Experimental and numerical investigation of highly absorbing nonlinear organic chromophores

We have developed a mathematical/numerical framework based on computational transition modules and measured ultrafast laser light propagating through nonlinear materials. The numerical framework can be applied to a broad set of photo-activated materials and lasers, and can optimize photo-physical parameters in multi-photon absorbers. Two photon (TPA) processes are particularly useful in many applications including fluorescence imaging, optical data storage, micro-fabrication, and nanostructured quantum dots for optical limiters. Laser transmission measurements of the organic molecular chromophore, AF455-known TPA material-were taken with a 175 fs, λ0=780nm, plane-polarized light pulses from Ti:S regenerative amplifier into a 5.1mm thick PMMA slab doped with the chromophore. The range of input energies (intensities) in this experiment was 0.01μJ (0.97 GW/cm2) to 25 μJ (2.4 x103 GW/cm2). Experiments showed that for intensities beyond several μJ, the material did not saturate as predicted by traditional theory. We included excited-state absorption (ESA), as demonstrated by the absorption spectrum, which still could not account for the deviation. To understand this result we used our framework to show that an unexpected/unknown higher energy level was being populated. We calculated the entire experimental curve from 0.01μJ (0.97 GW/cm2) to 25 μJ (2.4 x103 GW/cm2) and found excellent agreement with the experimental data.

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