The radiation intensity from the intrinsic Josephson junction high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ terahertz emitters (Bi2212-THz emitters) is one of the most important characteristics for application uses of the device. In principle, it would be expected to be improved with increasing the number of intrinsic Josephson junctions N in the emitters. In order to further improve the device characteristics, we have developed a stand alone type of mesa structures (SAMs) of Bi2212 crystals. Here, we understood the radiation characteristics of our SAMs more deeply, after we studied the radiation characteristics from three SAMs (S1, S2, and S3) with different thicknesses. Compar-ing radiation characteristics of the SAMs in which the number of intrinsic Josephson junctions are N ∼ 1300 (S1), 2300 (S2), and 3100 (S3), respectively, the radiation intensity, frequency as well as the characteristics of the device working bath temperature are well understood. The strongest radiation of the order of few tens of microwatt was observed from the thickest SAM of S3. We discussed this feature through the N²-relationship and the radiation efficiency of a patch antenna. The thinner SAM of S1 can generate higher radiation frequencies than the thicker one of S3 due to the difference of the applied voltage per junctions limited by the heat-removal performance of the device structures. The observed features in this study are worthwhile designing Bi2212-THz emitters with better emission characteristics for many applications.

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.

We studied the etching of small crystals of the high-T_c superconductor Bi₂Sr₂CaCu₂O_8+δ (Bi2212) with various dilute compositions of hydrochloric and nitric acids. A particular mixture of those acids was chosen to simultaneously fabricate multiple rectangular stand-alone Bi2212 mesa structures from a large, doubly-cleaved and doubly metallic-coated single crystal. The radiation characteristics of these devices were found to be very similar to stand-alone devices fabricated previously using dry-etching techniques. The greatly reduced time and cost of fabrication of stand-alone Bi2212 devices using our wet-etching technique should facilitate the mass production of a large number of identical stand-alone devices from a large single-crystalline Bi2212 substrate.