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 are seen to induce the regime of optical limiting to one of increasing transmittance. This is in direct agreement with recent experimental measurements.

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The picosecond optical limiting characteristics of the optical limiting dyes hexamethylindotricarbocyanine iodide (HITCI) and chloroaluminum phthalocyanine (CAP) are compared and contrasted at 532 nm. From single pulse transmittance experiments, HITCI is shown to possess a serious limitation in its optical limiting behavior, where the regime of induced absorption becomes one of induced transmission for high irradiances. CAP, on the other hand, although possessing a smaller ratio of the excited-state to ground-state absorption cross-section, continues to exhibit optical limiting over a much broader dynamic range. From cross-polarized time-resolved excitation-probe measurements, HITCI exhibits a sharp negative spike around zero delay which originates from orientational coherent grating effects, while CAP does not exhibit any spike. These nonlinearities are explained theoretically as a complex interplay between a series of excited manifolds for the dyes and the radiation properties of the interacting laser pulse.