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

We report on THz emission measurements and low temperature scanning laser imaging of Bi(2)Sr(2)CaCu(2)O(8) intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the cold'' part of the stack, coexist. By changing bias current and bath temperature, the emission frequency can be varied by more than 40%; the variation matches the Josephson-frequency variation with voltage. The linewidth of radiation is much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.