Atomistic-Continuum Study of an Ultrafast Melting Process Controlled by a Femtosecond Laser-Pulse Train

Abstract

In femtosecond laser fabrication, the laser-pulse train shows great promise in improving processing efficiency, quality, and precision. This research investigates the influence of pulse number, pulse interval, and pulse energy ratio on the lateral and longitudinal ultrafast melting process using an experiment and the molecular dynamics coupling two-temperature model (MD-TTM model), which incorporates temperature-dependent thermophysical parameters. The comparison of experimental and simulation results under single and double pulses proves the reliability of the MD-TTM model and indicates that as the pulse number increases, the melting threshold at the edge region of the laser spot decreases, resulting in a larger diameter of the melting region in the 2D lateral melting results. Using the same model, the lateral melting results of five pulses are simulated. Moreover, the longitudinal melting results are also predicted, and an increasing pulse number leads to a greater early-stage melting depth in the melting process. In the case of double femtosecond laser pulses, the pulse interval and pulse energy ratio also affect the early-stage melting depth, with the best enhancement observed with a 2 ps interval and a 3:7 energy ratio. However, pulse number, pulse energy ratio, and pulse interval do not affect the final melting depth with the same total energies. The findings mean that the phenomena of melting region can be flexibly manipulated through the laser-pulse train, which is expected to be applied to improve the structural precision and boundary quality.

Keywords: femtosecond laser; laser-material interaction; molecular dynamics; two-temperature model; ultrafast melting process.

Figure 1

Schematic diagram of the experimental setup of double-pulse laser processing.

Figure 2

Simplified dimensional schematic diagram of the Al melting process under laser irradiation and the design of 1D equivalent computational domain.

Figure 3

Flowchart of the MD-TTM model incorporating dynamic thermophysical parameters.

Figure 4

Schematic diagram of the laser-pulse train.

Figure 5

SEM images of melting region for SPs at fluences (a) $0.44 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$, (b) $0.50 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$, (c) $0.56 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$, (d) $0.63 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$, and (e) $0.67 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$.

Figure 6

SEM images of melting region for (a) an SP and DPs with pulse intervals of (b) $1 \mathrm{p}\mathrm{s}$, (c) $2 \mathrm{p}\mathrm{s}$, and (d) $3 \mathrm{p}\mathrm{s}$, at a total fluence of $0.63 \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$.

Figure 7

Temperature-dependent thermophysical parameters of Al. Comparison of (a) electron heat capacity $C_e$, (b) electron thermal conductivity $k_e$, and (c) the electron-phonon coupling factor $G_{e-ph}$ obtained by different calculation methods [17,46,54,55,56,57].

Figure 8

Atomic snapshots of the melting process on the Al surface region of $90 \mathrm{n}\mathrm{m}$, the left is the classical thermophysical parameters group for (a) SP, (b) DP, and (c) FP, while the right is the temperature-dependent thermophysical parameters group for (d) SP, (e) DP, and (f) FP. The black dotted line represents the boundaries between the solid and liquid phases.

Figure 9

Spatial distributions of (a–c) electron temperature $T_e$, and (d–f) lattice temperature $T_l$ at different pulse numbers for (a,d) SP, (b,e) DP, and (c,f) FP. Dashed lines represent the classical thermophysical parameters group results and the solid lines represent the temperature-dependent thermophysical parameters group results.

Figure 10

The morphology of the 2D melting region ($50 \mathrm{p}\mathrm{s}$) of the Al surface at (a) SP and (b) DP with a $2 \mathrm{p}\mathrm{s}$ interval, the left is the simulation results, and the right is the experimental results.

Figure 11

The morphology of the 2D melting region ($50 \mathrm{p}\mathrm{s}$) of the Al surface at FP with a $2 \mathrm{p}\mathrm{s}$ interval.

Figure 12

The longitudinal evolution of the melting region during the first $50 \mathrm{p}\mathrm{s}$ under both SP and DPs with varying pulse intervals. (a) atomic snapshots at $10 \mathrm{p}\mathrm{s}$, (b) atomic snapshots at $50 \mathrm{p}\mathrm{s}$, and (c) melting speed in different stages.

Figure 13

Evolution of electron temperature $T_e$ and lattice temperature $T_l$ under different pulse settings (a,d) SP, (b,e) DP with a $2 \mathrm{p}\mathrm{s}$ interval, (c,f) DP with a $3 \mathrm{p}\mathrm{s}$ interval.

Figure 14

The impact of sub-pulses energy ratio of DP on the melting process. Sub-figures for DPs with (a–c) $1 \mathrm{p}\mathrm{s}$, (d–f) $2 \mathrm{p}\mathrm{s}$, and (g–i) $3 \mathrm{p}\mathrm{s}$ intervals, respectively. The first and second columns are atomic snapshots of $0-10 \mathrm{p}\mathrm{s}$ and $0-50 \mathrm{p}\mathrm{s}$. The third column represents the melting speed vs. energy ratio at different stages.