Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime*

Having small sized active and tunable devices operating at frequencies up to the Terahertz (THz) range is one of the goals of modern electronics. However, there is still a lack of good active or passive devices, often referred to as the ``Terahertz gap.'' Intrinsic Josephson junctions formed by the layered crystal structure of high temperature superconductors such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ have the potential to operate in this regime. While for a long time the research on THz generation with this type of junctions was carried out with perhaps only modest success, recently synchronous emission, with an estimated output power in the $\mu $W range, of stacks consisting of several hundred intrinsic Josephson junctions was achieved [1]. We report on the investigation of THz electromagnetic wave generation in intrinsic junction stacks (mesas) of different geometries, using a combination of transport measurement, direct electromagnetic wave detection and Low Temperature Scanning Laser Microscopy [2,3]. At high enough input power a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent THz emission. We discuss possible scenarios of the hot spot/wave interaction and its relation to the generation of coherent THz radiation. \\[4pt] [1] L. Ozyuzer, et al., Science \textbf{318}, 1291 (2007). \\[0pt] [2] H.~B. Wang, et al., Phys. Rev. Lett. \textbf{102}, 017006 (2009). \\[0pt] [3] H. B. Wang, et al., Phys. Rev. Lett. \textbf{105}, 057002 (2010).