TST, Vol. 7, No. 3, PP. 108-127
(Invited paper) Efficient THz generation by femtosecond laser pulses and using THz radiation for plasma diagnostics
Andrey N. Stepanov 1*, Sergey B. Bodrov 1, 3, Igor E. Ilyakov 1, Aleksey I. Korytin 1, Nickolay L. Aleksandrov 2, and Nickolay V. Vvedenskii 1, 3
1 Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod 603950, Russia
2 Moscow Institute of Physics and Technology, Dolgoprudny 141700, Russia
3 University of Nizhny Novgorod, Nizhny Novgorod 603950, Russia
*1 Email: step@ufp.appl.sci-nnov.ru
(Received January 11, 2014）
Abstract: Optical-to-terahertz conversion of femtosecond laser radiation by optical rectification in the LiNbO3 crystal and two-color gas ionization in air is discussed. It is shown that a high energy conversion efficiency (~0.25%) is achieved by Čerenkov radiation of line-focused femtosecond pulses propagated in a sandwich structure with a thin LiNbO3 core covered with a Si prism. The effect of different substrates of the sandwich structure as well as laser pulse duration on the generation efficiency and terahertz spectrum is explored. The tilted pulse front technique for phase-matched THz generation using room-temperature and cryogenically cooled LiNbO3 is studied under conditions of weak and strong laser pump for different crystal lengths and pump pulse durations. It is shown that the optical-to-terahertz conversion efficiency saturates as a result of the Kerr self-phase modulation of the optical pump. Low-frequency THz generation by two-color femtosecond laser pulse ionization of the ambient air is studied both experimentally and theoretically in the case of the main field at the fundamental and a weak additional field tunable near the half-harmonic frequency. The dependences of the THz yield and polarization on the parameters of the two-color pulse are determined. The analytical formulas obtained using the model of the free-electron residual current density give an excellent agreement with the experimental results. For demonstration of the THz radiation application, we use scattering of terahertz radiation to measure the air plasma density decay in a filament produced by an intense femtosecond laser pulse in air. It is shown that the plasma density decreases by about two orders of magnitude within 2 ns after the ionization and the rate of plasma decay can be decreased by applying an additional high (several kV/cm) external electric field. Numerical simulation of the electron density decay and electron temperature evolution is performed taking into consideration dissociative and three-body electron-ion recombinations as well as the formation of complex positive ions, in good agreement with the experimental data.
Keywords: Efficient terahertz generation, Optical-to-terahertz conversion, Terahertz scattering, Plasma density decay
doi: 10.11906/TST.108-127.2014.09.09
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TST, Vol. 7, No. 3, PP. 128-138
(Invited paper) Terahertz pulse manipulation based on difference frequency generation
Dehua Li 1, 2*, Jianjun Ma 3, Wei Zhou 1, 2, Peng Zhao 1, 2, Junxiang Yu 1, 2, and Zhaoxin Li 1, 2
1 Terahertz Science and Technology Research Center, Shandong University of Science and Technology, 579 Qianwangang Rd, Huangdao, Qingdao, 266590, China
2 Qingdao Key Laboratory of Terahertz Technology, Qingdao, 579 Qianwangang Rd, Huangdao, 266590, China
3 Department of Physics, New Jersey Institute of Technology, 322 King Blvd., Newark, New Jersey, 07102, USA
*1 Email: jcbwl@sdust.edu.cn
(Received May 29, 2014)
Abstract: This paper presents a method of terahertz (THz) pulse shaping in which both the amplitude and polarity of the THz pulse can be manipulated. A pair of temporally separated and orthogonally polarized collinear propagating femtosecond pulses with the same central frequencies are focused onto a (110) oriented ZnTe crystal. By adjusting the relative time delay between the pulses, the amplitude and polarity of the generated THz pulse based on difference frequency generation (DFG) can be controlled. Theoretical derivation and simulation based on one-dimensional propagation equation of the THz wave have been carried out under simplified conditions of perfect phase matching, plane-wave approximation and no absorption. Considering the crystal absorption and phase mismatching condition in the DFG process, THz power spectrums from ZnTe, GaAs, GaP, InP, and CdTe crystals are conducted and compared. Besides, the crystal length dependence of THz waveform is discussed.
Keywords: Terahertz pulse, Pulse manipulation, Difference frequency generation, Crystal orientation
doi: 10.11906/TST.128-138.2014.09.10
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UCMMT Best Student Papers
The 7th European/UK-China Workshop on Millimeter Waves and Terahertz Technologies (UCMMT) was successfully held during 2nd - 4th September, 2014 in Chengdu, China. Many excellent research papers were presented at the workshop. Among them, some outstanding student presentations were shortlisted for the awards and four of them were selected by the Award Committee as the winners of “Best Student Paper Award” at UCMMT’ 2014. The awards are kindly supported by Shenggang Liu Science and Technology Development Foundation.
Here, we firstly publish the four Best Student Papers at the Workshop, and the rest short-listed student papers will be published in succession.
We hope the publication of the excellent student papers can encourage more and more young researchers to participate in the research and development of the terahertz science and technology in the future.
Here we wish to extend our thanks to UCMMT International Advisory Committee Co-chairs: Prof. Shenggang Liu and Prof. Alan Phelps, Conference Co-chairs: Prof. Xiaobo Yang, Prof. Caidong Xiong and Prof. Xiaodong Chen and all authors for their great support to TST.
TST, Vol. 7, No. 3, PP. 139-144
Investigation of a 0.2-THz magnetron using 3D particle simulation
Jiandong Lang *, Xiang Li, Yasir Alfadhl and Xiaodong Chen
School of Electronic Engineering and Computer Science, Queen Mary University of London
London, UK, E1 4NS.
* Email: j.lang@qmul.ac.uk
(Received May 31, 2014)
Abstract: The results of a 3D particle simulation of a 0.2-THz cold cathode magnetron are presented. It is operated at first order backward spatial harmonic of a π/2-1 mode. The saturated output power is calculated as 2.8 kW with 13.6 kV anode voltage and 0.944 T magnetic field.
Keywords: Magnetron, THz oscillator, High power, Vacuum tubes.
doi: 10.11906/TST.139-144.2014.09.11
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TST, Vol. 7, No. 3, PP. 145-149
Analysis and design of a quasi-optical monopulse antenna at sub-millimeter wavelengths
Long Zhang *, Wenbin Dou
State Key Laboratory of Millimeter Waves, Southeast University, Nanjing, China, 210096,
* Email: zhanglong0206@gmail.com
(Received May 31，2014 )
Abstract: A quasi-optical (QO) monopulse antenna operating at sub-millimeter wavelengths is described in this paper. The Finite Difference Time Domain (FDTD) method is adopted for investigating the radiation performance of sum-difference patterns. For suppressing the sidelobe levels of sum radiation patterns, a hyperbolic-plano dielectric lens with cutting segments is introduced. The simulated results show good monopulse performance with the sidelobe levels below -10.0 dB, the maximum null-depth approximately -30.0 dB, and the amplitude unbalance below 1.25 dB.
Keywords: Quasi-optical (QO), Monopulse antenna, Sub-millimetre wavelengths.
doi: 10.11906/TST.145-149.2014.09.12
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TST, Vol. 7, No. 3, PP. 150-153
THz modulation performance of VO2 film grown on Al2O3/Si substrate
Ying Xiong 1, Qi-Ye Wen 1*, Zhi Chen 2, Qing-Hui Yang 1, Huai-wu Zhang 1
1 State Key Laboratory of Electronic Films and Integrated Devices,
University of Electronic Science and Technology of China, Chengdu, 610054, China
2 National Key Laboratory of Science and Technology of Communication.
University of Electronic Science and Technology of China, Chengdu, 610054, China
*1 Email: qywen@uestc.edu.cn
(Received May 31, 2014)
Abstract: High quality VO2 thin film was successfully deposited on Si substrate with Al2O3 as a buffer layer. The electrical and optical performance of the film was great enhanced. Reflective terahertz modulation is characterized and the modulation amplitude of 55% is obtained. The VO2-on-Si buffered by Al2O3 can be widely used for THz devices.
Keywords: VO2 thin film, Terhertz, Modulation.
doi: 10.11906/TST.150-153.2014.09.13
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TST, Vol. 7, No. 3, PP. 154-159
Measurement and improvement of beam profile for terahertz quantum cascade laser
Yao Zhao 1*, Weihua Li 2, Dawei Yan 2, Zejian Lu 1 , Xiaoming Liu 1,
Yuan Yao 1, Junsheng Yu 1, and Xiaodong Chen 3
1 International Joint Laboratory of Electromagnetic Theory and Applications,
School of Electronic Engineering, Beijing University of Posts and Telecommunications.
No.10 Xitucheng Road, Haidian District, Beijing, China, 100876
2 Research Centre of Laser Fusion, China Academy of Engineering Physics, Mianyang, China, 621900
3 School of Electronic Engineering and Computer Science, Queen Mary, University of London
Mile End Road, London E1 4NS
*1 Email: zy715754427@139.com
(Received May 31, 2014)
Abstract: The divergence of the laser beam generated by Quantum Cascade Laser is relative large, which is not conducive to the process of experiment and the acquisition of the desirable results. Thus, a convex lens would be utilized in the propagation path to optimize the laser beam.
Keywords: Terahertz, Quantum cascade laser, Gaussian beam, Divergence
doi: 10.11906/TST.154-159.2014.09.14
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