Abstract

Stimulated Rayleigh–Bragg scattering (SRBS) in highly two-photon active chromophores’ solutions and in a doped polymer rod has been investigated. Under optimized experimental conditions, the energy conversion efficiency from the input 816-nm and $\sim\! 10 {\text -} {\rm ns}$ pump pulses to the backward SRBS pulses was up to $\eta = 58\%$ in the solution of AF450 chromophore in methyl methacrylate (MMA) liquid of 0.001 M concentration, while for the AF450 doped poly(methyl methacrylate) rod of the same concentration the measured value was only $\eta = 0.16\%$. Considering that the two-photon absorption properties of the chromophore molecules are basically the same in both cases, the above-mentioned results strongly suggested that the formation of the induced Bragg grating in a solution is mainly due to spatial redistribution of the dopant molecules driven by two-photon resonance enhanced gradient force. In contrast, the same dopant molecules are “frozen” in a solid matrix which prohibits the spatial redistribution of these molecules from forming an effective Bragg grating. Based on the high efficiency of backward SRBS generation as well as the coexisting two-photon absorption, the intense input laser beam will undergo a much stronger nonlinear attenuation. For the tested AF450/MMA solution sample of 0.004 M concentration, when the input laser pulse energy changed from 0.06 mJ to 3.7 mJ, the measured nonlinear transmissivity decreased from 0.82 to 0.11, showing a superior nonlinear attenuation performance with regard to the input pump laser beam.

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