Dynamics Of The Third-Order Optical Nonlinearity From Photo-Excitation Induced Reorientation In Dye-doped Liquid Crystal Systems

Optical control and modification of materials properties are an interesting and important problem. Guest-host interaction has long been a subject of interest in many disciplines. Emphasis is often on how a small amount of guest substance could significantly affect the properties of a host material. Guest-host interaction is particularly important. As is well known, the electric field of a light beam exerts a torque on anisotropic molecules to re-orientate molecules is the main mechanism for optical nonlinearity in transparent liquids. However, the phenomenon of light-induced molecular orientation itself can be strongly modified when the liquid is absorbing. In particular, as observed recently, mixing small fractions of some dyes in liquids can enhance the molecular orientation by orders of magnitude. That opened a door to expand the applications of the liquid crystals.This thesis emphasis on how the dyes, which can be photoisomerized, interact with the host molecules (such as polymer and liquid crystals) and realized the control or modification of the properties of the materials. By using various techniques, such as transient absorption spectra, transient optical Kerr Effect (OKE), time-resolved two color Z-scan methods, we investigated the dynamics of the isomerization enabling dye-Liquid Crystal system after being excited.This thesis focuses on following aspects:First, and most important, we studied the induced transient dichroism and the transient optical Kerr effect (OKE) response of azo-doped isotropic liquid crystals inthe picosecond time scale. Photoisomerization occurs when the azo-dye molecules are resonantly excited. As a result, significant enhancement of OKE signal was found in the dye doped liquid crystals with respect to pure liquid crystal sample. The enhancement was attributed to the extra torque exerted on liquid crystals after anisotropic orientational distribution of trans and cis isomers are generated by linearly polarized light. A modified mean field theory model was raised to treat interactions of trans and cis isomers to liquid crystals. The pump intensity relationship of the enhancement was also investigated.Second, in order to make clear the transient orientation torque direction in isotropic azo-dye doped liquid crystals, we carried out the two color time-resolved Z-scan experiment. The technique allows us to obverse both the sign and magnitude of the nonlinear refractive index lens effect of the liquid crystal. The photon-induced reorientation enhancement in the Azo-dye-doped liquid crystals was also observed. It shows that the molecular orientation will be along the pump light after the reorientation.Third, we also study the influence of the photoisomerization on the THG in dye doped polymer films. The phase matched THG measurements are also preformed in the nematic LC and AQ doped LC. And we also found it is very difficult to reorient in the planar nematic LC box even with the help of the AQ dye.