| Nematic liquid crystals possess wonderful physical and optical properties which are suitable for the reaserch and application for optical nonlinearity.In nematic liquid crystals doped with absorbing azo dye molecules,studies have shown that it is an effective mechanism for creating large optical nonlinearity.Azo dye doped nematic liquid crystals have attracted much attention owing to their promising applications in holography display,optical storage,optical limiting and nonlinear optical devices.Azo dye doped nematic liquid crystals have many merits,such as low cost,easy fabraction and low power.In this manuscript,we investigate the temperature dependent photoinduced birefringence and diffraction properties of photoinduced grating in Disperse Red 1(DR1)doped nematic liquid crystals.Otherwise,we investigated the off-resonant nonlinear optical properties and holographic grating in DR1 doped nematic liquid crystals.In the first section,we introduce the nonlinear optics,its application and the property of the nonlinear optical materials.Then we introduce the optical nonlinearity of liquid crystals and the application in holography display.Third we introduce the property of azo dye and the impact of optical nonlinearity in azo dye doped liquid crystals.The development of holography display in azo dye doped liquid crystals is introduced.Finally the main content of this manuscript are outlined.In the second section,we introduce the property of Disperse Red 1 doped 5CB nematic liquid crystals.Then the principle of self-phase modulation method for optical nonliearity,photoinduced birefringence and holography are introduced.In the third section,the temperature dependence of birefringence in azo dye doped nematic liquid crystals with homogeneous alignment was investigated.As the sample temperature approached to the phase transition temperature,oscillations signals of transmittance in the system could be observed.Otherwise,the reversible photoinduced transmitted signals resulted from azo dye molecules could be controlled by temperature,which can be used in reversible thermo-optical controller.Diffraction properties of photoinduced gratings recorded by overlapping two coherent beams at 532 nm in nematic liquid crystals doped with Disperse Red 1 were investigated with a probe beam at 632.8 nm.The diffraction efficiency of the photoinduced grating was found to increase rapidly when the sample temperature was close to the clearing point in the nematic phase and a nearly 30-fold enhancement of the first-order diffraction efficiency was obtained.The pretransitional enhancement of the diffraction efficiency was discussed in terms of the reorientation of liquid crystals,optical nonlinearity effects and the onset of critical opalescence near the nematic-isotropic phase transition.Moreover,a peak shift of diffraction efficiency towards the lower temperature was observed with the increase of recording light intensity,which was attributed to laser induced photochemical disordering.In the fourth section,nonlinear optical properties of azo dye doped nematic liquid crystals were investigated with a He-Ne laser(λ = 632.8 nm)as off-resonant light irradiation using spatial self-phase modulation method.Compared with the experimental results of resonant light(λ = 532 nm),the sample irradiated by He-Ne laser exhibited lower optical threshold and the number of diffraction rings was sensitive to the laser polarization.The mechanism of nonlinear optical response of azo dye doped nematic liquid crystals at 632.8 nm could be mainly attributed to photoisomerization effect.Moreover,the diffractions of holographic gratings recorded by He-Ne laser in azo dye doped liquid crystals were consistent with the grating formation based on photoisomerization effect.Holographic gratings in nematic liquid crystals doped with Disperse Red 1 were investigated using a linearly polarized He-Ne laser.Although the electric transitions of the samples were almost off-resonant,the diffraction of the transient holographic grating was found to exhibit a sinusoidal feature with changing the polarization of recording beams and the diffraction intensities and response time of the gratings were strongly dependent on DR1 concentration.The results revealed that the diffraction of holographic grating depends on the rate of trans-cis isomerization of DR1 molecules.Moreover,the temperature dependence of diffraction indicated that the grating formation mechanism was attributed to photoinduced reorientation of photoisomerization effect. |
