Direct imaging of hot spot in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ mesa terahertz sources

Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature superconductors such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) are a promising source of coherent continuous-wave terahertz radiation. When electrical power is applied to these devices, it is thought that hot spots may form due to resistive self-heating, and that these spots may be highly beneficial for the generation of high levels of THz power from Bi-2212 stacks. In order to better understand these hot spots, we have performed a thermal imaging study of BSCCO stacks which generate approximately 50 microwatts of radiation power at 0.59 THz. Utilizing the temperature-dependent 612nm fluorescence line of Eu$^{3+}$, we are able to directly measure the temperature distribution at the top surface of these stacks with a resolution of $+$/- 1K. The images reveal a highly non-uniform temperature distribution in which the temperature in the middle of the stack can exceed the superconducting transition temperature by tens of Kelvin under biasing conditions typical for THz-emission.