The z-scan measurement technique [1, 2] is often used for measuring the strength of the Kerr nonlinearity (i.e. the magnitude of the nonlinear index n2) of an optical material. Essentially, a sample of the material under investigation is moved through the focus of a laser beam, and the beam radius (or the on-axis intensity) is measured at some point behind the focus as a function of the sample position. These quantities are affected by the self-focusing effect. If the nonlinear index is positive, and the sample is placed behind the focus (as in Figure 1), self-focusing reduces the beam divergence and thus increases the detector signal. If the sample is moved to the left-hand side of the focus, the focus is moved to the left, and the stronger divergence after the focus decreases the detector signal. From the measured dependence of the detector signal on the sample position, it is possible to calculate the magnitude of the nonlinear index.
Note that nonlinear absorption, e.g. due to two-photon absorption, can also affect the measured signal. This, however, can be measured separately by recording the power of the whole transmitted beam. With these data, the measurement of nonlinearity can be corrected.
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