Multilevel rate-equation analysis to explain the recent observations of limitations to optical limiting dyes

A theoretical rate-equation analysis is presented to investigate the molecular properties that are important for achieving high-fluence optical limiting. Critical conditions for achieving induced absorption are derived in terms of the material and laser parameters by employing a range of hierarchical energy-level models. The influences of stimulated emission and saturation of the excited-state absorption an seen to induce the regime of optical limiting to one of increasing transmittance. This is in direct agreement with recent experimental measurements.