We have extended the conventional <i>Z</i>-scan theory by employing an aberration-free approximation of a Gaussian beam through a nonlinear medium and derived a simple analytical formula for <i>Z</i>-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 <i>Z</i>-scan theory in an amorphous chalcogenide As<sub>2</sub>S<sub>3</sub> thin film by measuring the <i>Z</i>-scan transmittance with both open and closed apertures. The nonlinear refractive index γ=7.6×10<sup>−5</sup> cm<sup>2</sup>/W and the nonlinear absorption coefficient β=1.6 cm/W of As<sub>2</sub>S<sub>3</sub> were measured at subbandgap 633-nm illumination.
© 1999 Optical Society of America
(160.2750) Materials : Glass and other amorphous materials
(160.4330) Materials : Nonlinear optical materials
(190.3270) Nonlinear optics : Kerr effect
(190.5940) Nonlinear optics : Self-action effects
Chong Hoon Kwak, Yeung Lak Lee, and Seong Gyu Kim, "Analysis of asymmetric Z-scan measurement for large optical nonlinearities in an amorphous As2S3 thin film," J. Opt. Soc. Am. B 16, 600-604 (1999)