TRANSIENT OPTICAL PROPERTIES OF TARGET INDUCED LASER SUPPORTED ABSORPTION WAVES

Abstract
Efficient energy transfer from short duration high-power laser pulses to metallic as well as dielectric targets causes serious problems as strongly absorbing plasmas are initiated when the power density applied exceeds a threshold of the order of 10⁶ to 10⁷ W/cm². The purpose of our investigations was to determine temporal relaxation processes of the optic transparency of plasmas produced by powerful CO₂ laser pulses. The laser system was capable of giving single or double pulses with energies (in both pulses) up to 150 J. The time delay could be varied arbitrarily. The formation and decay of the plasma absorption waves were investigated up to power densities of some 10⁸ W/cm² by means of a low power cw CO₂ probe laser. The relaxation time the plasma needs for reattaining transparency which was thus determined, is a most important parameter which has to be considered if, for example, optical energy has to be transferred to a target by repetitively pulsed lasers.