Coherent flux-flow emission from stacked Josephson junctions: Nonlocal radiative boundary conditions and the role of geometrical resonances

I derive simple nonlocal dynamic boundary conditions, suitable for modeling of radiation emission from stacked Josephson junctions in an arbitrary dynamic state. Coherent flux-flow emission from intrinsic Josephson junctions in high-T c superconductors is analyzed. It is shown that due to the lack of Lorentz contraction of fluxons in stacked junctions, high-quality geometrical resonances are prerequisite for high power flux-flow emission from the stack. This leads to a dual role of the radiative impedance: on one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced radiative losses reduce the quality factor of geometrical resonances, which may decrease the total emission power. Optimal conditions are achieved when radiative losses are comparable to resistive losses inside the stack.