Determination of n₂ by direct measurement of the optical phase

We determine the nonlinear refractive index, n₂, in an optical material through the direct measurement of the phase of an ultrashort optical pulse, using the technique of frequency-resolved optical gating. This method results in the accurate measurement of n₂, with the error dominated by the uncertainty in the fluence measurement. We measure n₂ in fused silica and in potassium dihydrogen phosphate at 804 and 402 nm. These results are consistent with those from previous measurements of n₂ in these materials, and the measured dispersion of n₂ in fused silica agrees with theoretical predictions.