A Multi-Scale Approach to Simulate the Nonlinear Optical Response of Molecular Nanomaterials

Nonlinear optics is essential for many recent photonic technologies. Here, we introduce a novel multi-scale approach to simulate the nonlinear optical response of molecular nanomaterials combining ab initio quantum-chemical and classical Maxwell-scattering computations. In this approach, the first hyperpolarizability tensor is computed with time-dependent density-functional theory and translated into a multi-scattering formalism that considers the optical interaction between neighboring molecules. A novel object is introduce to perform this transition from quantum-chemistry to classical scattering theory: the Hyper-Transition(T)-matrix. With this object at hand, the nonlinear optical response from single molecules and also from entire photonic devices can be computed, incorporating the full tensorial and dispersive nature of the optical response of the molecules. To demonstrate the applicability of our novel approach, the generation of a second-harmonic signal from a thin film of a Urea molecular crystal is computed and compared to more traditional simulations. Furthermore, an optical cavity is designed, which enhances the second-harmonic response of the molecular film by more than two orders of magnitude. Our approach is highly versatile and accurate and can be the working horse for the future exploration of nonlinear photonic molecular materials in structured photonic environments.