Nonlinear absorption of femtosecond laser pulses (800 nm) by atmospheric air and water vapour

Quantitative data on the nonlinear absorption cross sections of femtosecond Ti : Sapphire laser pulses in air and water vapour have been obtained. A photoacoustic spectrometer calibrated based on the calculated value of linear absorption of laser pulses with a wavelength of 800 nm and a spectral width of 17.7 nm is used to find the nonlinear absorption cross sections of water vapour and air: σ₂^w = (2.6±0.4) × 10^-55 cm⁴ s and σ₂^a = (8.7±1.0) × 10^-56 cm⁴ s, respectively. Based on measuring the absorption of femtosecond Ti : Sapphire laser pulses with a photoacoustic detector calibrated with the known linear absorption of ruby laser radiation by water vapour in air, the air nonlinear absorption cross section is found to be (8.2±0.9) × 10^-56 cm⁴ s.