Time-Resolved Shadowgraphs of Material Ejection in Intense Femtosecond Laser Ablation of Aluminum

The dynamic process of intense 50 fs laser ablation of aluminum is investigated by ultrafast time-resolved microscopy. A stripe pattern preceding phase explosion is clearly seen in the shadowgraph of 1 ns time delay. Intermittent material ejections are observed within the ejected plume after 2.5 and 7 ns time delay, respectively, which may be attributed to the material response to the generation of an extremely strong thermoelastic wave. Similar processes are also recorded in the ablation of silicon and glass samples, except for the glass samples, the intermittent material ejections are not found.

Figure 1

Experimental setup.

Figure 2

Time-resolved shadowgraphs of material ejection at the indicated time delays after an aluminum target is ablated by a 50 fs laser pulse of $40\mathrm{J}/{\mathrm{cm}}^2$. Frame size: $170\times 170\mu \mathrm{m}$.

Figure 3

Time-resolved shadowgraphs of material ejection at larger time delays after the same aluminum sample is ablated by a 50 fs laser pulse of $40\mathrm{J}/{\mathrm{cm}}^2$. Frame size: $200\times 200\mu \mathrm{m}$.

Figure 4

Measured radii of the plume fronts (open circles) and the shock wave fronts (solid squares) as a function of time delay. The solid line represents the radius of the shock wave front calculated according to Sedov’s theory.

Figure 5

Calculated time-dependent stress distribution in the interior of the aluminum target as a result of thermoelastic effect. The coordinate $x$ originates from the surface and points towards the target interior.