  We study the diffraction produced by a slab of purely reflective PT-symmetric
volume Bragg grating that combines modulations of refractive index and
gain/loss of the same periodicity with a quarter-period shift between them.
Such a complex grating has a directional coupling between the different
diffraction orders, which allows us to find an analytic solution for the first
three orders of the full Maxwell equations without resorting to the paraxial
approximation. This is important, because only with the full equations can the
boundary conditions, allowing for the reflections, be properly implemented.
Using our solution we analyze unidirectional invisibility of such a grating in
a wide variety of configurations.
