Zscan

Zscan, short for Z-scan, is an optical technique used to characterize third-order nonlinear optical properties of materials. It was introduced in the early 1990s to provide a simple, sensitive method for measuring nonlinear refractive index (n2) and two-photon absorption (or other nonlinear absorption coefficients). The method relies on a focused Gaussian laser beam and an axially movable sample, translated along the beam (z) through the focal region.

In a typical Z-scan experiment, a pulsed laser is focused by a lens onto a thin sample, which is mounted on a motorized stage that scans along the z-axis. The transmitted light is detected by a photodetector, with optional aperturing in the far field. Two configurations are used: open-aperture Z-scan, which is sensitive to nonlinear absorption, and closed-aperture Z-scan, which is sensitive to nonlinear refraction. By recording transmittance as a function of z, a characteristic curve is obtained: open-aperture scans reveal valley-type or peak-type transmittance at focus depending on whether two-photon absorption or saturable absorption dominates, while closed-aperture scans reveal a peak or valley at focus depending on the sign of the nonlinear refractive index.

Data from Z-scan can be analyzed to extract quantitative values for the nonlinear refractive index n2 and the nonlinear absorption coefficient beta, often within established models that account for beam geometry, pulse duration, and sample thickness. Z-scan is widely used in materials science and photonics to assess organic and inorganic crystals, polymers, semiconductors, and thin films, aiding the search for efficient optical switches, limiters, and other nonlinear devices.

Considerations include keeping the sample within the Rayleigh range, minimizing thermal effects, and selecting pulse durations that probe the desired electronic or thermal nonlinearities.