Determination of the inertial contribution to the nonlinear refractive index of air, N-2, and O-2 by use of unfocused high-intensity femtosecond laser pulses

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 N-2, O-2, 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. (C) 1997 Optical Society of America.