Non-reciprocal linearly polarized light in simple media.

Reciprocity-the principle that a response is identical along the forward and backward paths-is a fundamental concept across physics. Non-reciprocity occurs when this symmetry is broken, resulting in direction-dependent behaviour. Achieving optical non-reciprocity typically requires complex metamaterials, exotic media or strong fields. Researchers have overlooked the possibility that conventional materials could support optical non-reciprocity. Here, through the Stokes-Mueller formalism, we predict a pathway to non-reciprocal absorption and emission of orthogonal linear polarizations. We test this idea using solution-processed films of CdS, CdSe and CdTe magic-size clusters with comparable circular and linear dichroism, and demonstrate non-reciprocal absorption and emission of linearly polarized light. Based on these findings, several design rules and practical applications are presented. Our work reveals that non-reciprocal linear dichroism and emission can be achieved in readily processable materials by harnessing chiral-linear optical interference, providing opportunities within polarization-based quantum optics and photonics such as direction-dependent optical routing or polarization-multiplexed encryption.