Intrinsic Josephson junction Bi₂Sr₂CaCu₂O₈ terahertz sources: Strategies for increasing power output above 77 Kelvin

The high-temperature superconductor Bi₂Sr₂CaCu₂O₈ contains stacked 'intrinsic' Josephson junctions, with very high packing density and a large superconducting gap energy. Rectangular ‘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 technological applications of such devices is their cryocooling requirements, and it is therefore highly desirable to optimize their performance at temperatures that can be achieved while using liquid nitrogen cryogenics. Here we report 0.15 milliwatts of coherent emission power at 0.5 THz, at a bath temperature of 77 Kelvin. We achieved this 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 possible future strategies for the design of Bi₂Sr₂CaCu₂O₈ THz devices that are intended to operate at 77 Kelvin or above.