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Towards Practical Applications of THz Josephson Oscillators with Sub-mW Power and 500 GHz Frequency Tunability
H. B. Wang1, 2, *, J. Yuan2, D. Y. An1, 2, N. Kinev3, M.Y. Li1, 2, X. J. Zhou1, M. Ji1, 2, Y. Huang1, T. Hatano1, V. P. Koshelets3, D. Koelle4, R. Kleiner4, W. W. Xu2, and P. H. Wu2 National Institute for Materials Science, Tsukuba 3050047, Japan Research Institute of Superconductor Electronics, Nanjing University, Nanjing 210093, China 3 Kotel'nikov Institute of Radio Engineering and Electronics, Moscow 125009, Russia 4 Physikalisches Institut and Center for Collective Quantum Phenomena in LISA+, Universitaet Tuebingen, D-72076 Tuebingen, Germany *Email: hbwang1000@gmail.com
maximum detected power of 25 W, the frequency can be tuned by either bias current or bath temperature from 10 K to 70 K, over a frequency range of between 234 GHz and 876 GHz. The wide range tunability is related to the hotspot resulted from the self-heating4. Photon-assisted current steps or Shapiro steps have been observed either in an SIS Josephson junction or a grain boundary Josephson junction5, indicating the power is sufficient to pump most of the superconducting THz detectors.
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Abstract--Using a double-sided fabrication method, we fabricated Bi2Sr2CaCu2O8 (BSCCO) intrinsic Josephson junctions (IJJs) with a gold-BSCCO-gold structure. Coherent emission is observed at large dc input power, where a hot spot and "cold" part of the stack, coexist. The power, directly detected with a bolometer, is as high as 25 W, implying the integrated power should be already in the order of sub-mW. We have successfully pumped a Nb-based Josephson junction and a grain boundary Josephson junction. Also with a wide tunable frequency range, BSCCO THz emitters will find themselves many practical applications, e.g., local oscillators of SIS, HEB, and Josephson mixers.

I. INTRODUCTION A

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BACKGROUND

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N recent years coherent terahertz (THz) emission have been obtained from stacks of intrinsic Josephson junctions (IJJs) made of the high temperature superconductor Bi2Sr2CaCu2O8 (BSCCO). An IJJ is formed naturally in the BSCCO unit cell, as shown in the inset of Fig. 1(a), with the CuO2 layers forming the superconducting electrodes and the BiO and SrO layers forming the barrier layer1. A crystal of 1 m thickness consists of about 670 IJJs. In 2007 it was reported that such stacks can emit coherent radiation at frequencies up to 0.85 THz, however, with a directly detected power of only 10 nW2, which is much lower than that required for practical applications. For example, power of 100 nW is needed for pumping a hot electron bolometer or a superconductor-insulatorsuperconductor (SIS) mixer. Therefore, How to increase emission power becomes the most important mission in the IJJs community. In this talk, we reported THz emission from IJJs with a detected power as high as 25 W, three orders as high as that in the first report2.

Fig. 1. Properties of the THz emitter. (a) Current voltage characteristic and (b) THz emission power vs. bias current, as detected by a Si bolometer. The maximum detected power at 25 K is about 25 W. The inset in (a) shows the crystalline structure and how the Josephson junction array is biased with dc current.

REFERENCES
[ 1] [ 2] R. Kleiner et al., "Intrinsic Josephson effects in Bi2Sr2CaCu2O8 single crystals", Phys. Rev. Lett. 68, 2394, 1992. L. Ozyuzer et al., "Emission of coherent THz radiation from superconductors", Science 318, 1291, 2007. H. B. Wang et al., "Microwave responses of an insular intrinsic Josephson junction stack fabricated from Bi-Sr-Ca-Cu-O single crystal", IEEE Trans. on Appl. Supercond., 11(1), 1199, 2001. H. B. Wang et al., "Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime", Phys. Rev. Lett. 105, 057002, 2010. D. Y. An et al., "Terahertz emission and detection both based on h igh-Tc superconductors: Towards an integrated receiver", Appl. Phys. Lett. 102, 092601, 2013.

II. RESULTS The fabrication process of the IJJ emitter is very similar to the one for a Au/BSCCO/Au structure developed earlier3. The BSCCO emitter, mounted on the Si lens, was characterized in terms of transport and THz emission measurements. Fig. 1 shows the IVC, as measured near 25 K, together with the emission power detected by the Si bolometer. Besides the

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