Nonlinear refractive index measurements of solids and the impact of B-integral on the phase of generated high-order harmonics

We present an interferometric method that employs a birefringent common-path interferometer to measure absolute nonlinear refractive indices in 5 mm thick CaF₂, SiO₂, NBK-7 and KBr for femtosecond pulses at 804 nm center wavelength. The measured nonlinear refractive indices are found to be in agreement with previous literature, over a broad range of pulse energies. In addition, we show and highlight the importance of thorough B-integral characterization when using this birefringent common-path interferometer for high-harmonic generation driven extreme-ultraviolet interferometry. The accumulated third-order nonlinear phase is shown to have significant impact on the harmonic phase after up-conversion to higher orders, but can be accounted for if correctly characterized.