Unveiling High Electro‐Optic Performance in a Proton–π‐Electron‐Coupled Ferroelectric Crystal

Abstract The convergence of electronics and photonics is attracting attention for its potential to surpass performance limitations of existing information‐processing devices. In particular, the electro‐optic (EO) effect plays a critical role in high‐speed and low‐power conversion between electrical and optical signals, which is demanded for future communication networks. Here, a novel class of EO material is demonstrated, the organic ferroelectric crystal of croconic acid (CRCA). The recently developed birefringence field‐modulation imaging technique enables high‐throughput evaluation of the EO coefficient for as‐grown bulk crystals, unveiling a figure of merit of >400 for CRCA, which exceeds that of 320 in the conventional EO material LiNbO 3 in the visible‐light range. Analyses in conjunction with theoretical calculations clarify that its remarkable EO performance is attributable to deformation of the π‐orbital coupled with the proton displacement. This finding provides a new route for the molecular design of high‐performance EO materials: proton–π‐electron‐coupled ferroelectrics.