Nonlinear Optical Properties Of π-conjugated Polymers And A Novel Z-scan Theory

Polyaniline (PAn) and Poly-phenylene vinylene (PPV) are organic polymers with π^-conjugated structure. Due to their good electric conduction and luminescence, people investigate mainly preparation of photoelectric devices and luminescent diodes in these years. Less studies on nonlinear optical properties were reported. However, because of the easily delocalized n electrons , they have more significantly different optical properties than common polymers, such as large third-order nonlinear optical susceptibilities , ultrafast optical response time , good chemical and thermal stabilities. The material properties can be improved through the modification of the backbone and side chain thanks to their easily being cut out. These organic polymers are tended to manufacture because of low price of materials. Therefore they have widely potential application in optical switch, optical storage, optical limiter, optical computation and communication.The third-order nonlinear optical properties of two kinds of π-conjugated polymer, Polyaniline and Poly-phenylene vinylene, were studied. Using N-methyl-pyrrolidone (NMP) as solvent, we prepared PAn/ NMP solution and methyl-replaced PAn/NMP solution. Using chloroform as solvent, we prepared 2-methoxy-5-butoxy-PPV /chloroform solution and 2-methoxy-5-decoxy-PPV /chloroform solution.On experimental, the absorption spectra of the samples were measured. We carried out closed-aperture and open-aperture Z-scan experiments. But we did not obtain the open-aperture Z-scan curve, which indicate that the samples' nonlinear absorption are extremely small, therefore the nonlinear absorption could be neglected. The nonlinear refraction index n2 were estimated according to the closed-aperture Z-scan curves. We also did the laser-induced self-diffraction experiment on 2-methoxy-5-butoxy-PPV. Nonlinear constant r| could be calculated according to the numbers of thediffraction rings. The sample's optical limiting properties were discussed. According to the experimental results, we estimated that the main mechanism of the optical limiting was from the thermo-optic effect.On theoretical, a diffraction model of a nonlinear optical medium to a Gaussian beam was built based on Fresnel-Kirchhoff diffraction theory . The propagation behavior of the Gaussian beam passing through the nonlinear optical medium was theoretically deduced and numerically simulated, which could explain the Z-scan phenomenon from a new approach. It provides a new way to calculate the nonlinear refraction index and the two-photon absorption coefficients. Numeric simulation showed that for the closed-aperture Z-scan considering the two-photon absorption, the simulation results were well consistent with the conventional theory. For a given open-aperture Z-scan curve, the two-photon absorption coefficient computed by the new Z-scan theory was 1/3 of the value fitted by the conventional Z-scan theory, whereas other conclusions were identical to the conventional Z-scan theory. This theory has better accuracy than the conventional Z-scan theory because the approximate conditions are only thin sample and small nonlinear absorption. From this theory, a unified equation was derived, which could describe both the closed-aperture Z-scan curve and the open-aperture Z-scan curve with consideration of the two-photon absorption. This can not be given by the conventional Z-scan theory. This theory is also a good method for studying propagation properties of the Gaussian beam passing through the nonlinear media.