Mesa sidewall effect on coherent terahertz radiation from Bi₂Sr₂CaCu₂O_8+δ intrinsic Josephson junctions

Coherent terahertz sources based on the stack of Bi₂Sr₂CaCu₂O_8+δ intrinsic Josephson junctions (IJJs) have been attracted in the academic and industrial fields. See Ref. [U. Welp et al., Nat. Photonics, vol. 7, p. 702 (2013)] for recent review. In this study, we demonstrate that tapered sidewalls produce a difference in the junction voltages along the stack and thus synchronization among the IJJs fails to occur with distributed oscillation frequencies. Also, our systematic experiment using shielding metal masks reveals that only the ac Josephson current flowing along the sidewall can generate the terahertz waves outside of the mesa. These results suggest that the condition of the sidewall is essentially important to control the radiation characteristics. In this talk, we will present the details of the fabrication process and experimental results and will discuss the role of the mesa sidewalls on the coherent radiation.