Ablation and plasma emission produced by dual femtosecond laser pulses

Abstract

Pairs of 80fs, 800nm laser pulses were used to ablate Si, Cu, and CaF2 in air. The spectrally resolved plasma emission was measured as a function of laser fluence and pulse delay. After an initial dip, the fluorescence was found to increase monotonically with pulse delay, reaching a plateau after some tens of picoseconds, depending on the material and fluence. The enhancement ratio (defined as the ratio of the fluorescence produced by the pulse pair to that produced by a single pulse of the same total fluence) reaches a maximum value of 6 and 11 at a fluence of ∼6J∕cm2 for Si and Cu, respectively, and declines to a value below 2 at higher fluences. In contrast, the enhancement for CaF2 increases slowly from zero near threshold to a broad maximum value of 2 near 50J∕cm2. Using reflectivity and atomic force microscopy measurements as diagnostics, we interpret the Si and Cu behavior in terms of a two phase mechanism, in which the first pulse melts the surface of the crystal and the second pulse ablates the resulting liquid film. A qualitatively different mechanism initiated by multiphoton absorption is involved in CaF2 ablation.

Femtosecond laser ablation of silver foil with single and double pulses

“…In addition, we have used double pulses with separations of up to 3.4 ns in a pump probe experiment to study a transition region between two logarithmic fluence regimes. Fairly extensive experimental results have been reported on ''single pulse'' femtosecond metal ablation, though few specifically on silver [3,7] but results on dual (or multiple) pulse ablation have been rather more limited [18][19][20][21][22][23][24][25][26][27][28][29][30][31]. By dual pulse it is always understood that the pulse separation is much less than the interpulse period (typically 1 ms) of the train of amplified femtosecond pulses used for ablation.…”

Atomistic Modeling of Short Pulse Laser Ablation of Metals: Connections between Melting, Spallation, and Phase Explosion

“…Recent observation of a pronounced plasma emission enhancement in dual pulse femtosecond laser ablation of Cu and Si has been attributed to melting of a surface region of the target by the first pulse, resulting in greater localization of the energy deposited by the second pulse in the melted layer. 44 The rates of cooling and resolidification are also playing an important role in defining the morphology of the resolidified surface and contribution of the melt expulsion to the total amount of the ablated material. Geometrical limitations imposed on the heat transfer, such as two-dimensional lateral heat transfer in thin metal films, [41][42][43]45,46 or relatively low thermal conductivity of nonmetallic targets 37,40 are the factors that can prolong the time to complete resolidification and facilitate the effects associated with the melt redistribution and expulsion.…”

Optimally enhanced optical emission in laser-induced breakdown spectroscopy by combining spatial confinement and dual-pulse irradiation

“…In DP-LIBS using orthogonal geometry, the first pulse is introduced onto the sample surface and the second pulse is focused parallel to the surface to reheat the plasma ablated by the first pulse. An increase in optical emission from the re-excited plasma can be observed [26][27][28]. At the same time, along with the generation and expansion of the laser-induced plasma in air by the first pulse, a shock wave is produced.…”