Abstract-All-dielectric metamaterial analogue of electromagnetically induced transparency and its sensing application in terahertz range

Tian Ma, Qiuping Huang, Hongchuan He, Yi Zhao, XIaoxia Lin, and Yalin Lu

Fig. 2 (a) Schematic of the all-dielectric metamaterial composed of two asymmetric split ring resonators. Inset: top view of the unit cell. All dimensions shown here are $h=100\mu m,$ $t=30\mu m,$ $P_x=400\mu m,$ $P_y=400\mu m,$ $R_1=75\mu m,$ $R_2=75\mu m,$ $\alpha =160°,$ and $\beta =120°.$ (b) Microscopy of the fabricated sample. Bars refer to 500μm. (c) Transmission spectra and (d) corresponding group delay of the proposed metamaterial

http://aoip.osa.org/oe/abstract.cfm?uri=oe-27-12-16624

A novel electromagnetically induced transparency (EIT) all-dielectric metamaterial is proposed, fabricated, and characterized. The unit cell of the proposed metamaterial comprises of two asymmetric split ring resonators (a-SRRs) positioned with a mirror symmetry. The asymmetric nature of a-SRRs results from the length difference of two arcs. Optical properties of the fabricated metamaterial are investigated numerically using finite difference method, as well as experimentally using a terahertz time-domain spectroscopy. The results confirm that the proposed metamaterial exhibits an EIT transparent window in the frequency range around 0.78THz with a Q-factor of ~75.7 and a time-delay up to ~28.9ps. Theoretical investigations show that EIT effects in our metamaterial are achieved by hybridizing two bright modes in the same unit cell, which are aroused by the excitation of magnetic moments. We also confirm that the proposed metamaterial has great potential for sensing applications with high sensitivity and high figure of merit (FOM), which guarantees potential applications in in situ chemical and biological sensing.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-High power THz coherent Cherenkov radiation based on a separated dielectric loaded waveguide

Shimin Jiang, Weiwei Li, Zhigang He, Ruixuan Huang, Qika Jia, Lin Wang, Yalin Lu,



https://www.sciencedirect.com/science/article/pii/S0168900219301202

In this paper, we propose a new type of dielectric loaded waveguide structure named separated dielectric loaded waveguide (SDLW), whose dielectric layer and metal layer are separated with each other. The characters of wakefield inside SDLW are studied in details by theory analysis and numerical simulation. Compared with the ordinary dielectric loaded waveguide, the peak power of Terahertz coherent Cherenkov radiation (CCR) excited by the short relativistic electron bunch can be enhanced by over one order. Therefore, this new structure offers a promising candidate for high power THz source

Abstract-Design of a Pre-Bunched THz Free Electron Laser

Ruixuan Huang, Weiwei Li,  Zhouyu Zhao, Heting Li, Jigang Wang, Tian Ma, Qiuping Huang, Zhigang He, Qika Jia, Lin Wang, Yalin Lu

https://www.mdpi.com/2571-712X/1/1/21/htm

Terahertz (THz) radiation has attracted much attention in new scientific and industrial applications. There has been significant recent progress in generating THz with accelerators. To investigate the collective behavior of electron dynamics, we have proposed a new high throughput material characterization system, which supplies a multiple light source. The system includes a pre-bunched THz free electron laser (FEL), which is a high-power narrow-band THz source with a wide tuning range of frequency. The physical design with the main components of the facility is introduced, and the simulation results are illustrated. Radiation of 0.5–3.0 THz is obtained by the fundamental wave of the pre-bunched beam, and radiation covering 3.0–5.0 THz is realized by second harmonic generation. As the simulation shows, intense THz radiation could be achieved in a frequency from 0.5–5.0 THz, with a peak power of several megawatts (MWs) and a bandwidth of a few percent.

Abstract-A broadband and switchable VO2-based perfect absorber at the THz frequency

Yi Zhao, Qiuping Huang, Honglei Cai, Xiaoxia Lin, Yalin Lu,



https://www.sciencedirect.com/science/article/abs/pii/S0030401818304772

We demonstrate a broadband and switchable THz metamaterial absorber by utilizing the phase transition of VO2, which is a stacked structure composed of VO2 periodic array, dielectric layer, VO${}_2$ film, Au periodic array, dielectric layer and Au reflective layer, respectively. The absorption band from 0.76 THz to 0.86 THz at room temperature can be changed into the absorption band from 1.12 THz to 1.25 THz when temperature increases above the phase change temperature of VO2, with the absorptivity in both bands over 90%. Furthermore, the effective medium theory is introduced to explain the perfect absorption mechanism. Such absorber can work well over a wide range of incidence around 40°. Thickness of the absorber is only about one twentieth of the working wavelength. The proposed structure can be applied to absorbers working at other frequencies.

Abstract-Optical tuning of dielectric properties of La0.7Sr0.3MnO3/SrTiO3 superlattices in the terahertz range

Honglei Cai, Haoliang Huang, Qiuping Huang, Xiang Hu, Jie Zhang, Xiaofang Zhai,  Yalin Lu

Fig. 2 (a) Schematic diagram of the home-made THz-TDS system. The green arrow indicates the excitation under 532 nm continuous waves. (b) Schematic diagram of the home-made OPTP system. Here, λ/4 and W.P. refer to a quarter-wave plate and a Wollaston prism, respectively.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-26-6-7842&origin=search

Two (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices with different superlattice period but the same total thickness were deposited on LaAlO3 substrates by pulsed laser deposition. Dielectric properties of these samples were investigated by means of terahertz time-domain spectroscopy (THz-TDS) under external continuous wave green laser excitation and optical-pump terahertz-probe spectroscopy (OPTP) at room temperature. Experimental results show that the real part of the permittivity for both superlattices increases significantly with increasing green laser pump power, which indicates the decrease of the plasma frequency, along with the increase of the electron scattering rate, soft mode eigenfrequency and oscillator strength in the Drude-Lorentz model. Furthermore, it’s observed that the insulating superlattice exhibits a more significant dielectric tunability than the metallic superlattice. Besides, the carrier lifetime of superlattices is much shorter than the La0.7Sr0.3MnO3 thin film in the OPTP measurements, indicating that the electrons excited in the La0.7Sr0.3MnO3 layers may be trapped by the defects located in the interfaces of La0.7Sr0.3MnO3 and SrTiO3 or the SrTiO3 layers. With the optical field-induced tunability of dielectric properties, (La0.7Sr0.3MnO3)n/(SrTiO3)m superlattices show great potential in the actively tunable devices in the THz range.

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

Abstract-Multicolor Terahertz Frequency Mixer Using Multibunching of Free-Electron Beams

Weihao Liu, Linbo Liang, Qika Jia, Lin Wang, and Yalin Lu

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.10.034031

We propose a free-electron-beam (FEB)-driven frequency mixer for generating terahertz-wave radiation. It employs an initial steady-current FEB to drive two cascaded gratings. On the first grating, the FEB is macrobunched by interacting with the self-stimulated backward slow waves. On the second grating, which operates at high harmonics of the first grating, it is further microbunched at high frequencies, producing mixed bunching components within the FEB. The multibunched FEB then generates a series of superradiant Smith-Purcell radiations from the second grating, achieving multicolor radiations in the terahertz region. The radiation can be tuned by adjusting the beam energy, covering the frequency range from 0.8 to 1.8 THz. It is a promising broad-tunable and multicolor terahertz source.

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Abstract-Multifunctional Hybrid Metasurfaces for Dynamic Tuning of Terahertz Waves

Honglei Cai,  Shi Chen,   Chongwen Zou,   Qiuping Huang,   Yu Liu,  Xiang Hu,   Zhengping Fu,  Yi Zhao,  Hongchuan He,   Yalin Lu,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201800257


Actively tuning optical transmission through hybrid metasurfaces incorporated with multifunctional active media holds great promise for the next generation optical devices. In the terahertz (THz) range, they remain rare due to the lack of dynamic and multifunctional designs and materials. Here, a vanadium dioxide (VO2)‐based hybrid metasurface is proposed to present multifunctional control of THz waves via electrically triggering and ultrafast optical excitation. By minimizing the thermal mass of VO2 and optimizing the VO2 patterns within two side gaps of the asymmetric split‐ring resonators, a hybrid metasurface which can tune the THz wave with an absolute modulation depth up to 54% and a figure of merit as high as 138% is hereby presented. The hybrid metasurface achieves a switching time of 2.2 s under the electrically triggering and offers an ultrafast modulation within 30 ps under the femtosecond pulse excitation. More interestingly, owing to the intrinsic hysteresis behavior of VO2, the hybrid metasurface exhibits distinguishing multistate transmission amplitudes with a single electrical input. In short, this study paves the way for robust multifunctionality in electric‐controlled terahertz switching, photonic memory, and ultrafast terahertz optics.

Abstract-High-harmonic terahertz Smith-Purcell free-electron-laser with two tandem cylindrical-gratings

Linbo Liang, Weihao Liu, Qika Jia, Lin Wang, and Yalin Lu

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-3-2960

A modified Smith-Purcell free-electron-laser based on two tandem cylindrical-gratings is proposed. The preset grating with larger size, operating in the slow-wave condition, is to prebunch the initial continuous electron-beam, and the postpositive grating with smaller size, operating in the fast-wave condition, is used as the main radiator. Compared with traditional Smith-Purcell free-electron-lasers operating at the second harmonic of the bunched-beam, the present scheme operates at much higher harmonics, fifth and sixth harmonics have been achieved, and the radiation frequency is greatly increased consequently. And also the radiation power is enhanced by tens of times. Thus it could be developed as an efficient terahertz source with frequency being over 0.5 THz in practice.

© 2017 Optical Society of America

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Abstract-Harmonics generation of a terahertz wakefield free-electron laser from a dielectric loaded waveguide excited by a direct current electron beam

Weiwei Li, Yalin Lu, Zhigang He, Qika Jia, and Lin Wang
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-41-11-2458

We propose to generate high-power terahertz (THz) radiation from a cylindrical dielectric loaded waveguide (DLW) excited by a direct-current electron beam with the harmonics generation method. The DLW supports a discrete set of modes that can be excited by an electron beam passing through the structure. The interaction of these modes with the co-propagating electron beam results in micro-bunching and the coherent enhancement of the wakefield radiation, which is dominated by the fundamental mode. By properly choosing the parameters of DLW and beam energy, the high order modes can be the harmonics of the fundamental one; thus, high frequency radiation corresponding to the high order modes will benefit from the dominating bunching process at the fundamental eigenfrequency and can also be coherently excited. With the proposed method, high power THz radiation can be obtained with an easily achievable electron beam and a large DLW structure.

© 2016 Optical Society of America

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Abstract-A multimode terahertz-Orotron with the special Smith–Purcell radiation

Weihao Liu1,a), Yalin Lu1, Lin Wang1 and Qika Jia1

1 National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, China

a) Electronic mail: liuwhao@ustc.edu.cn

Appl. Phys. Lett. 108, 183510 (2016); http://dx.doi.org/10.1063/1.4949015

http://scitation.aip.org/content/aip/journal/apl/108/18/10.1063/1.4949015

We proposed and investigated a terahertz Orotron, which is based on the recently revealed special Smith–Purcell radiation. It overcomes the main obstacles of the traditional Orotron in the terahertz region—unreachable high starting-current-density and low radiation power. With the experimentally available electron beam and facilities, its average output power can reach hundreds of milliwatts and even several watts in terahertz region, which is many orders of magnitude higher than that of the traditional Orotron. Additionally, it can be controlled to operate in ether the first or the second order mode, and the radiation frequency can extend from 0.1 THz to 1 THz. These remarkable advantages make it a promising terahertz source for practical applications.

Abstract-Broad-tunable terahertz source with over-mode waveguide driven by train of electron bunches

Weihao Liu, Yalin Lu, Zhigang He, Weiwei Li, Lin Wang, and Qika Jia

A broad-tunable free electron terahertz radiation source is proposed. In this source, a train of electron bunches with tunable bunching frequency is produced by a photocathode based DC-gun under excitation of a train of tunable laser pulses. These electron bunches are then applied to excite an over-mode waveguide, in which diverse guided modes are coupled into radiation with frequency determined by the bunching frequency. By this means, the tunable radiation with frequency extending from 0.1 THz to 1.2 THz can be obtained from one single structure model. In addition, compared with other sources, the proposed source is compact and easily achievable.

© 2016 Optical Society of America

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