  Optical superlattices with sublattice symmetry subjected to a synthetic
imaginary gauge field undergo a topological phase transition in the Bloch
energy spectrum, characterized by the change of a spectral winding number. For
a narrow gap, the phase transition is of universal form and described by a
non-Hermitian Dirac equation with Lorentz-symmetry violation. A simple photonic
system displaying such a phase transition is discussed, which is based on light
coupling in co-propagating gratings.
