Interplay between self-focusing and high-order multiphoton absorption

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Abstract
We study the distortion of optical beams that is due to the combined effects of strong self-focusing and three and four-photon absorption, a situation that exists, for example, in the polydiacetylene bis-paratoluene sulfonate (PTS). The characteristic nonlinear distances were defined for each process. Theoretical analysis of the beam propagation leads to two distinct limits, one limit dominated by self-focusing and the other by higher order absorption. Propagation was studied analytically and numerically for cw and pulsed beams in these two limits and for cases in which both nonlinear effects are important. It was found that beam distortion caused by multiphoton absorption and refraction leads to situations in which diffraction plays an important role, even for input beams whose diffraction length is much larger than the sample length. It was concluded that, for the typical intensities used in Z-scan measurements, nonlinear processes and diffraction contribute simultaneously to beam distortion and must be taken into account.