Effect of the stimulated transparency of a superconducting plate due to the nonlinear wave interaction

A novel nonlinear effect in the electrodynamics of hard superconductors is predicted and studied theoretically. It is shown that an electromagnetic wave (we call it base signal) can pass through a superconducting plate due to its interaction with another wave (an assisting signal) propagating in the opposite direction, even at small amplitudes of the base wave when it cannot reach the opposite boundary of the sample without assistance. The dependence of the surface impedance of the sample at the base frequency on the amplitude of the assisting wave is studied within the framework of the critical state model. The surface impedance is found to be drastically increased when the regions of wave penetration overlap and the wave interaction takes place. The real part of the surface impedance is shown to be negative (or higher than the impedance of vacuum) for some values of the parameters of the problem. This phenomenon occurs due to a considerable redistribution of energy between the interacting waves. We use the obtained results for a qualitative description of available experimental data.