A Route to Pure Optical Rotation in Self-Assembled Materials through Energetic Nondegeneracy

Abstract

Achieving large optical rotation while preserving linear polarization and minimizing loss─“pure” optical rotation─remains a central challenge in chiral photonics. Solution-processed self-assembled materials can exhibit exceptional chiroptical responses (g-factor > 1) and are promising candidates with near-UV operation and scalability. However, their potential for optical rotation applications is limited because their regions of large circular birefringence (CB, which generates rotation) overlap with regions of absorption and circular dichroism (CD, which makes linear polarization elliptical). Here, we identify energetic nondegeneracy between constituent chromophores as a design principle for pure optical rotation in self-assembled systems. Using a coupled-oscillator framework, we show that detuning chromophore resonances produces collective states that generate CB in spectral regions where CD and absorption are naturally weak. We experimentally validate this mechanism using mixed assemblies of nondegenerate α- and β-CdS magic-size clusters, which exhibit the predicted emergent, off-resonant CB. With our model, we show that maximizing nondegenerate neighbors through ABAB ordering enhances the pure rotation performance. Using optical parameters from inorganic nanoclusters, simulations predict a 50 meV (12 THz) window with >20° optical rotation, >40% transmission, and <1° ellipticity at micron-scale thicknesses, performance metrics typically associated with lithographic metamaterials. Because the mechanism relies only on dipolar coupling and energetic detuning, it is broadly generalizable across wavelengths and chromophore platforms; for example, we show that in gold nanorod assemblies, aspect-ratio tuning can induce the required nondegeneracy. These results establish energetic nondegeneracy as a general mechanism for engineering chiroptical response and realizing pure optical rotation in self-assembled systems.