  The evolution of a pair of resonant Bragg modes through a medium
characterized by a complex one-dimensional $\mathcal{PT}$-symmetric periodic
permittivity is thoroughly investigated. Analytic solutions of Maxwell's
equations are derived beyond the paraxial approximation to investigate the
periodic energy exchange that occurs between the Bragg modes for the Hermitian
lattices as well as for complex lattices. Three regimes defined by the symmetry
breaking point are discussed: below it, above it and at it. These regimes are
determined by the existence of four real eigenvalues in the symmetric phase,
which collide and coalesce into a pair at the breaking point. Above the
critical value each member of the pair bifurcates into a pair of complex
values. Therefore, the complex lattice reveals a variety of wave dynamics
depending on the gain/loss balance. In all regimes of the transition the
signature of $\mathcal{PT}$-symmetric systems is present, as the evolution is
always nonreciprocal and unidirectional.
