Determination of molecular contributions to the nonlinear refractive index of air for mid-infrared femtosecond laser-pulse excitation

Numerical simulations of the rotational contribution of oxygen and nitrogen molecules to the Kerr refractive index change in air are performed for ultrashort laser pulses in the near- and mid-infrared wavelength region. The calculated molecular response is parametrized by means of a damped harmonic oscillator model that is easily tractable in numerical simulations of long-distance propagation of ultrashort laser pulses. Our simulations show that pulses from available mid-infrared laser systems are long enough to be dramatically affected by the molecular response of air during propagation.