Nonlinear suppression of time reversals in PT -symmetric optical couplers

We reveal a generic connection between the effect of time reversals and nonlinear wave dynamics in systems with parity-time (PT ) symmetry, considering a symmetric optical coupler with balanced gain and loss where these effects can be directly observed experimentally. We show that for intensities below a threshold level, the amplitudes oscillate between the waveguides, and the effects of gain and loss are exactly compensated after each period due to periodic time reversals. For intensities above a threshold level, nonlinearity suppresses periodic time reversals, leading to the symmetry breaking and a sharp beam switching to the waveguide with gain. Another nontrivial consequence of linear PT symmetry is that the threshold intensity remains the same when the input intensities at waveguides with loss and gain are exchanged.