THz emission from a stacked coherent flux-flow oscillator: non-local radiative boundary conditions and the role of geometrical resonances

TL;DR

This paper develops non-local boundary conditions to model THz radiation from stacked Josephson junctions, revealing that optimal emission occurs when radiative and resistive losses are balanced, with geometrical resonances playing a crucial role.

Contribution

It introduces simple non-local boundary conditions for modeling radiation from stacked Josephson junctions and analyzes the impact of geometrical resonances and radiative impedance on emission efficiency.

Findings

High-quality geometrical resonances are essential for high power emission.

Optimal emission occurs when radiative losses are comparable to resistive losses.

Radiative impedance has a dual role, affecting efficiency and resonance quality.

Abstract

I derive simple non-local dynamic boundary conditions, suitable for modelling of radiation emission from stacked Josephson junctions, and employ them for analysis of flux-flow emission from intrinsic Josephson junctions in high-$T_c$ superconductors. It is shown that due to the lack of Lorenz contraction of fluxons in stacked junctions, high quality geometrical resonances are prerequisite for high power emission from the stack. This leads to a dual role of the radiative impedance: on the 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. Therefore, the optimal conditions for the coherent flux-flow oscillator are achieved when radiative losses are comparable to resistive losses inside the stack.