Abstract-A power-adjustable superconducting terahertz source utilizing electrical triggering phase transitions in vanadium dioxide

L. Y. Hao1,2, X. J. Zhou1,2, Z. B. Yang1, H. L. Zhang1, H. C. Sun1,2, H. X. Cao3, P. H. Dai3, J. Li1, T. Hatano2, H. B. Wang1,2,a), Q. Y. Wen3,b) and P. H. Wu1
http://scitation.aip.org/content/aip/journal/apl/109/23/10.1063/1.4971408
1 Research Institute of Superconductor Electronics, Nanjing University, Nanjing 210023, China
2 National Institute for Materials Science, Tsukuba 3050047, Japan
3 State Key Laboratory of Electronic Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a) Electronic mail: hbwang1000@gmail.com
b) Electronic mail: qywen@163.com
Appl. Phys. Lett. 109, 233503 (2016); http://dx.doi.org/10.1063/1.4971408

We report a practical superconducting terahertz (THz) source, comprising a stack of Bi2Sr2CaCu2O8intrinsic Josephson junctions (IJJs) and a vanadium dioxide (VO2) tunable attenuator with coplanar interdigital contacts. The electrical triggering phase transitions are observed not only at room temperature, but also at low temperatures, which provides a proof of the electrical triggering. Applying this, the VO2 attenuator is implemented for the independent regulations on the emission powers from the IJJ THz emitter, remaining frequencies and temperatures unchanged. The attenuation can be tuned smoothly and continuously within a couple of volts among which the maximum is, respectively, −5.6 dB at 20 K or −4.3 dB at 25 K. Such a power-adjustable radiation source, including the VO2 attenuator, can further expand its practicability in cryogenic THz systems, like superconducting THz spectrometers.

Abstract-Three-Dimensional Simulations of the Electrothermal and Terahertz Emission Properties of Bi2Sr2CaCu2O8 Intrinsic Josephson Junction Stacks

F. Rudau, R. Wieland, J. Langer, X. J. Zhou, M. Ji, N. Kinev, L. Y. Hao, Y. Huang, J. Li, P. H. Wu, T. Hatano, V. P. Koshelets, H. B. Wang, D. Koelle, and R. Kleiner

Phys. Rev. Applied 5, 044017 – Published 27 April 2016

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.5.044017

We use 2D coupled sine-Gordon equations combined with 3D heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a 250×70  μm2intrinsic Josephson junction stack. The 700 junctions are grouped to 20 segments; we assume that in a segment all junctions behave identically. At large input power, a hot spot forms in the stack. Resonant electromagnetic modes oscillating either along the length [(0, n) modes] or the width [(m, 0) modes] of the stack or having a more complex structure can be excited both with and without a hot spot. At fixed bath temperature and bias current, several cavity modes can coexist in the absence of a magnetic field. The (1, 0) mode considered to be the most favorable mode for terahertz emission can be stabilized by applying a small magnetic field along the length of the stack. A strong field-induced enhancement of the emission power is also found in experiment for an applied field around 5.9 mT.

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Abstract-Tuning the Terahertz Emission Power of an Intrinsic Josephson-Junction Stack with a Focused Laser Beam

X. J. Zhou, J. Yuan, H. Wu, Z. S. Gao, M. Ji, D. Y. An, Y. Huang, F. Rudau, R. Wieland, B. Gross, N. Kinev, J. Li, A. Ishii, T. Hatano, V. P. Koshelets, D. Koelle, R. Kleiner, H. B. Wang, and P. H. Wu

Phys. Rev. Applied 3, 044012 – Published 21 April 2015

http://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.3.044012

We report on tuning the THz emission of a Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson-junction stack by a focused laser beam which is scanned across the stack. The emission power Pe increases by up to 75% upon laser irradiation for a bath temperature near 22 K. The laser-induced changes in the voltage Vdc across the stack and in the emission power are measured simultaneously. The maximum of the laser-induced changes in emission power ΔPe is achieved by irradiating the stack on the location where the local temperature is about the critical temperature Tc. However, ΔPe is found to be proportional to the laser-induced global voltage change ΔVdc, irrespective of the laser position. This unexpected global response is likely to be related to a change in the average stack temperature and is consistent with the change in Pe when increasing the bath temperature by about 0.2 K. This tuning method can be employed in the application of BSCCO THz sources.

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Abstract-Modeling the linewidth dependence of coherent terahertz emission from intrinsic Josephson junction stacks in the hot-spot regime

B. Gross, J. Yuan, D.Y. An, M. Y. Li, N. Kinev, X. J. Zhou, M. Ji, Y. Huang, T. Hatano, R.G. Mints,V. P. Koshelets, P.H. Wu, H. B. Wang, D. Koelle, R. Kleiner

http://arxiv.org/abs/1307.3391

Recently it has been found that, when operated at large input power, the linewidth of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime a hot spot coexists with regions which are still superconducting. Surprisingly, the linewidth was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T_b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T_b. In the presence of a spatial gradient of the junction parameters critical current and resistance, the linewidth deceases with increasing T_b, similar to the experimental observation.