Research On The Nonlinear Optical Properties And Polarization Holography In Azo-containing Polymers

Organic polymers possess large nonlinear optical coefficients, fast response time, highdamage threshold and good flexibility to combination and modification. Thus, much emphasishas been put on organic polymers in recent years due to the great application background inlaser technology, optical communication, optical storage and optical information processing.As one kind of typical organic polymer materials, azo-containing materials have attractedconsiderable interest because of their excellent nonlinear optical properties in wide responsewave band. Meanwhile, when excited with linear polarized light at a wavelength lying in theabsorption region, the azo molecules undergo trans-cis-trans isomerization cycles and reorientperpendicular to the polarized light. This photoinduced orientation process results inmacroscopic anisotropy. Therefore, azo-containing materials are used not only in the field ofnonlinear optics, but also in holographic optical recordings. This thesis focuses on thenonlinear optical properties and polarization holography recordings in azo-containingpolymers.In the first section, we describe the second/third-order nonlinear optical phenomenon andtheir prospecting applications in modern optical technology. In the second part of this section,we introduce the structure characteristics and basic optical properties of azo-containingmaterials, especially the physical mechanism for the photoinduced isomerization cycle of theazo molecules. The research progress of azo-containing polymers in applications of nonlinearoptics and polarization holographic recording is also discussed. At the end, the structure andnovelties of this paper are presented. In the second section, the optical properties of the azo-containing polymers areintroduced in detail. Several experimental methods are analyzed, including single Z-scantechnique, femtosecond Optical Kerr Effect (OKE) measurements, polarization holographicrecording method, etc. The single Z-scan method and femtosecond OKE measurements areused to investigate the nonlinear optical properties of materials. Photoinduced birefringenceand polarization holographic recording methods are employed to indicate the optical storageproperties of azo-containing polymers.In the third section, the nonlinear optical properties of an azobenzene-containing polymerare investigated under different pumping conditions and different molecular environment. Thenonlinear absorption coefficient and nonlinear refractive index of the polymer are determinedby the single Z-scan technique. Different nonlinear responses are observed when excited by fs,ps and ns laser, and the mechanisms for the different optical response inazobenzene-containing polymer are discussed. The polymer also shows different nonlinearoptical response in different environment. The response time of the polymer in film is fasterthan that in solution.In the fourth section, the photoinduced anisotropy of an azo-containing liquid crystalpolymer is investigated. When the azo group was combined with the liquid crystalline (LC)character, anisotropy could be further enhanced by exploiting the inherent anisotropy of LCmaterials. The polymer possesses large photoinduced dichroism (D~0.36) and birefringence(10-2). We investigate the influence of pumping intensity on birefringence value, and anoteworthy observation is that the birefringence did not show any decay, but stronglyincreased after switching off the pumping light. A discussion about the mechanism accountingfor the abnormal relaxation process of birefringence is presented.In the fifth section, based on the photoinduced anisotropy of azo-containing polymers,polarization holograms are recorded by two488nm beams with orthogonal linear and circularpolarizations. For the gratings recorded by two orthogonal linear polarized lights, the diffraction efficiency is strongly dependent on the polarization direction of the probe beam asa consequence of the spatial modulation of the linear and circular photoinduced birefringence.While the gratings recorded by two orthogonal circular polarized lights presents higherdiffraction efficiency, and no circular birefringence is induced in circular polarizationrecording method. The polarization properties of the gratings are analyzed theoretically.In the sixth section, we investigate the polarization induced fluorescence modulationbehavior of a chiral coordination complex using femtosecond laser pulses. The variations ofthe total two-photon-induced fluorescence (TPF) intensity are found to be strongly modulatedby different polarized incident lights and tightly depend on the linearly polarized componentof the excited light. The polarization-induced modulation efficiency of the TPF undergoesintensity-dependent decrease, which can be attributed to the two-photon induced excited stateabsorption.