Electromagnetically induced grating with maximal atomic coherence

We describe theoretically an atomic diffraction grating that combines an electromagnetically induced grating with a coherence grating in a double-Λ atomic system. With the atom in a condition of maximal coherence between its lower levels, the combined gratings simultaneously diffract both the incident probe beam as well as the signal beam generated through four-wave mixing. A special feature of the atomic grating is that it will diffract any beam resonantly tuned to any excited state of the atom accessible by a dipole transition from its ground state.