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

Abstract

Achieving large optical rotation with minimal ellipticity and absorption, 'pure' optical rotation, remains a central challenge in chiral photonics. Solution-processed self-assembled materials can exhibit exceptional chiroptical responses (g-factors > 1), yet their circular birefringence (CB) typically overlaps with circular dichroism (CD) and resonant loss (Absorption), leading to elliptical, attenuated signals. Here, we establish a general, theory-guided design principle showing that non-degeneracy provides a route towards pure optical rotation in self-assembled systems. Using a generalized coupled-oscillator framework, we demonstrate that breaking degeneracy between the excited states of interacting chromophores produces CB in spectral regions where CD and absorption are naturally weak. We experimentally validate this mechanism using mixed assemblies of $\alpha$- and $\beta$-CdS magic-sized clusters, which exhibit the predicted off-resonant, emergent CB. Guided by this principle, we design a layered architecture that maximizes non-degenerate neighbors through alternating chromophore planes. This structural architecture results in optical response lineshapes optimized for pure rotation. Because the mechanism relies solely on dipolar coupling and energetic detuning, it is generalizable across wavelengths, including in the ultraviolet (~310 nm), where suitable nanocrystal and organic chromophores are readily available. Simulations predict a 50 meV (12 THz) window exhibiting low-dispersion optical rotation of ~20{\deg}, >40% transmission, and <1{\deg} ellipticity-strong performing benchmarks typically associated with lithographic metamaterials. These results establish non-degenerate coupling as a general mechanism for engineering chiroptical response and provide a strategy for realizing pure optical rotation in self-assembled systems.