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The origin and limit of asymmetric transmission in chiral resonators

The interaction of light with suitably designed artificial nanostructures, such as metasurfaces or photonic crystals, can lead to a strong chiral response. Such chirality is not only essential for realizing miniature components to manipulate polarization, but also opens new avenues in chiral sensing, emission control, and topological photonics. An extreme possible consequence of a system's chirality is asymmetric transmission (AT): the difference in total transmittance when polarized light impinges from opposite sides of the system [1]. AT in nanostructures relates directly to potential functionalities such as spin-dependent light emission and enantioselective sensing. To realize AT for linearly polarized light is a significant challenge, as it strictly requires broken mirror symmetry in the propagation direction [2]. Important open questions remain, like how to introduce an efficient symmetry breaking, what value of AT could be maximally achieved, and how to design structures that can offer this maximum AT. In this work, we explain the origin of AT in terms of eigenmodes of a chiral structure and reveal a fundamental limit to AT in any resonant system based on the principle of reciprocity. Using the uncovered design principles, we show a structure that can offer AT as high as 84% for normally incident linearly polarized light.