We show that it is possible, in Z-scan measurements of thin films, to obtain data that closely resemble typical results for nonlinear optical materials, but which actually arise from linear optical effects caused by sample damage. Z-scan measurements on a silica-based thin film yielded the expected peak–valley signature of Z-scan data, but subsequent analysis and microscopic examination of the film indicated that the data resulted from an ablation hole produced in the film when it was near the laser focus. The resulting spatial variation of the linear refractive index of the film produced a lensing effect that mimicked the typical Z-scan response. Scalar diffraction theory was used to model the effects of a spatially varying refractive index and gave results that qualitatively agreed with the Z-scan measurements.
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