Study On Z-Scan Diffraction Theory For Large Optical Nonlinear Phase Shift

This paper built a diffraction model of nonlinear optical media to Gaussian beam for larger optical nonlinear absoption phase shift which is based on Fresnel-Kirchhoff diffraction theory.Closed aperture z-scan experiment was finished with CS₂ solution that was in accord with our theroy. Numerical results show that the peak of the transmittance is greatly enhanced and the valley is suppressed with larger refraction phase shift.While the larger absorption phase shift makes the peak of transmittance severely suppressed and the valley grately enhanced.The ratio of these two phase shifts decide whether the valley-peak structure is existed.When the ratio varies from minus 0.3 to 0.3,the walley-peak structure is so obviously.The far-field output power is not completely enhanced with the increasing nonlinear absorption coefficient.Besides,open-aperture z-scan theory is also theoretically deduced and numerically simulated.The theory model is further improved when the factor M is introduced.Experiment was finished with C₆₀toluene solution which was in accord with our theory.Numerical results show that the output power is attenuated and surged with the increased input laser power whenΓis equal or samller than 0.15.The optical limiting phenomenon is observed whenΓis larger than 0.15.This theory model is more accurately than the conventional z-scan theroy because the applicable condition is only thin sample.The nonlinear absorption properties of polyaniline/carbon natotube composite films were investigated using 8 ns pulses at 532nm.The excited absorption cross sections of 3.5%and 7.0%PANI/MWCNT films areσ(ex)=2.89×10¹⁹cm² andσ(ex)=2.76x10¹⁹cm²,both larger than their groud-state cross sections.The nonlinear absorption coefficient of these two materials areβ=1.11×10^-7cm/W andβ=1.51×10^-7cm/W,respectively.The experimental results show that the compoud own strong reverse saturable absorption and could be applied in the area of optical limiting.The main origin of its nonlinearity isÏ€-Ï€interaction between MWCNT and PANI,which makes Frenkel exciton polarizated and coupled.