Angle-dependent asymmetric transmission in gradient 3D photonic crystals

Asymmetric transmission (AT) materials allow light to pass through differently depending on its direction, which is important for optical devices like isolators, encryption, and solar cells. Here, we propose and experimentally demonstrate an approach to achieve AT at optical frequencies by using three-dimensional gradient spiral photonic crystals (PCs). By analyzing the iso frequency surface's response of spiral photonic crystals, we predict the AT of light at certain incident angles. We fabricate dielectric spiral PCs with different gradients of 1%, 11%, and 28% in the z-direction using the femtosecond laser direct writing technique. The predicted AT of light and the dependence of AT on the structural gradient amplitude due to the complex dispersion relations in PCs have been revealed through experimental measurements. These results demonstrate the potential of gradient 3D PCs in the development of diverse (AT) optical devices.