Laser-induced plasma by two-color excitation

With a superposition of a nanosecond infrared-laser pulse and its second harmonic, the threshold of plasma formation in gasses as well as the energy transfer efficiency from the laser field to the plasma can be significantly improved in comparison to purely monochromatic radiation. We present experimental results in air at pressures between 0.4 and 3 bar; in this pressure range, the threshold energy for the superposition is reduced by a parts per thousand 30% as compared to that of pure infrared radiation, while the energy transfer slope efficiency is equal to the IR efficiency at high pressures and somewhat less at lower pressures. Possible mechanisms responsible for this effect are discussed.