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.

Tunneling measurements are reported for break junctions on Bi 2 Sr 2 CaCu 2 O 82d single crystals with various oxygen concentrations. Superconducting energy gaps D are observed in the underdoped samples which are considerably larger ͑ϳ30%͒ than found in optimal doped crystals. The trend of decreasing D and 2D͞kT c with increasing hole doping is continued into the overdoped region. Thus the superconducting gap and strong-coupling ratio change monotonically and dramatically over a narrow doping region where T c exhibits a maximum. [S0031-9007(97)04904-1] PACS numbers: 74.50. + r, 74.25.Dw, 74.62.Dh, 74.72.Hs Tunneling spectroscopy and angle-resolved photoemission spectroscopy (ARPES) have emerged as powerful, complementary probes of the superconducting gap D in high-T c superconductors (HTS). ARPES has made important contributions in Bi 2 Sr 2 CaCu 2 O 82d (Bi2212), including strong support for d-wave symmetry of the gap and the evolution of the superconducting gap into a pseudogap which persists well above T c for underdoped samples [1][2][3]. At present there is no generally accepted model for the ARPES spectral function A͑k, E͒ in HTS, and this leads to uncertainty in the magnitudes of energy gaps, but also limits the ability to distinguish superconducting gaps from those arising from other electronic correlations such as charge density waves. It is therefore crucial to establish a correspondence to other traditional probes of superconductivity such as tunneling spectroscopy. We report here tunneling measurements in Bi2212 crystals using superconductor-insulator-superconductor (SIS) break junctions for which the peak in tunneling conductance is a direct measure of 2D. The Bi2212 crystals each have a T c 95 K at optimal doping and by changing the oxygen concentration they span a range from overdoped to moderately underdoped. We find a surprisingly strong, monotonic dependence of the superconducting energy gap and strong coupling ratio 2D͞kT c on doping concentration. Furthermore, we find that the magnitude of D measured by tunneling agrees with the low-temperature peak position of A͑k, E͒ measured in ARPES along the ͑p, 0͒ momentum direction [1-3] and that the temperature and doping dependence of the tunneling spectra display the same trends as found in ARPES. Thus we confirm the superconducting origin of the gap in ARPES below T c .This work builds upon previous tunneling studies of Bi2212 in both the SIS and SIN (N: normal metal) configurations [4-10], but here we present a detailed, sys-tematic examination of the doping dependence. The reproducibility of the SIS spectra indicates that the crystals have a homogeneous oxygen composition. In SIN tunneling the conductance at T 0 K is proportional to SA͑k, E͒jT k j 2 where the summation is over quasiparticle momentum k. Assuming that the tunneling matrix element T k varies only weakly with k, it follows that tunneling probes the quasiparticle density of states (DOS), r͑E͒ SA͑k, E͒ and this leads to a natural connection to ARPES [11]. A significant advantage of S...