We demonstrate that the filamentation process is strongly influenced by the polarization state of the driver laser. When the laser polarization changes from linear to circular, the critical power for the self-focusing of a Ti:Sapphire laser (800 nm, 40 fs) in air increases from about 9.6±1.0 to 14.9±1.5 GW, while the second nonlinear refractive index n2 of air decreases from 9.9 × 10 20 to 6.4 × 10 20 cm2/W. We also demonstrate that the luminescence from the neutral nitrogen molecules at 337 nm is dependent on both the laser intensity and plasma density inside the filament.