Radiation from a Josephson STAR-emitter

TL;DR

This paper models the angular dependence and polarization of terahertz radiation emitted from Josephson junction stacks, revealing the role of spatial current fluctuations and cavity modes in shaping the emission characteristics.

Contribution

It introduces a comprehensive model for the radiation from Josephson junction stacks, accounting for spatial current fluctuations and substrate effects, which explains experimental observations.

Findings

Fundamental mode locking to cylindrical cavity modes causes resonance.

Surface electric current acts as an antenna source for harmonic radiation.

Resonance leads to non-uniform magnetic surface current density and polarized output.

Abstract

We calculate the angular dependence of the radiation-zone output power and electric polarization of stimulated terahertz amplified radiation (STAR) emitted from a $dc$ voltage applied across cylindrical and rectangular stacks of intrinsic Josephson junctions. During coherent emission, a spatially uniform $ac$ Josephson current density in the stack acts as a surface electric current density antenna source, leading to an harmonic radiation frequency spectrum, as in experiment, but absent in all cavity modesl of cylindrical mesas. Spatial fluctuations of the $ac$ Josephson current cause its fundamental mode to lock onto the lowest finite energy cylindrical cavity mode, causing it to resonate, leading to a non-uniform magnetic surface current density radiation source, and a non-trivial combined fundamental frequency output power with linear polarization We also present a model of the superconducting substrate, and present results for rectangular mesas.