Terahertz emission from stacked intrinsic Josephson junction Bi₂Sr₂CaCu₂O₈ sources: (θ, φ) angle-dependence of the emission power

The extremely anisotropic high-temperature superconductor Bi₂Sr₂CaCu₂O₈ contains stacked 'intrinsic' Josephson junctions. Mesa-shaped devices constructed from this material are consequently 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 usually been investigated at emission frequencies ranging from 0.3 THz to 1.0 THz, corresponding to free space wavelengths ranging from 900 microns to 300 microns. Since these free space wavelengths are comparable to the dimensions of the stack of Josephson junctions, the far-field THz power radiated by the device is relatively smoothly distributed over 2π steradians of available solid angle.



For a large rectangular stack of optimally-doped Bi₂Sr₂CaCu₂O₈ Josephson junctions, we have studied the angular distribution of the emitted THz power in (θ, φ)-space, where θ is the poloidal angle with respect to the crystalline c-axis, and φ is the azimuthal angle (i.e. within the plane of the CuO₂ layers). We find that different cavity modes excited within the stack result in very different (θ, φ)-distributions of the emitted THz power. 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.