Origin of thickness-dependent circularly polarized luminescence in chiral polymer films

An initially achiral polymer, when blended with small chiral molecules, can adopt a chiral structure and become optically active. The resulting material exhibits circularly polarized luminescence (CPL), which has numerous applications in display technologies, optical quantum computing, and biosensing. A recent experimental study reported fabrication of a chiral polymer film with exceptionally large CPL [L. Wan et al. ACS Nano 13, 8099 (2019)]. Interestingly, this CPL can be made to invert handedness by merely changing the thickness of the film. Here, we present a comprehensive analytical theory that explains this unusual dependence of the polymer’s CPL on film thickness. The chiral structure of the polymer is modelled as helically twisted fibres, in which circularly polarized light is emitted by excitations propagating along the twisted polymer chains. We reveal that the thickness dependence of CPL stems from the interplay between the spatial dispersion of light and strong delocalization of excitons in the polymer. We further explore the possibility of maximising CPL through alignment of the polymer chains within the film. The results of our study will prove useful in designing chiral polymers with tunable chiroptical properties for photonic applications.