Optimized Photothermal Lens Determination of Nonlinear Absorption
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An optimized pump–probe mode-mismatched photothermal lens experiment aimed at determination of nonlinear absorption of an optical sample is reported. The pump beam generates a local thermal gradient or thermal lens that is tested by the probe light. The pump beam is tightly focused, and the probe beam is highly collimated. Changes in the probe light transmission through a small aperture located at some distance from the sample provide the signal. Scanning of the sample around the focal point yields a single-peaked Z-scan signature with a width several times larger than the pump Rayleigh range for linear absorption. If nonlinear absorption is dominant, the width of the peak is significantly smaller and of the order of the Rayleigh range of the pump field. If linear and nonlinear absorption are present simultaneously, a double-peaked Z-scan signature is obtained. In this situation, the linear and nonlinear absorption contributions can be easily separated and compared to each other for calibration purposes. Using the known values of linear absorption, nonlinear absorption coefficients can be estimated with good accuracy. The method is tested by studying nonlinear absorption in nitrobenzene and iron oxide water colloids. The values of the effective nonlinear absorption coefficients are determined. The physical origin of nonlinear absorption in both types of samples is also discussed.
KeywordsNonlinear absorption Photothermal lens Two-photon absorption Z-scan
This research has been possible thanks to the support of the National Science Foundation (NSF-CREST grant 0630388 and NSF-MRI grant 0922587) and of the National Aeronautics and Space Administration (NASA URC 5 grant NNX09AU90A). The author would like to thank Dr. Raymond Edziah from the Department of Physics and Pre-Engineering at Delaware State University for critical reading of the manuscript.
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