  Intrinsic Josephson junctions in high-temperature superconductor
Bi2Sr2CaCu2O8 are known for their capability to emit high-power terahertz
photons with widely tunable frequencies. Hotspots, as inhomogeneous temperature
distributions across the junctions, are believed to play a critical role in
synchronizing the gauge-invariant phase difference among the junctions, so as
to achieve coherent strong emission. Previous optical imaging techniques have
indirectly suggested that the hotspot temperature can go higher than the
superconductor critical temperature. However, such optical approaches often
disturb the local temperature profile and are too slow for device applications.
In this paper, we demonstrate an on-chip in situ sensing technique that can
precisely quantify the local temperature profile. This is achieved by
fabricating a series of micro "sensor" junctions on top of an "emitter"
junction and measuring the critical current on the sensors versus the bias
current applied to the emitter. This fully electronic on-chip design could
enable efficient close-loop control of hotspots in BSCCO junctions and
significantly enhance the functionality of superconducting terahertz emitters.
