Characterization of laser-induced air plasmas by third harmonic generation

Third harmonic generation by a weak femtosecond probe pulse intersecting a pump laser-induced plasma in air is investigated and a general model is developed to describe such signal, applicable to a wide range of focusing and plasma conditions. The effect of the surrounding air on the generated signal is discussed. The third-order nonlinear susceptibility of an air plasma with electron density N(e) is determined to be χ(p)((3)) = χ(a)((3)) + γ(p)N(e) with γ(p) = 2 ± 1 × 10(-49) m(5) V(-2) and χ(a)((3)) being the third-order susceptibility in air. Lateral scans of the probe through the plasma are used to determine electron density profiles and the effect of focusing and phase mismatch is discussed.