Twisted Molecular Excitons as Mediators for Changing the Angular Momentum of Light

The molecular absorption of photons with angular momentum can result in twisted excitons with a well-defined quasi-angular momentum. Although they represent different physical properties, photonic and excitonic quanta can both be described in terms of topological charge, a conserved quantity. Multiple absorption events can be used to create a wide range of excitonic topological charges. Subsequent emission produces photons that exhibit this same range. The molecule can thus be viewed as a mediator for changing the orbital angular momentum of light. This sidesteps the need to exploit nonlinear light-matter interactions based on higher-order susceptibilities. A tight-binding paradigm is used to establish topological charge conservation and demonstrate how it can be exploited to combine, subtract, and change the nature of the angular momentum of light. The approach is then extended to a time-dependent density functional theory setting where the key results are shown to hold in a many-body, multi-level setting.