  We show that parity-time (PT) symmetric Bragg periodic structures, near the
spontaneous PT - symmetry breaking point, can act as unidirectional invisible
media. In this regime, the re flection 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.
