Photoacoustic Z-scan for the measurement of third-order nonlinear absorption coefficient

The third order optical nonlinearities cover a vast and diverse area in nonlinear optics. The third order nonlinear susceptibility χ⁽³⁾ is a complex quantity and its real and imaginary components represent nonlinear refraction and nonlinear absorption, respectively. Measurement of these parameters is important for many practical applications. Z-scan is a simple optical technique for determining these characteristics with high accuracy. Here we present a novel photoacoustic Z-scan (PAZ-scan) technique that combines the advantages offered by the conventional Z-scan method and the sensitivity of the photoacoustic detection. In PAZ-scan, instead of measuring the transmitted optical signal as in the case of traditional Z-scan, we record the generated photoacoustic (PA) signal using a 10 MHz focused ultrasound transducer while the sample is translated along the focused laser beam. Since the photoacoustic signal strength is directly proportional to the optical absorption, PAZ-scan displays nonlinear behavior depicting the nonlinear optical absorption of the material. For reverse saturable absorber (RSA) materials, absorption and hence the PA signal increases with the increase of input intensity. Similarly for saturable absorber (SA) materials, absorption and the associated PA signal decreases with increase in input intensity. We studied the nonlinear absorption properties of SA and RSA materials. Our calculations of nonlinear absorption coefficient are in good agreement with conventional the Z-scan data.We believe that PAZ-scan will be a valuable tool for material characterization and has potential for applications in the fields of chemistry, physics, material science, biomedical, and manufacturing.