Stacked intrinsic Josephson junction Bi₂Sr₂CaCu₂O₈ terahertz sources: Design issues for achieving high power output close to <i>T</i>_c

The high-temperature superconductor Bi₂Sr₂CaCu₂O₈ contains stacked 'intrinsic' Josephson junctions, with unrivalled packing density and a high superconducting gap energy. Cuboid ‘mesa’ devices constructed from this material are consequently a promising technology for coherent, continuous-wave radiation in the 'terahertz gap' range, spanning from approximately 0.3 – 1.5 THz.
A key issue for practical applications of such devices is their cryocooling requirements, and it is therefore highly desirable to optimize their performance at temperatures that can be achieved by nitrogen cryogenics. Here we report generation of 0.15 milliwatts of coherent emission power at 0.5 THz, at a bath temperature of 77 Kelvin. This was achieved by exciting the (3, 0) cavity mode of a stack containing 580 junctions, and T_c of 86.5 Kelvin. In order to minimize self-heating, the THz source was mounted on a copper substrate using PbSn solder.
We will discuss the choice of mesa dimensions and cavity mode, and implications for the design of devices which are intended to operate close to the material’s superconducting critical temperature.