Study of radiation spectrum of Bi₂Sr₂CaCu₂O_8+d high-T_c superconducting terahertz emitters.

In order to understand the emission characteristics of Bi₂Sr₂CaCu₂O_{8+ d} (Bi2212) high-T_c superconducting THz emitters, it is very important to clarify the Joule heating characteristics of the devices. Recently, in order to reduce the self-heating from the Bi2212-THz emitters, we have proposed a stand-alone mesa structure of Bi2212 single crystals and its sandwich structures. The radiation characteristics obtained from these devices indicate clearly that the thermal management of the THz emitters is most important issue in order to obtain good radiation performance and reproducibility.
In order to understand the details of the radiation characteristics of above thermal managed Bi2212-THz emitters, we have studied not only radiation frequency characteristics but also radiation linewidth characteristics. We measured the radiation spectrum ranging from 0.6 to 0.9 THz by using a sub-harmonic mixer. The observed radiation linewidths at low bias conditions seem to be broader than those of at high bias conditions. The radiation spectrum at lower bias point have several peaks, however, the radiation spectrum at high bias condition seems to be single peak. The details of the these radiation characteristics will be discussed.