Local heating effects on the radiation intensity of high-T_c superconducting terahertz emitters

The terahertz electromagnetic waves can be generated by the mesa device made of a piece of high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ single crystal, which is known to be comprised of multi-stacks of intrinsic Josephson junctions. In the case of a thick mesa containing the large number of junctions, large amount of Joule heat is generated, resulting in the formation of the inhomogeneous temperature distribution T(r) in the mesa. It is known from previous studies that T(r) strongly affects the emission intensity and line width. When the mesa is irradiated partly by a focused laser beam, the terahertz radiation intensity grows almost two times as compared with the standard biases without laser irradiation as was found previously. It is also found that the enhanced emission remains without losing the intensity even after switching off the laser power. This result suggests that the normal current distribution in the mesa is modified irreversibly when the device is biased under laser irradiation, according to the thermoreflectance measurement.