Asymmetric diffraction based on a passive parity-time grating

Optical structures with balanced loss and gain provide an efficient platform to study the features of light propagation under non-Hermitian parity-time symmetry. Here, we report a feasible design of one-dimensional parity-time symmetric diffraction grating, where the real and imaginary parts of refractive index are separately modulated. Due to the spontaneous breaking of parity-time symmetry at the exceptional point, asymmetric diffractions are observed between a pair of oblique incident light. This asymmetric phenomenon, determined by the modulation direction of the introduced parity-time symmetry, is also polarization-dependent. The coupled-mode theory is implemented to theoretically analyze the polarization dependent asymmetric diffraction, showing consistence with numerical simulations. Our findings may provide a feasible way for manipulating light and instructively inspire the development of diffraction optics.