The quintic nonlinear response of fused silica was investigated using a modified z-scan technique and a cubic-quintic propagation model at femtosecond pulse intensities up to \(10 \ \mathrm {TW/cm^{2}}\) . A statistically significant deviation from the purely cubic Kerr response was observed only at the highest intensities. Interpreting this deviation within the perturbative cubic-quintic model yields effective quintic nonlinear refractive indices \(n_4=-(4...7)\cdot 10^{-6}\ \mathrm {cm^4/TW^2}\) at a wavelength of 1033 nm and \(n_4=-(8...16)\cdot 10^{-6}\ \mathrm {cm^4/TW^2}\) at 517 nm. Since plasma-induced refractive effects may contribute to the observed higher-order response, these values should be regarded as the upper bounds of the negative quintic nonlinear refractive index of fused silica. The obtained bounds are approximately 1–2 orders of magnitude lower than the previously reported values.
