In-line holographic imaging and electron density extraction of ultrafast ionized air filaments

Abstract

In-line spatial holographic methods are used to image the optical diffraction patterns of ultrafast laser produced plasma filaments in air. Using femtosecond laser pump-probe methods the plasma filament is produced and subsequently probed by a time-delayed probe beam traversing the filament at a right angle. The resulting probe beam image is recorded as a holographic diffraction pattern on a charge coupled device camera as the probe beam is diffracted due to the optical index change caused by the plasma filament. Extraction of the electron density in the filament down to 1016 cm−3 is obtained by using the Helmholtz wave equation beam propagation model to simulate the data. By varying the pump-probe optical time delay, a plasma electron recombination time of approximately 250 ps is measured and agrees with our rate equation population kinetics model.