Bulk Optical Fréedericksz Effect: Non Linear Optics of Nematics Liquid Crystals in Capillaries

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Abstract
We investigated the behavior of a beam experiencing the Optical Fréedericksz effect while traveling through a nematic liquid crystal confined in a capillary and doped with a dye. The source is a specially tapered fiber that delivers a narrow and well collimated beam. We present the results we obtained with two different nematics, having different birefringence and aligned in two different ways along with the capillary wall. In the case of an alignment normal to the wall (radial escaped structure), we observed an optically induced displacement of the homeotropic central part of the structure and even a splitting of it. In the case of a planar alignment (fully axial structure), we observed a self focusing and a « selfwaveguiding » behavior, different from the usual filaments. We also give an experimental evidence of the difference between the Fréedericksz threshold and self trapping threshold. In both cases, the light acts on a larger volume than the so far reported non linear optics experiments, therefrom the title. Our results are compared with those obtained in a similar geometry and a pure nematic by other authors. Finally, the non linear behavior of the liquid crystal is discussed.

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