Self-heating, phase coherence, and sidewall angle in stacked intrinsic Josephson junction Bi₂Sr₂CaCu₂O₈ terahertz sources

The extremely anisotropic high-temperature superconductor Bi₂Sr₂CaCu₂O₈ contains stacked 'intrinsic' Josephson junctions with a large superconducting gap energy. Mesa-shaped devices constructed from this material are therefore a promising source of coherent, continuous-wave radiation in the 'terahertz gap' range.

To optimize the THz emission power and linewidth performance of these devices, it is essential to create a phase-coherent state among the stacked IJJs. This in turn requires minimizing the spread in their drive voltages, which is far from trivial when the IJJs are current-biased and shunted in series. This is further complicated by self-heating in the mesa, due to the poor c-axis thermal conductivity of Bi₂Sr₂CaCu₂O₈ .

The drive voltage spread can be minimized by the correct choice of mesa sidewall angle. Here we calculate the optimal sidewall angle as a function of device geometry, doping state, emission frequency, and intended operating temperature. We find that the optimal sidewall angle varies dramatically depending on the above parameters, and we discuss its implications for THz mesa source engineering.