Abstract
We report on THz emission measurements and low temperature scanning laser imaging of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ 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.
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