Anomalous Circular Birefringence in Isomorphous Mixed Crystals

ABSTRACT The anomalous linear birefringence of mixed crystals, inconsistencies between optical and morphological symmetries, have been studied more extensively for mixed crystals of isomorphic cubic nitrates of strontium, barium and lead than for any other family of laboratory‐grown crystals. Anomalous circular birefringence, an unanticipated dissymmetry leading to different refractivities for circularly polarized rays, on the other hand, is harder to detect and has not been described in two centuries during which the etiologies of optical anomalies were debated. We quantify anomalous circular birefringence in Ba x Pb 1‐ x (NO 3 ) 2 and Sr x Pb 1‐ x (NO 3 ) 2 mixed crystals using polarimetric imaging. This circular birefringence is not natural optical activity but arises from the non‐commutativity of successive linearly anisotropic sub‐volumes that intersect the incident wave vector. The signal is best described as an emergent optical activity generated by orientation gradients across linearly birefringent domains. The effect was simulated by treating each growth sector as a uniaxial crystal misaligned with respect to the adjacent sector oriented along an adjacent cube body diagonal. The magnitude and spatial distribution of the experimentally observed linear and circular retardance are reproduced with fidelity.