Asymmetric light propagation based on complex all-dielectric graded photonic crystals

We demonstrate an asymmetric light propagation at a telecommunication frequency by designing two-dimensional all-dielectric complex graded photonic crystals (GPCs) consisting of rod dimers with a rotational gradient layer by layer. The GPC enables beam splitting for the left-hand side incidence, while for the right-hand side incidence beam focusing is observed due to the breaking of spatial inversion symmetry. The functionality is operable both in the near field range at a subwavelength scale and in the far field range with a relatively larger configuration. The merit of complex GPCs lies in the fact that they can offer more degrees of freedom in engineering the dispersion curves, giving rise to flexible manipulation on light propagation. In particular, a complex GPC with a simultaneous rotational and size gradient—namely, a combined gradient—is proposed, which exhibits asymmetric propagation with better performance and less layers, facilitating the miniaturization of the photonic structures. Besides, the idea can be generalized to PC slab systems, favourable for realizing compact optical elements and optical integration. (paper)