Abstract-Compact high- T c superconducting terahertz emitter operating up to 86 Kelvin

Hancong Sun, Raphael Wieland, Zuyu Xu, Zaidong Qi, Yangyang Lv, Ya Huang, Huili Zhang, Xianjing Zhou, Jun Li, Yonglei Wang, Fabian Rudau, Johannes S. Hampp, Dieter Koelle, Shigeyuki Ishida, Hiroshi Eisaki, Yoshiyuki Yoshida, Biaobing Jin, Valery P. Koshelets, Reinhold Kleiner, Huabing Wang, Peiheng Wu,

https://journals.aps.org/prapplied/accepted/c4071A2fE601520eb1472e406dee8430aae0a3f93

We report on a Stirling-cooled compact Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ intrinsic Josephson-junction stack with very high critical current density and improved cooling, operating at bath temperatures $T_b$ up to 86\,K. The square stand-alone stack is embedded between two sapphire substrates. For bath temperatures between 27.8\,K and 86\,K emission was observed at frequencies from 0.356\,THz to 2.09\,THz. The emission power exceeded 1\,$\mu$W at bath temperatures between 60\,K and 80\,K for emission frequencies between 0.5\,THz and 0.88\,THz. A record high value of 0.577\,THz was obtained for the emission frequency at $T_b$ = 80\,K, which is important for potential applications using liquid nitrogen as coolant. We also compare our experimental results to numerical simulations based on 3D coupled sine-Gordon equations combined with heat diffusion equations.

Abstract-Self-Mixing Spectra of Terahertz Emitters Based on Bi 2 Sr 2 CaCu 2 O 8 + δ Intrinsic Josephson-Junction Stacks

Ya Huang, Hancong Sun, Deyue An, Xianjing Zhou, Min Ji, Fabian Rudau, Raphael Wieland, Johannes S. Hampp, Olcay Kizilaslan, Jie Yuan, Nickolay Kinev, Oleg Kiselev, Valery P. Koshelets, Jun Li, Dieter Koelle, Reinhold Kleiner, Biaobing Jin, Jian Chen, Lin Kang, Weiwei Xu, Huabing Wang, and Peiheng Wu

https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.8.054023

Josephson junctions can serve as mixers for electromagnetic radiation, producing difference frequencies $\left|m{f}_s–n{f}_{\mathrm{LO}}\right|$ of the signal frequency $f_s$ and the local oscillator frequency $f_{\mathrm{LO}}$, where the latter can be provided by ac Josephson currents, and $m$ and $n$ are natural numbers. In order to obtain a better understanding of the purity of the terahertz radiation generated by stacks of intrinsic Josephson junctions (IJJs), we study self-mixing—i.e., $f_s$ is also produced by Josephson currents inside the stacks—in the difference-frequency range between 0.1 and 3.0 GHz. Simultaneously, we perform off-chip terahertz emission detection and transport measurements. We find that at high-bias currents, when a hot spot has formed in the stack, the power level of self-mixing can be low and sometimes is even absent at the terahertz emission peak, pointing to a good phase locking among all IJJs. By contrast, at low-bias currents where no hot spot exists, the self-mixing products are pronounced even if the terahertz emission peaks are strong. The mixing products at high operation temperature, at which the temperature variation within the stack is moderate, are minor, indicating that the low junction resistance, perhaps in combination with the lowered Josephson critical current density, may play a similar role for synchronization as the hot spot does at low temperature. While these observations are helpful for the task to synchronize thousands of IJJs, the observation of self-mixing in general may offer a simple method in evaluating the coherence of terahertz radiation produced by the IJJ stacks.