Precise Measurements of the Absolute Nonlinear Refractive Index of Fused Silica Using High-Repetition-Rate Femtosecond Laser and Nonlinear Ellipse Rotation Signal.

Here, we present a single-beam technique based on nonlinear ellipse rotation (NER) that, we believe, enables a more accurate determination of the absolute value of the electronic nonlinear refractive index of fused silica. We show that using high-repetition-rate femtosecond pulses enables NER to achieve a good trade-off among nonlinear signal, peak, and average laser power while remaining completely immune to thermal contributions. Also, NER can retrieve the laser pulse duration, linear chirp, and the nonlinear response under an identical experimental setup, ensuring fully self-consistent refractometry. We measured five commercial fused silica samples using ultrafast laser pulses with about 200 fs, at three wavelengths: 343, 514, and 1028 nm, reinforcing its role as the metrological benchmark for next-generation photonic materials and devices. All tested fused silica samples showed no significant differences in their nonlinear refractive properties, yielding n 2 = (1.9 ± 0.3) × 10-20 m2/W at 1028 nm, n 2 = (2.3 ± 0.3) × 10-20 m2/W at 514 nm, and n 2 = (2.7 ± 0.5) × 10-20 m2/W at 343 nm. All values are consistent with those reported in the literature but slightly smaller than those usually accepted as references.