Pseudo-nonlinear absorption in z-scan measurements

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In the present study, pseudo-nonlinear absorption characteristics are numerically demonstrated in z-scan measurements even for samples of pure linear absorption materials. The pseudo-nonlinear absorption curves look similar to the nonlinear absorption curves in an open-aperture z-scan measurement. Numerical results reveal that the plausible nonlinear effects stem from two sources, namely the changes in the incident angle and azimuth polarization of the focused beam during the z-scan measurement. It is found that when the laser beam is focused by a lens with a large numerical aperture, the influence of the pseudo-nonlinear absorption due to the change in incident angle becomes substantial and cannot be ignored, whereas if a lens with a low numerical aperture is used, the deviation caused by the pseudo-nonlinear absorption is negligible. On the other hand, for a laser beam focused by a lens with a large numerical aperture, the polarization azimuth angle of the incident beam can be adjusted to the corresponding critical angle, so that the plausible nonlinearity is eliminated. Finally, the theoretical analysis is verified experimentally with a single BK7 glass slice sample and objective lens with numerical apertures of 0.09, 0.45, and 0.65 using a z-scan setup with a laser at 488 nm wavelength. The findings are useful for improving the z-scan measurement of the nonlinear absorption coefficient in terms of sensitivity and accuracy.