Intense femtosecond laser-induced second-harmonic generation in atmospheric-pressure air

Second-harmonic generation (SHC) in atmospheric-pressure air is experimentally studied using single focused linear-polarized Ti:sapphire intense femtosecond laser pulses at 810 nm. The efficiency of SHG is found to reach a maximum at the optical breakdown threshold of approximate to 2.9 x 10(14) W/cm(2). The spectral distribution and polarization property of the second harmonic are investigated. The contribution to SHG from electric-field-induced third-order mixing plays the main role even after the optical breakdown had occurred.