Lop-sided Raman-Nath diffraction in PT-antisymmetric atomic lattices

Two-dimensional (2D) optical lattices of driven cold atoms can provide a useful platform to construct 2D electromagnetically induced grating (EIG) with parity-time (PT) anti-symmetry. This atomic grating is achieved by the spatial modulations of the atomic density and frequency detunings in the four-level double-Lambda atomic system. Gain-assisted PT antisymmetry allows us to realize lop-sided Raman-Nath diffraction with high diffraction efficiency at the exception point. It is shown that the nontrivial phenomenon originates from non-Hermitian degeneracy of PT antisymmetry. Our scheme may provide the possibility for active all-optical control and conversion of the spatial beam in optics. (C) 2019 Optical Society of America