This paper discusses the efficiency of absorption of sub-picosecond laser pulses during the formation of ablation plasma plumes from a silicon solid target. Such plasmas are normally used to fabricate thin films on a heated substrate. Properties of the plasma such as temperature, density, energy, and species can affect growth mechanisms and the final film quality. The energy and the associated flux density in ablation plasmas are a function of the percentage of energy extracted from the incoming laser pulse at the target. It is shown in this work that the critical density condition of a plasma initially formed by a single pulse can be subsequently modified by the pumping action of a secondary time-delayed pulse. Figure 1 shows the results of measuring total ion yield and average energy in a pre-formed ablation plume generated by a 120 femtosecond (780 nm) laser pulse followed by an identical time delayed pulse. The double pulses are seen to increase the ion yield and energy compared to a single pulse with twice the energy of each double pulse (or the double pulses with zero time delay). The maximum enhancement above the single pulse result occurs at a 5-picosecond delay. These results imply that 5 picoseconds are required to allow time for sufficient expansion of the initial overdense plasma to reach critical density, whereupon it can again become an efficient optical absorber. Based on a self-similar model calculation this implies that the initial plasma density in these experiments was 6 × 1021 cm−3, which is approximately 3 times higher than critical density, at the wavelength being used.

© 2002 Optical Society of America

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