PT symmetry breaking in multilayers with resonant loss and gain locks light propagation direction

Using the Maxwell-Bloch equations for resonantly absorbing and amplifying media, we study the temporal dynamics of light propagation through the PT-symmetric structures with alternating loss and gain layers. This approach allows us to precisely describe the response of the structure near the exceptional points of PT symmetry breaking phase transition and, in particular, take into account the nonlinear effect of loss and gain saturation in the PT symmetry broken state. We reveal that in this latter state the multilayer system possesses a lasinglike behavior, releasing the pumped energy in the form of powerful pulses. We predict locking of pulse direction due to the PT symmetry breaking, as well as saturation-induced irreversibility of phase transition and nonreciprocal transmission.