Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 . In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to~50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

The measurement of the Cu-O distances by a local and fast probe, polarized Cu K-edge extended x-ray absorption fine structure (EXAFS) in La 1.85 Sr 0.15 CuO 4 crystal shows two different conformations of the CuO 6 octahedra below 100 K assigned to two types of stripes with different lattice. This experiment supports a model of "two components" spatially separated in a superlattice of quantum stripes for the anomalous properties of cuprate superconductors. [S0031-9007(96)00119-6] PACS numbers: 74.72. Dn, 61.10.Ht, 78.70.Dm Experimental methods probing the local structure have shown that the structure of the metallic CuO 2 plane in high T c cuprate superconductors is not homogeneous at a mesoscopic scale length [1][2][3][4]. It has been proposed that an anharmonic 1D modulation of the CuO 2 plane is a key feature for the mechanism of high T c superconductivity [5]. A superstructure of the type q pb ‫ء‬ 1 ͑1͞n͒c ‫ء‬ , in the orthorhombic notation, seems to be a common feature of the superconducting cuprates close to the optimum doping. It has been observed in Bi 2 Sr 2 CaCu 2 O 81d (Bi2212) [6,7] and in Bi 2 Sr 2 Cu 2 O 61d (Bi2201) [8] with p ϳ 0.21 and n 2 considering doubling of the c axis; in La 2 CuO 4.1 (LCO) with p 0.22 and n 3 [9] and with p 0.2 and n 3 [10]; in La 22x Sr x Cu 2 O 4 (LSCO) for x ϳ 0.075 with p ϳ 0.16 and n ϳ 2.5 [11] and a similar superstructure for x 0.1, 0.15 but much weaker in intensity for the overdoped sample, i.e., x 0.2 [12]; in Tl 2 Ba 2 CaCu 2 O 8 (Tl2212) with p ϳ 0.2 [13] and a similar one in Tl 2 Ba 2 Ca 2 Cu 3 O 10 (Tl2223) [14]. This superstructure is difficult to identify in some of the compounds (for example, in the case of LSCO it could be identified only after about 9 yr of the discovery of high T c superconductivity in this material), and it is more clear at temperatures lower than 100-200 K (e.g., in Tl2212, Tl2223, LCO, LSCO). On the other hand, the superstructure is stable even at high temperatures in Bi2212. The c-axis modulation, different from sample to sample, is due to ordering of dopants in the rock-salt block layers as it is clear in the isostructural compounds, e.g., La 2 NiO 41d . The long wavelength incommensurate modulation of the CuO 2 plane along the 45 ± direction from the Cu-Cu direction, involving ϳ10 Cu sites, appears to be a common feature of cuprate superconductors at optimum doping.A "two-component" model has been proposed [5] where at optimum doping (0.2 hole per Cu sites) a first component with hole density d i ϳ 1 1 0.16 coexists with a second component of impurity states, with hole density d ᐉ ϳ 0.04, spatially separated in two different types of stripes forming a superlattice of quantum wires. A com-mensurate superstructure with lower period (4 Cu sites) was predicted [5] where all doped holes form a single electronic component, a pinned Wigner polaronic charge density wave (CDW), that will suppress superconductivity, and it has been observed at the 1͞8 critical doping and in the nickelates [15].In the case of Bi2212 we have shown [5] that th...