Determination of the inertial contribution to the nonlinear refractive index of air, N2, and O2 by use of unfocused high-intensity femtosecond laser pulses

Abstract : We analyze the spectral changes that occur when unfocused intense femtosecond pulses at 800 nm propagate through different atomic and molecular gases. The observed red shift in the self-phase-modulated spectra after propagation through N2, O2, and air is attributed to a delayed change of the nonlinear refractive index, a consequence of the molecular response to impulsive excitation of rotational coherences. We compare these results on the nonlinear refractive index with those obtained from Ar, Xe, and SF6.