Abstract

We present a simple yet highly sensitive technique for determining the sign and magnitude of nonlinear refraction. Using a single focused Gaussian beam with a constant input pulse energy, the far field transmittance is measured through a finite aperture as a function of the sample position with respect to the focal plane along the propagation path (z). The result is a dispersion-shaped curve with a maximum (peak) and a minimum (valley) corresponding to the far field beam narrowing or broadening, respectively. The ∓ sign of the nonlinear refraction is immediately deduced from a z-scan depending on whether a peak occurs before (−z) or after the focus (+z), respectively. The magnitude of the refractive nonlinearity is easily obtained from a simple linear relation between the normalized peak-to-valley transmittance change (ΔT_p−v) and the on-axis nonlinear phase shift at the focus (ΔΦ₀): This relation was obtained by using a simple wave optics formalism. We have employed this technique to investigate the nonlinear refraction in various types of material such as semiconductors (InSb, ZnSe, CdTe), glasses (BaF2, MgF2), semiconductor doped glasses and liquids (CS2) at various laser wavelengths. A phase distortion resolution of ≃λ/300 was obtained in n₂ measurements of BaF₂ at 532 nm.

© 1989 Optical Society of America