| Beam dynamics in synthetic optical media with Parity (P) and Time (T) symmetries imposed by a balanced arrangement of gain and loss is investigated. We find that the temporal behavior of the total power in linear media is insensitive to microscopic details of the system and follows three distinct universal laws which depends only on the magnitude of the gain/loss parameter. We further show that PT-symmetric Bragg grating structures, at the spontaneous PT-symmetry breaking (exceptional) point, can act as unidirectional invisible media. In this regime the reflection from one end is diminished while it is enhanced from the other. At the same time the transmission coefficient and phase, are indistinguishable from those expected in the absence of a grating. The phenomenon is robust even in the presence of Kerr non-linearities, and it can also effectively suppress optical bistabilities. Away from the exceptional point, the interplay of optical nonlinearities with PT-symmetries, allow for diode action. Such PT-synthetic unidirectional optical valves may find promising applications in integrated photonic systems. |
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