Ab initio calculations of the linear and nonlinear susceptibilities of N2, O2, and air in midinfrared laser pulses

We present first-principles calculations of the linear and nonlinear susceptibilities of N2, O2, and air in the midinfrared (MIR) wavelength regime from 1-4μm. We extract the frequency-dependent susceptibilities from the full time-dependent dipole moment that is calculated using time-dependent density functional theory. We find good agreement with curves derived from experimental results for the linear susceptibility and with measurements for the nonlinear susceptibility up to 2.4μm. We also find that the susceptibilities are insensitive to the laser intensity even in the strong field regime up to 5×1013W/cm2. Our results will allow accurate calculations of the long-distance propagation of intense midinfrared laser pulses in air.