Third and fifth harmonic generation by tightly focused femtosecond pulses at 2.2 μm wavelength in air.

We report experiments on the generation of third and fifth harmonics of millijoule-level, tightly focused, femtosecond laser pulses at 2.2 μm wavelength in air. The measured ratio of yields of the third and fifth harmonics in our setup is found equal to 2 · 10(-4). This result contradicts the recent suggestion that the Kerr effect in air saturates and changes sign in ultra-intense optical fields.