Output from a Josephson stimulated terahertz amplified radiation emitter

Abstract

The angular dependence of the radiation-zone output power and electric polarizationof stimulated terahertz amplified radiation (STAR) emitted from a dc voltageapplied across cylindrical and rectangular stacks of intrinsic Josephson junctions iscalculated. The boundary conditions are obtained from Love’s equivalence principles.During coherent emission, a spatially uniform ac Josephson current density inthe stack acts as a surface electric current density antenna source, leading toa harmonic radiation frequency spectrum, as in experiment, but absent in allcavity models of cylindrical mesas. Spatial fluctuations of the ac Josephson currentallow its fundamental mode to lock onto the lowest finite energy cylindrical cavitymode, causing it to resonate, leading to a non-uniform magnetic surface currentdensity radiation source, and a non-trivial combined fundamental frequency outputpower with linear polarization for general radiation directions, which may be fullyor partially coherent. The higher ac Josephson harmonics do not excite othercylindrical cavity modes. For rectangular mesas, the lowest energy modes areempirically not excited, but the non-uniform ac Josephson current can excitethe harmonic sequence of modes with spatial variation across the rectangularwidths, leading to combined radiation outputs both for the fundamental andthe higher harmonics, which combinations also may be either fully or partiallycoherent. The superconducting substrate is modeled as a perfect magnetic conductor,greatly reducing the STAR emitter power and modifying its angular dependence,especially parallel to the substrate. Based upon this substrate model, existingBi2Sr2CaCu2O8 + δ crystals atop perfect electric conductors could have STAR emitter power in excess of 5 mW,acceptable for many device applications.