Using Image Analysis to Determine Ideal Aperture Conditions for Optimal Z-Scan Signal Quality

An image analysis technique that is insensitive to power fluctuations and beam steering issues, and can be used with non TEM00 Gaussian beams, is introduced to determine the nonlinear properties of materials using the well-known Z-scan experimental technique. Instead of measuring the transmission through or around a physical aperture, this method records the complete beam profile digitally at each step as the sample is translated. Various apertures of any desired shape are then applied to the recorded data set to allow accurate nonlinear characterization under identical experimental conditions which means that both open aperture and closed aperture measurements can be performed on the same data set. Through varying the aperture shape, the signal to noise and sensitivity of the Z-scan measurement can be optimized allowing weak nonlinearities, such as the nonlinear absorption of toluene to be measured. The technique is used to simultaneously perform closed aperture Z-scan or eclipsing aperture Z-scan measurements, with the signal dependence on aperture size agreeing well with that theoretically expected. A novel ring aperture is introduced that allows accurate determination of the laser beam radius without requiring a nonlinear reference sample to calibrate this important Z-scan parameter.