiation sources. In this paper, based on terahertz time-domain spectroscopy, the effects of a 400- eV electron beam on graphene conductivity are presented. The results indicate that exposure to a 400-eV electron beam can cause n-doping in the graphene from the interaction of the electron beam with the graphene film. After a long-time electron beam irradiation, the graphene conductivity decreases, indicating that defects and damages are created in the graphene. The findings are valuable for the further experimental study to realize graphene-based terahertz radiation sources excited by an electron beam.

Abstract-The Enhanced Third Harmonic Superradiation of Smith Purcell Terahertz radiation source

 Sen Gong, Tao Zhao, Shenggang Liu, Zhenhua Wu, Pengfei Hu, Min Hu, Yueheng Cao, Xiaoqiuyan 

Zhang

https://ieeexplore.ieee.org/document/8510515

Here we propose a new method to enhance the third harmonic of superradiant Smith-Purcell. In this paper, the specific designed grating structure enhances the power of the third harmonic by suppressing the second harmonic. Therefore, the higher frequency radiation signal (25 mW, 326 GHz) is generated by the lower energetic electron beam (9 keV)passing through the grating with larger size (0.4 mm), compared with normal superradiation by using PIC simulation. Accordingly, this mechanism is expected to achieve more efficient compact and tunable terahertz radiation sources with higher frequency.

Abstract-Preliminary experimental study of graphene-based terahertz radiation excited by an electron beam

Xiaodong Feng,   Min Hu,   Jun Zhou,   Seng Gong, Tao Zhao,   Renbin Zhong,  Shenggang Liu, 

https://ieeexplore.ieee.org/document/8391577/

There are several proposed graphene-based coherent terahertz radiation sources excited by an electron beam in the terahertz frequency band. The properties of graphene such as graphene conductivity are of great importance to achieve these graphene-based terahertz radiation sources. In this paper, based on terahertz time-domain spectroscopy, the effects of a 400- eV electron beam on graphene conductivity are presented. The results indicate that exposure to a 400-eV electron beam can cause n-doping in the graphene from the interaction of the electron beam with the graphene film. After a long-time electron beam irradiation, the graphene conductivity decreases, indicating that defects and damages are created in the graphene. The findings are valuable for the further experimental study to realize graphene-based terahertz radiation sources excited by an electron beam.

Abstract-Terahertz radiation in graphene hyperbolic medium excited by an electric dipole

Xiaodong Feng, Sen Gong, Renbin Zhong, Tao Zhao, Min Hu, Chao Zhang, and Shenggang Liu

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-5-1187#Abstract

In this Letter, the enhanced and directional radiation in a wide terahertz (THz) frequency range in a graphene hyperbolic medium excited by an electric dipole is presented. The numerical simulations and theoretical analyses indicate that the enhanced radiation comes from the strong surface plasmon couplings in the graphene hyperbolic medium, consisting of alternative graphene and dielectric substrate layers. The simulation results also show that the peak power flow of the enhanced THz radiation in the graphene hyperbolic medium is dramatically enhanced by more than 1 order of magnitude over that in a general medium within a certain distance from the dipole, and the electromagnetic fields are strongly concentrated in a narrow angle. Also, the radiation fields can be manipulated, and the fields’ angular distributions can be tuned by adjusting the dielectric permittivity and thickness of the substrates, and the chemical potential of graphene. Accordingly, it provides a good opportunity for developing miniature, integratable, high-power-density, and tunable radiation sources in the THz band at room temperature.

© 2018 Optical Society of America

Abstract-Observation of graphene surface plasmon polaritons excited by free electron beam

Min Hu,  Sen Gong, Tao Zhao,  Renbin Zhong,  Chengpeng Yu,   Shenggang Liu

http://ieeexplore.ieee.org/document/8066948/

In previous work, we propose to develop a novel kind of terahertz radiation source which utilizing free electron beam to excited graphene surface plasmon polaritons (GSPPs) combining vacuum electronics and new 2D material graphene. The previous theoretical analysis show that this coherent radiation source can be tuned covering almost whole THz frequency range and work at room temperature. In this paper, we demonstrate the conductivity mapping of GSPPs with free electron beam excitation using TDS system. The characteristics of GSPPs can be derived from the calculated conductivity results and be compared with the theoretical Drude model, which would guide the next step of the experiment to realize the new kind of THz source.

Abstract-Graphene loaded double ridge plasmon Terahertz waveguide

https://link.springer.com/article/10.1140%2Fepjd%2Fe2017-70223-y

In this paper, a single-mode graphene loaded double ridge plasmon waveguide (GDRW) with long propagation length and strong mode confinement is proposed. The characteristics of the guided modes are investigated in detail, and tunable single-mode transmission with good performance can be realized either by changing the Fermi energy level or by optimizing the key structural parameters. High figures of merit and much lower crosstalk are obtained due to the dramatically suppressed interference between two parallel placed GDRWs, enabling thereby more tightly stacking in terahertz integrated circuits. Further investigation on fabrication errors, such as the horizontal misalignment of the two symmetric ridges, fabrication distortion of rectangular ridges, and variation of ridge tip curvature radius, indicates that the proposed structure has enough fabrication error tolerance. The results are greatly expected to facilitate the application of high-density terahertz integrated circuits.

Abstract-Calculation and Study of Graphene Conductivity Based on Terahertz Spectroscopy

Xiaodong Feng, Min Hu, Jun Zhou, Shenggang Liu

http://link.springer.com/article/10.1007/s10762-017-0362-5


Based on terahertz time-domain spectroscopy system and two-dimensional scanning control system, terahertz transmission and reflection intensity mapping images on a graphene film are obtained, respectively. Then, graphene conductivity mapping images in the frequency range 0.5 to 2.5 THz are acquired according to the calculation formula. The conductivity of graphene at some typical regions is fitted by Drude-Smith formula to quantitatively compare the transmission and reflection measurements. The results show that terahertz reflection spectroscopy has a higher signal-to-noise ratio with less interference of impurities on the back of substrates. The effect of a red laser excitation on the graphene conductivity by terahertz time-domain transmission spectroscopy is also studied. The results show that the graphene conductivity in the excitation region is enhanced while that in the adjacent area is weakened which indicates carriers transport in graphene under laser excitation. This paper can make great contribution to the study on graphene electrical and optical properties in the terahertz regime and help design graphene terahertz devices.

Abstract-Optical bistability induced by nonlinear surface plasmon polaritons in graphene in terahertz regime

Matthew Sanderson¹, Yee Sin Ang¹, Sen Gong^2,3, Tao Zhao^2,3, Min Hu^2,3, Renbin Zhong^2,3, Xiaoxing Chen^2,3, Ping Zhang^2,3, Chao Zhang^1,3,a) and Shenggang Liu^2,3
http://scitation.aip.org/content/aip/journal/apl/107/20/10.1063/1.4936232?TRACK=RSS
a) czhang@uow.edu.au
Appl. Phys. Lett. 107, 203113 (2015); http://dx.doi.org/10.1063/1.4936232

We demonstrate optical bistability in a prism-air-graphene-dielectric structure. Under a moderate electric field in the terahertz frequency regime, the third order nonlinear optical conductivity is comparable to the linear conductivity. The nonlinear conductivity enhances the energy of surface plasmonpolaritons. Both the energy and frequency of the surface plasmonpolaritons depend on the strength of the nonlinear current in the graphene layer. When considering excitation in the Kretschmann configuration, the reflectance as a function of frequency exhibits bistability. The origin of the bistability is the field dependence of the plasmon mode. We have determined the parameter regime for the occurrence of bistability in this structure.

Abstract-Coherent and Tunable Terahertz Radiation from Graphene Surface Plasmon Polarititions Excited by Cyclotron Electron Beam

Tao Zhao

, Sen Gong

, Min Hu

, Renbin Zhong

, Diwei Liu

, Xiaoxing Chen

, Ping Zhang

, Xinran Wang

,Chao Zhang

, Peiheng Wu

 & Shenggang Liu

http://www.nature.com/articles/srep16059

Terahertz (THz) radiation can revolutionize modern science and technology. To this date, it remains big challenges to develop intense, coherent and tunable THz radiation sources that can cover the whole THz frequency region either by means of only electronics (both vacuum electronics and semiconductor electronics) or of only photonics (lasers, for example, quantum cascade laser). Here we present a mechanism which can overcome these difficulties in THz radiation generation. Due to the natural periodicity of 2π of both the circular cylindrical graphene structure and cyclotron electron beam (CEB), the surface plasmon polaritions (SPPs) dispersion can cross the light line of dielectric, making transformation of SPPs into radiation immediately possible. The dual natural periodicity also brings significant excellences to the excitation and the transformation. The fundamental and hybrid SPPs modes can be excited and transformed into radiation. The excited SPPs propagate along the cyclotron trajectory together with the beam and gain energy from the beam continuously. The radiation density is enhanced over 300 times, up to 105 W/cm2. The radiation frequency can be widely tuned by adjusting the beam energy or chemical potential. This mechanism opens a way for developing desired THz radiation sources to cover the whole THz frequency regime.

Abstract-Gbps terahertz external modulator based on a composite metamaterial with a double-channel heterostructure

Yaxin Zhang , Shen Qiao , Shixiong Liang , Zhenhua Wu , Ziqiang Yang , Zhihong Feng , Han Sun , Yucong Zhou , Linlin Sun , Zhi Chen , Xianbing Zou , Bo Zhang , Jianhao Hu , Shaoqian Li ,Qin Chen , Ling Li , Gaiqi Xu , Yuncheng Zhao , and Shenggang Liu

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/acs.nanolett.5b00869

Publication Date (Web): April 28, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00869

The past few decades have witnessed a substantial increase in terahertz (THz) research. Utilizing THz waves to transmit communication and imaging data has created a high demand for phase and amplitude modulation. However, current active THz devices, including modulators and switches, still cannot meet THz system demands. Double-channel heterostructures, an alternative semiconductor system, can support nano-scale two-dimensional electron gases (2DEGs) with high carrier concentration and mobility and provide a new way to develop active THz devices. In this article, we present a composite metamaterial structure that combines an equivalent collective dipolar array with a double-channel heterostructure to obtain an effective, ultra-fast and all-electronic grid-controlled THz modulator. Electrical control allows for resonant mode conversion between two different dipolar resonances in the active device, which significantly improves the modulation speed and depth. This THz modulator is the first to achieve a 1-GHz modulation speed and 85% modulation depth during real-time dynamic tests. Moreover, a 1.19-rad phase shift was realized. A wireless free-space-modulation THz communication system based on this external THz modulator was tested using 0.2-Gbps eye patterns. Therefore, this active composite metamaterial modulator provides a basis for the development of effective and ultra-fast dynamic devices for THz wireless communication and imaging systems.

Coherent and tunable terahertz radiation from graphene surface plasmon polaritons excited by an electron beam

Shenggang Liu^1,2,a), Chao Zhang^1,2,3, Min Hu^1,2, Xiaoxing Chen^1,2, Ping Zhang^1,2, Sen Gong^1,2, Tao Zhao^1,2 and Renbin Zhong^1,2
 HIDE AFFILIATIONS
1 Terahertz Research Centre, School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu, Sichuan 610054, China
2 Cooperative Innovation Centre of Terahertz Science, Chengdu, Sichuan 610054, China
3 School of Physics and Institute for Superconducting and Electronic Materials, University of Wollongong, New South Wales 2522, Australia
a) liusg@uestc.edu.cn
Appl. Phys. Lett. 104, 201104 (2014); http://dx.doi.org/10.1063/1.4879017
http://scitation.aip.org/content/aip/journal/apl/104/20/10.1063/1.4879017

Although surface plasmon polaritons (SPPs) resonance in graphene can be tuned in the terahertz regime, transforming such SPPs into coherent terahertz radiation has not been achieved. Here, we propose a graphene-based coherent terahertz radiation source with greatly enhanced intensity. The radiation works at room temperature, it is tunable and can cover the whole terahertz regime. The radiation intensity generated with this method is 400 times stronger than that from SPPs at a conventional dielectric or semiconducting surface and is comparable to that from the most advanced photonics source such as a quantum cascade laser. The physical mechanism for this strong radiation is presented. The phase diagrams defining the parameters range for the occurrence of radiation is also shown.

Abstract-Free electron terahertz wave radiation source with two-section periodical waveguide structures

http://jap.aip.org/resource/1/japiau/v111/i6/p063107_s1?isAuthorized=no

Weihao Liu, Sen Gong, Yaxin Zhang, Jun Zhou, Ping Zhang, and Shenggang Liu

Terahertz Science and Technology Research Center, University of Electronic Science and Technology of China, Chengdu 610054, China 

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(Received 20 December 2011; accepted 24 February 2012; published online 26 March 2012)

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We analyze a free electron terahertz wave radiation source with two-section periodical waveguide structure (PWS), where the first section (section-I) is used to pre-modulate the electron beam and the second section (section-II) is for terahertz wave generation. By means of theoretical analysis and numerical simulations, we demonstrate that the starting current density of the beam-wave interaction in section-II can be significantly reduced provided that the operation frequency is the harmonic of electron beam’s pre-modulation frequency. This kind of source can generate relatively high power terahertz wave radiation but only need moderate beam current density. And it may have great potential application in developing the compact and high power terahertz wave radiation source.

© 2012 American Institute of Physics