Self-induced transparency in laser-aerosol interactions

Abstract

The interaction of high-power laser beams with aerosol particles is important to the study of atmospheric propagation. We have investigated the interaction of the CO2 laser with a single H2O aerosol particle of controlled size and obtained some interesting experimental results. The H2O particles were generated by a commercial generator (TSI, Inc.) with particle sizes ranging from 20 to 100 µm. For each particle size, the transmitted laser pulse, the acoustic signal generated by the shock wave, and the production of a breakdown plasma were measured as a function of the laser intensity. It was found that (1) the breakdown threshold of the CO2 laser was reduced by a factor of 100 with respect to clean air in the presence of the H2O droplets; however, the droplet size did not affect the reduced threshold; (2) the acoustic signal appeared at intensities as low as 10 MW/cm2; the onset of the acoustic signal depended strongly on the droplet size; (3) before the onset of plasma formation, the aerosol became totally transparent to the laser beam. A plot of the intensity output vs intensity input into the drop also shows an optical bistability type hysteresis.