Simple And Sensitive Technique For Determining Refractive Nonlinearities

A simple, highly sensitive method is proposed for studying nonlinear refraction in materials. In a tight-focus lighting geometry, the transmission of a nonlinear medium through a finite aperture in the far field as a function of the sample position z, measured with respect to the focal plane, is measured at a fixed input energy. Such a trace, referred to as a z-scan , contains information on the sign, magnitude, and order of nonlinearity. By controlling the aperture diameter, refractive nonlinearities can be distinguished from absorptive ones. Such z-scans have been performed on a number of materials using IR (10-μm) and visible (532-nm) laser pulses. For example, thermal self-defocusing was observed in CS2 using 50-300-ns CO2 laser pulses, whereas self-defocusing due to the reorientational Kerr effect was dominant using 100-ps pulses at the same wavelength. The z-scan technique has also been used to analyze and optimize optical power and fluence-limiting devices that operate on nonlinear refraction