Monochromatic THz radiation caused by vortex-antivortex pair production in a current-driven Josephson stack

We report numerical simulations of the Cherenkov instability of Josephson vortices driven by a dc current density J in layered superconductors. Dynamic equations for a stack of up to 321 coupled interlayer junctions were solved self-consistently together with an equation for a mean temperature T(N,J) of the stack. It is shown that the Cherenkov wake behind a trapped vortex bouncing back and forth along the stack can trigger proliferation of multiple counter-propagating vortices and antivortices which get synchronized and eventually form large-amplitude standing electromagnetic waves causing temporal oscillations of the magnetic moment M(t) of the stack. This happens as J exceeds a critical value J_p(η) which can be well below the Josephson interlayer critical current density J_c if the quasiparticle damping parameter η is small. The magnitude of oscillating M(t) has sharp peaks at certain values of J at which the spectrum of M(t) in the THz region is nearly monochromatic. The peak radiation power P caused by the oscillating M(J,t) increases rapidly (P∝ N⁶) with N at N<261 and tends to slow down at larger N. For a Bi-2212 mesa with 261 CuO₂ layers the peak power P can reach a few μW while overheating remains well below T_c.