No abstract

Using a modified mesa structure of high-T c superconducting Bi 2 Sr 2 CaCu 2 O 8þd with a thin underlaying base superconductor ($3 lm), the effective working temperature of the continuous and monochromatic terahertz emitter is extended up to 70 K, and the maximum power of $30 lW at 0.44 THz is achieved at the relatively high temperature of T b ¼ 55 K in a low bias current retrapping region. The diverging behavior of the intensity occurring at 55 K in the low current regime without hot spot formation may provide us an important clue for the stronger THz radiation from intrinsic Josephson junction devices. V

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature T c superconductor Bi 2 Sr 2 CaCu 2 O 8+δ , the local surface temperature T (r) was directly measured during simultaneous sub-THz emission from the N ∼ 10 3 intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents I and low bath temperatures T bath 35 K, the center of a large elliptical hot spot with T (r) > T c jumps dramatically with little current-voltage characteristic changes. The hot spot does not alter the ubiquitous primary and secondary emission conditions: the ac-Josephson relation and the electromagnetic cavity resonance excitation, respectively. Since the most intense sub-THz emission was observed for high T bath 50 K in the low I bias regime where hot spots are absent, hot spots cannot provide the primary mechanisms for increasing the output power, the tunability, or promoting the synchronization of the N IJJs for the sub-THz emission, but can at best coexist nonmutualistically with the emission. No T (r) standing waves were observed.