Nonlinear exciton equation factorization for non-perturbative absorption modelling

Various types of optical spectra of molecular systems are often analyzed via perturbative series expansion in the powers of optical field. The simplest absorption is related to the linear optical response. However, observed spectral features can be mislabelled if higher orders are not vanishing. High-intensity excitation field breaks the established assumption of quickly converging perturbative regime. Non-perturbative quantum methods can solve these problems. However, they lead to endless hierarchies of equations that, in general, cannot be solved analytically. Dropping terms at a specific order or factorizing (expressing high-order terms as products of several lower-order terms) can be used to close the hierarchy. We propagate the nonlinear exciton equations (NEE) with exciton–exciton annihilation (EEA) non-perturbatively in a high-excitation regime and calculate absorption spectra of a molecular aggregate using various factorization schemes. The results demonstrate that the solution is weakly sensitive to the factorization method when EEA is included.