| With the rapid development of nonlinear optical techniques, the research on nonlinearoptical materials is growing faster and faster. Research on nonlinear optical materialsdepend on convenient and reliable measurement methods. In the past several decades, avariety of different experimental methods have been used to measure the nonlinear opticalproperties of materials. They mainly include nonlinear interferometry, degenerate four-wave mixing, nearly degenerate three-wave mixing, ellipse rotation, and beam distortionmeasurements. Z-scan technique is the represent method of the beam distortionmeasurements. It is a measurement to measure the third-order nonlinear susceptibility ofmaterials by a single-beam. It has a simple experimental setup, high sensitivity, and canmeasure the magnitude and sign of nonlinear absorption and nonlinear refraction.In this paper, we introduce a new measurement technique-a modified top-hat Z-scan(with a combination of aperture and disk) based on the traditional Z-scan technique. Thepaper gives the theoretical derivation and numerical simulation of the technique. Comparedto the traditional Z-scan, the curves of modified top-hat Z-scan for the nonlinear refractionshowed a single peak rather than a peek-valley curve. Furthermore, the sensitivity of thisnew technique can be more than two orders of magnitude enhanced.In this paper, we report a kind of copper phthalocyanine film fabricated by the electro-deposition. We use the modified top-hat Z-scan technique to investigate the nonlinearrefractive response of the film with19picoseconds pulses at wavelength of532nm. Theresearch shows that the nonlinear refraction can not be measured by the traditional Z-scan，but the modified top-hat Z-scan technique can measure the nonlinear refraction. The filmhas a big nonlinear refraction. The research shows that the modified top-hat Z-scantechnique has a good prospect in measuring the nonlinear refraction. |
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