Since the discovery of continuous, coherent THz radiation from intrinsic Josephson junctions (IJJs) constructed in the single crystals of Bi₂Sr₂CaCu₂O_8+δ in 2007, various types of device structures have been developed. Thermal management of Joule heat in the IJJ mesa structure is the most important issue to improve the radiation characteristics of the IJJ-THz emitters. The radiation frequencies ranging from 0.3 to 2.4 THz, the emission power of $ \sim 30$ pW/mesa and the radiation linewidth of 0.2 GHz at $ \sim 0.5$ THz were obtained from the thermal managed device structures so far. We show a brief overview of the IJJ-THz emitters in our group.

Using our recent design of thermally managed sandwich device structures, we studied the radiation frequency characteristics of three such devices of the same rectangular dimensions made from the same single crystal of the high-T_c superconductor Bi₂Sr₂CaCu₂O_{8+ δ}, and all three devices exhibit similar characteristics. Their observed radiation intensities appear to be enhanced at many transverse magnetic TM_n,m cavity mode frequencies, possibly including some higher TM_0,m modes with waves solely along the rectangular length, none of which have previously been reported. In addition, the temperature dependences of the radiation frequencies correspond strongly to the temperature dependences of the maximum bias voltages applied to the devices. The excitations of many cavity modes higher in frequency than that of the usually observed TM_1,0 mode and the high reproducibility of the radiation frequency characteristics both appear to originate from the reduction in the Joule self-heating of the thermally managed sandwich structures. The information provided here should aid in the design of future devices to obtain the desired emission frequency ranges.