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Experimental study of laser-induced air plasma using a Nomarski interferometer

The spatial and temporal evolution of plasma generated via laser-induced breakdown in air was investigated. The plasma was produced in air using a focused Nd:YAG Q-switched laser (λ = 1064 nm; pulse duration ∼5 ns; pulse energy ∼60 mJ). The interference patterns of the resulting plasma were measured as a function of time using a Nomarski interferometer. The elapsed times were in the range 59–232 ns. A frequency-doubled Q-switched Nd:YAG laser (λ = 532 nm; pulse duration ∼10 ns; pulse energy ∼10.5 mJ) was coupled with the Nomarski interferometer to form an interferometric probe beam. The electron density was inferred from Abel inversion and fast Fourier transformation analysis of the recorded interference patterns. The measured electron densities were on the order of ∼1018 cm−3. Using the Saha equation, assuming that the plasma is in local thermodynamic equilibrium, and that the ionization reaction is (N → N+ + e), the electron temperatures were estimated to be in the range 17600–15500 K.