Superradiant THz emission stimulated by vortex-antivortex pair production in layered superconductors.

We report numerical simulations of coupled sine-Gordon and heat diffusion equations describing superradiant states stimulated by a trapped vortex driven by dc current in a stack of up to $N=321$ Josephson junctions. It is shown that the Cherenkov wake behind the vortex shuttle trapped in the stack can trigger proliferation of counter-propagating vortices and antivortices which get synchronized and form large-amplitude standing electromagnetic waves if the dc current density $J$ exceeds $J_s$ which can be well below the Josephson interlayer critical current density $J_c$ for underdamped junctions. The cavity modes stimulated by the vortex-antivortex pair production cause peaks in the radiated power $P_N(J)$ with a nearly monochromatic spectrum at discrete values of $J$. For small stacks, the highest peak in $P_N(J)$ increases rapidly, $P_N\propto N^6$, with the number of junctions at $N\leq 81$ and gradually slows down to $P_N\propto N^2$ at $161\leq N\leq 321$. For stacks larger than the radiated wavelength, we obtained $P_N\propto N^5$ at $N\lesssim 200-300$ and $P_N\propto N^2$ at larger $N$. For stacks with $N\leq 321$ and parameters of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, we observed moderate overheating and no hotspots. The vortex-antivortex pair production can amplify THz radiation from Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesas for which trapping Josephson vortices could be used to stimulate THz emission at subcritical currents and optimize the radiation output.