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Opto-dielectric effect on a nematic liquid crystal doped with a photoactive azo mesogen Dynamic holography and polarimetric techniques are used to study the photoisomerization kinetics of D2 bisazo dye and the excitation transfer from dye to nematic liquid crystal host. As a result of this transfer, transient modulation of the orientational order parameter, three-dimensional long-scale molecular transient rotation and equilibrated two-dimensional molecular rotations are identified in the host. Amplified rotation of nematic molecules is observed which is much more efficient for the dye photoexcitation with a wavelength in the red rather than in the blue side of its absorption band. The possible role of dye photoisomerization mechanism in these processes is discussed. The dynamic character of the dye induced torque and the presence of long-living intermediate states during photoisomerization and their diffusion are suggested.

Wc discuss the occurrencc of thc dynamic Stark cffect in resonant CARS spectroscopy for a homogeneously broadcned medium. We use a four-level systcm interaction with threc monochromatic ficlds of different frcqucncics. The equations of the density matrix are obtaincd in a rotating frame and solved numerically for the steady-state solutions. Analytical expressions are derived for five cases and show a splitting or a broadening of the CARS linc. Stark shifts due to intensc off-resonance fields are discussed.Nous discutons d'apparition de I'cffet Stark dynamique en speetroscopie CARS resonantc. pour un milieu tlargi de f a~o n homogene. Nous utilisons un systbme a quatre niveaux cn interaction avec trois ehamps monochromatiques de frCqucnces difftrentes. Nous obtenons les equations de la matricc densitt dans un rtftrentiel tournant ct les rCsolvons numtriquement en rtgime stationnairc. Pour cinq cas, nous dtrivons des cxprcssions analytiqucs qui dtmontrent un dtdoublement ou un Clargisscment dc la raie CARS. Lcs dtplaccmcnts de la raie dus h dcs champs intcnscs et nonrtsonants sont discutts.

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Dynamic holograms are created and studied in a D2 azo-dye-doped nematic liquid crystal. The dependence of diffraction on the excitation and probe polarizations is observed. The efficiency of excitation and the corresponding relaxation time depend also on the period and orientation of the light-interference pattern with respect to the initial molecular orientation. The excitation and diffusion of dye molecules are suggested to be the origin of these phenomena.