Analysis of asymmetric Z-scan measurement for large optical nonlinearities in an amorphous As_2S_3 thin film

We have extended the conventional Z-scan theory by employing an aberration-free approximation of a Gaussian beam through a nonlinear medium and derived a simple analytical formula for Z-scan transmittance, including the effects of both nonlinear absorption and nonlinear refraction, which could be applicable to the sample with large nonlinear phase shifts. We verified the extended Z-scan theory in an amorphous chalcogenide As2S3 thin film by measuring the Z-scan transmittance with both open and closed apertures. The nonlinear refractive index γ=7.6×10-5 cm2/W and the nonlinear absorption coefficient β=1.6 cm/W of As2S3 were measured at subbandgap 633-nm illumination.

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