DIRECT PROBE FOR PLASMA FORMATION DURING EXCIMER LASER ABLATION OF MATERIALS.

Abstract

Summary form only given. The authors report the use of a Michelson interferometer to make real-time measurements of the index of refraction of the plume of ablated material. Free electrons are easily distinguished from neutral species since the index of refraction of a free electron gas is less than one. Thus, the technique provides a direct time-resolved quantitative probe for plasma formation. Several types of information can be obtained from the experiment. Free electron desities can be calculated from the magnitude of the interferometer fringe shift, and velocity distributions can be obtained from the time dependence of the signal. Thresholds for plasma formation as a function of excimer laser wavelength and fluence can be obtained and compared with thresholds for optical emission from atoms, ions, or diatomic molecules under identical materials including sapphire, aluminum, silicon, and Kapton.