Excited State Absorption Drives Low-Energy Optical Limiting in Oligothiophenes

Optical limiting (OL), a crucial mechanism for protecting human eyes and sensitive sensors from intense radiation, relies on understanding the optical nonlinearities acting on the systems. Assessing and disentangling the effects at play is crucial to predict and control the nonlinear optical response in real materials. In this ab initio study based on real-time time-dependent density-functional theory, we investigate non-perturbatively the absorption spectra of a set of thiophene oligomers, the building blocks of technologically relevant organic semiconductors, excited by broadband radiation of increasing intensity. Under strong electric fields, the absorption cross section grows significantly below the onset of linear excitations, exhibiting saturation typical of OL. By exciting the oligothiophenes with a train of pulses targeting the first and second excited states of each moiety and analyzing the resulting population dynamics, we reveal excited-state absorption (ESA) in the near-infrared to visible region. Our results indicate ESA as the driving mechanism for OL in oligothiophene molecules, thereby providing important insight to design novel compounds with optimized nonlinear optical characteristics.