The Z-scan measurement is a technique which helps to determine intrinsic photo-physical properties of photo-activated materials by tightly focusing a single laser beams onto the surface of the material samples. The laser beam is directed perpendicular to the plane of the sample. The sample is moved along the z direction (the laser beam propagation direction) in and out of the laser focal point. The transmitted signal goes either directly to a detector or through an aperture to a detector. The magnitude of the detected signal is plotted as a function of the sample position.

radiative relaxation

In the case of no aperture (called “open aperture” measurements), the laser pulse energy can remain constant, but the beam intensity at the sample varies as the sample position is moved in the z-direction due to the changing beam diameter at the sample. It allows determining the nonlinear absorption of the material. If the measurements are done with the beam going through an aperture (called “closed aperture” measurements), then the magnitude and sign of the nonlinear index of refraction resulting from the Kerr effect can be determined. Changes in the nonlinear index of refraction lead to nonlinear distortions of the beam. A positive Kerr effect leads to self-focusing of the laser beam. A negative Kerr effect leads to defocusing. The simple Z-scan technique is sensitive to all types of nonlinear mechanisms, so it is sometimes difficult to determine the physical mechanisms behind the results from a single set of measurements. Usually measurements need to be taken at different laser pulse energies or different laser pulse lifetimes in order to determine what nonlinear processes are present.

See also: Single-photon absorption, Two-photon absorption, Kerr effect.

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