THz emission peak at low voltage bias from 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 therefore show promise as a source of coherent, continuous-wave radiation in the 'terahertz gap' range.
THz emission from these devices has previously been typically observed at frequencies between around 0.4 THz – 2 THz, corresponding to Josephson voltages of between 0.8 mV and 4 mV per intrinsic junction. However, we have recently observed a new type of emission which occurs at bias currents below the retrapping current of the stacked Josephson junctions. This unusual THz emission state is highly reproducible, is metastable over a timescale of hours, and shows systematic temperature dependence. The maximum emitted THz power that can be generated from this mode is comparable to that generated by the more well-established and better-understood THz emission modes in Bi₂Sr₂CaCu₂O₈ mesa sources.
We will discuss possible mechanisms for the unusual emission mode, together with future directions of related research.