Cavity mode dependence of terahertz emission power from stacked intrinsic Josephson junction Bi₂Sr₂CaCu₂O₈ 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, which spans from around 0.3 THz to 2.0 THz.

Bi₂Sr₂CaCu₂O₈ THz sources have been most typically studied at emission frequencies ranging from 0.3 THz to 1.0 THz, corresponding to free space wavelengths ranging from approximately 900 microns to 300 microns. Since the emission wavelength is comparable to or larger than the dimensions of the stack of Josephson junctions, the far-field THz power radiated by the device is broadly distributed over 2π steradians of solid angle.

For a large rectangular stack of optimally-doped Bi₂Sr₂CaCu₂O₈ Josephson junctions, we have mapped the angular distribution of the emitted THz power in (θ, φ)-space. We find that the (θ, φ)-dependence of the emitted THz power depends strongly upon which THz-frequency cavity mode is being excited within the stack. We also find that the total integrated THz power for these modes ranges from tens of microwatts to a few hundred microwatts for this device. Our results provide direct confirmation of estimates for the emitted THz power from stacked Bi₂Sr₂CaCu₂O₈ sources that have been previously reported in the literature.