Study On The Nonlinear Optical Mechanism And Photoinduced Anisotropy In Azo-containing Polymers

The field of nonlinear optics has continued to achieve new advances both in fundamental physics and in practical applications in recent years. Nonlinear optics'enormous application background in optical communication, optical information processing, optical storage and laser technology has been showed in the last decades. Therefore, it is of importance to design and synthesize new materials which have strong and quick nonlinear optical response. Azo-containing material is one kind of typical organic materials which have excellent nonlinear optical property. Azo-containing polymers have attracted considerable interest for their conjugated molecular structures, easy processing, good chemical stability, etc. Especially, azo molecules undergo trans-cis-trans isomerization cycles under the irradiation at a wavelength lying in the absorption region. This photoinduced orientation results in macroscopic anisotropy. Thus, azo-containing polymers can be widely used not only in the field of nonlinear optics, but also in optical storage. This thesis mainly focused on the nonlinear optical property of azo-containing polymers. We discussed the nonlinear optical mechanism photoinduced anisotropy in azo-containing molecules in detail.In the first section, the nonlinear optics, nonlinear optical material and several experimental methods for second/third-order nonlinear optics are introduced. The nonlinear optical property and photoinduced isomerization in azo-containing materials are also induced in this section. Furthermore, several mechanisms for the third-order nonlinear optical response in materials (e.g. azo-containing molecules) are listed. At the end of the section, the structure and novelties in this thesis are presented.In the second section, Properties of azo-containing polymers, including of their common optical properties, thermal and chemical stabilities, etc. are introduced. Especially the characteristics of nonlinear optics and photoinduced anisotropy are emphasized. Then some experimental techniques are discussed in the section. Z-scan method is used to investigate the nonlinear optical property of materials. The experimental methods of two ways to indicate the photoinduced anisotropy: photoinduced dichroism and photoinduced birefringence are introduced.In the third section, the theory and experimental method of Z-scan technique, which is the most common way to measure the third-order nonlinearity of materials, was introduced. Research advances on this technique in recent years was reviewed. And then, based on Fresnel-Kirchhoff diffraction theory, we set up a new model to calculate the propagation behavior of near top-hat beams through the nonlinear optical medium. The accurate z-scan curves under near top-hat beams are obtained and the characteristics of curves are discussed. In the last part of this section, we introduced a new design way for Z-scan experimental set-up under pulse laser irradiation. The progress for the building up Z-scan system was introduced briefly.In the fourth section, three kinds of materials: the liquid crystalline azo-dendrimer, a series of azobenzene liquid-crystalline materials which have different side-chain lengths and a series of azobenzene polymer which have different substitutes in their molecular structure from one to another are investigated for their third-order nonlinear optical properties. Different pump laser conditions, such as different laser pulse's widths and wavelengths, and different molecular environment (e.g. in solution or in film) are employed to investigate mechanism of materials'nonlinearity. We calculated the contribution of laser heating induced nonlinearity in materials under irradiation of longer pulse laser. The nonresonant electronic nonlinearity which is relevant to the molecular structure is discussed. Effects of some factors, such as density of chromospheres, aggregation of chromospheres, length of molecular chain and substitute group, are studied in detail. Trans-cis isomerization in azo-group is also mentioned in the origin of nonlinearities of azo-containing polymers.In the fifth section, the photoinduced anisotropy which comes from trans-cis isomerization in azo molecules is studied. In the first part of the section, a methyl red doped polymer thin film is employed. Its third-order nonlinear refraction is investigated using lasers which have different polarization states. The mechanism responsible for the process of nonlinear refraction was discussed in the process of photoisomerization. And then, we studied the photoinduced dichroism and photoinduced birefringence in an azobenzene ionic liquid crystalline polymer. The photoinduced birefringence of liquid crystalline azo-dendrimer is also investigated under picosecond and CW regimes.In the sixth section, a series of polythiophenes were firstly investigated using Z-scan technique to further study on the mechanism of third-order nonlinearity inπ-conjugated polymers. The results indicated that some important factors, such as intensity, density and length ofπ-bonds, symmetry of molecules and substituents on the chain, can affect the nonlinear optical response. Secondly, nanoparticles materials doped in azo-containing molecules can induce the enhancement of nonlinear optical response. To study the mechanism of nonlinearity's enhancement, the third-order nonlinearity and fluorescent property in metal Ag colloidal solutions are investigated. The fluorescent spectra indicate the effects of nanoparticles'size and structure. Moreover, the data fitting result of optical limiting (OL) response of metal Ag colloidal solution indicated that the nonlinear absorption was attributed to two-photon absorption effect at 532 nm.