Superconductivity is one of the most striking macroscopic quantum phenomena because the coherence of the wave-functional order parameter appears as a sharp drop in resistivity at critical temperature Tc. In curate superconductors, rich varieties of quantum phenomena have been observed in the c-axis transport properties because their crystal structures are alternations of a unit of CuO2 planes, where the superconducting order parameter is localized, and a unit of insulating metal-oxide planes participating as a charge reservoir for their superconductivity. This alternation of superconducting and insulating layers, refereed as a stack of intrinsic Josephson junction (IJJ) , provides emissions of terahertz electromagnetic waves with powers up to 0.6 mW, which are potentially applied for quantum devices.
This talk reviews our recent work on terahertz continuous-wave emission from superconducting IJJs. An IJJ emitter generates terahertz electromagnetic waves of which frequency strongly depends on device temperature and bias voltage . Simultaneous measurements of local temperature distribution and emission intensity have revealed that less temperature rise arises more intensive emitting power . By utilizing the patch antenna theory, emission of circularly polarized waves with degree of circular polarizations more than 99 % were observed .
This work was supported by JSPS KAKENHI Grant Nos. JP23681030 and JP26286006. My visit at LPA has been supported by JSPS KAKENHI for the Promotion of Joint International Research Grant No. JP15KK0204.
 R Kleiner, F Steinmeyer, G. Kunkel and P. Müller Phys. Rev. Lett. 68 2394 (1992).
 L. Ozyuzer, et al., Science 318 1291 (2007) ; I. Kakeya and H. B. Wang, Superconductor Science and Technology 29 073001 (2016).
 I. Kakeya et al., J. App. Phys. 117 043914 (2015).
 M. Tsujimoto et al., Phys. Rev. App. 2 044016 (2014).
 A. Elarabi et al., in preparation.