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 lasing-like 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.