Truth in Nonlinearity: Measurement of n2 and Multiphoton Absorption Coefficients in Fused Silica from 420 to 1600 nm

This paper presents quantitative measurements of the nonlinear refractive index n2 and multiphoton absorption coefficients (from three- to six-photon) of fused silica (SiO2) in the wavelength range of 420–1600 nm, performed by Z-scan technique with femtosecond pulses (~150 fs). It was found that the nonlinear susceptibility is caused predominantly by the instantaneous electronic response, as confirmed by the agreement of the experimental data with the Kramers–Kronig relations. The observed nonmonotonic spectral dispersion, including an anomalous enhancement of four-photon absorption at 490 nm and a maximum n2 in the near IR at 1300 nm, is interpreted as a consequence of resonant enhancement of virtual transitions near UV‑edges of the fundamental absorption band and the possible contribution of phonon modes in the infrared region. The obtained data provide a consistent and reliable set of parameters for modeling the nonlinear interaction of ultrashort laser pulses with SiO2 in bulk photonics and laser micromachining applications.