Photobleaching and reorientational dynamics of dyes in a nematic liquid crystal

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Abstract
The polarized fluorescence of excited dyes in a prototype nematic liquid crystal is studied as a function of temperature, polarization of the light, and laser wavelength. We show explicitly the coexistence of photobleaching and dye diffusion through the nematic host as two mechanisms influencing the magnitude of the fluorescence signal. In addition, we exhibit clear evidence of the presence of a torque at low input laser powers that twist the dye molecules with respect to the director orientation of the liquid crystal, if the dyes are resonantly excited. The fluorescence emission in this latter case is able to perceive the birefringence of the nematic liquid crystal host, and this is shown as clear oscillations in the polarized fluorescence as a function of temperature. Extensive qualitative comparisons of the experimental results with the mean-field Maier-Saupe theory of the nematic state are presented, and a model is proposed to account for the observations.