Nonlinear absorption spectra of transparent organic crystals for optical limiting applications at visible wavelengths

We have studied nonlinear absorption spectra and optical limiting propertie s of transparent molecular organic crystals. Measurements performed in the 450-650 nm range, using nanosecond pulses, show that these materials have l arge and broadband nonlinear absorptions. Efficient optical limiting proper ties are obtained due to crystal packing effects: high active molecule conc entration, specific orientation, and enhanced local field factor. In the nanosecond regime, the nonlinear absorption is a two-step process: t wo-photon absorption followed by an excited-state absorption. At resonance, the nonlinear response of studied crystals spans over two orders of magnit ude in agreement with quantum chemistry calculations. (C) 2000 Elsevier Sci ence S.A. All rights reserved.