Optimized energy coupling at ultrafast laser-irradiated metal surfaces by tailoring intensity envelopes: Consequences for material removal from Al samples

We present results describing the efficiency of energy coupling in laser-irradiated metallic surfaces by ultrashort laser pulses with different intensity envelopes. Subsequently, we discuss probable thermodynamic paths for material ejection under the laser action. Ion and neutral emission from the excited sample is used as a sensitive method to probe the efficiency of energy deposition in the material. With support from numerical simulations of the hydrodynamic advance of the excited matter, consequences of optimized energy coupling relevant for applications in material processing are revealed. Despite the reduced sensitivity to intensity-dependent effects for linear materials, the overall absorption efficiency can be elevated if the proper conditions of density and temperature are met for the expanding material layers. In this respect, short sub-ps single pulse irradiation is compared with picosecond sequences. We show that in particular irradiation regimes, characterized by fluences superior to the material removal threshold, laser energy delivery extending on several picoseconds leads to significant superheating of the superficial layers as compared to femtosecond irradiation and to a swift acceleration of the emitted particles. Subsequently, the lifetime of the post-irradiation liquid layer is diminished, which, in turn, translates into a reduction in droplet ejection. In contrast, short pulse irradiation at moderate fluences generates a higher quantity of removed material that is ejected in a dense mixture of gas and liquid-phase particulates.

Figure 1

(Color online) Velocity distributions of Al ions emitted from aluminum targets irradiated with short (SP), optimized (OP), and long (LP) pulses at two different input average fluence levels. (a) $\mathrm{F}=0.45\mathrm{J}∕{\mathrm{cm}}^2$, approximately $3F_{\mathit{th}}$. (b) $\mathrm{F}=0.75\mathrm{J}∕{\mathrm{cm}}^2$, approximately $5F_{\mathit{th}}$. The optimization velocity window is also marked in the figure.

Figure 2

Normalized temporal optimal intensity profiles at similar energies (the two upper OP1 and OP2 curves) and the long laser pulse (LP) envelopes used in the experiment and, later, in the simulation procedure (bottom solid and dashed curves). The OP1 curve is used to illustrate the laser action under the optimal conditions.

Figure 3

(Color online) Velocity distributions of neutral ejecta emitted from aluminum targets irradiated with short (SP), optimized (OP), and long (LP) pulses. The inset shows a blowup of the high velocity range.

Figure 4

Simulation of the instantaneous reflectivity values and of the integrated absorbed energy density as a function of time for a $180\mathrm{fs}$ laser pulse (SP) exciting the aluminum sample at $800\mathrm{nm}$ wavelength. The incident fluence value is $1\mathrm{J}∕{\mathrm{cm}}^2$. The arrow indicates the maximum of the laser pulse.

Figure 5

Simulation of the instantaneous reflectivity values and of the integrated absorbed energy density as a function of time for a $15\mathrm{ps}$ laser pulse (LP) exciting the Al sample at $800\mathrm{nm}$ wavelength. The incident fluence value is $1\mathrm{J}∕{\mathrm{cm}}^2$. The arrow indicates the maximum of the laser pulse.

Figure 6

(Color online) Maximum transient ionic temperature as a function of the initial, unperturbed solid depth for different fluences and for the two sets of laser pulses. The critical temperature is $6300\mathrm{K}$.

Figure 7

(Color online) Maximum velocity of different expanding layers as a function of initial, unperturbed solid depth for different fluences and for the two sets of laser pulses, SP and LP.

Figure 8

(Color online) Density profiles at three different moments during the simulation of a $180\mathrm{fs}$ laser pulse interaction with aluminum. The solid-liquid and liquid-gas borders marked on the left side of the figure are an approximate indication of the respective transitions. The incident fluence is $1\mathrm{J}∕{\mathrm{cm}}^2$.

Figure 9

(Color online) Density profiles at three different moments during the simulation of a $15\mathrm{ps}$ laser pulse interaction time with aluminum. The solid-liquid and liquid-gas borders marked on the left side of the figure are an approximate indication of the respective transitions. The incident fluence is $1\mathrm{J}∕{\mathrm{cm}}^2$.

Figure 10

(Color online) (a) Thermodynamic trajectories of several simulation cells in the excited aluminum region in a $\left(\rho \text{−}T\right)$ diagram for a $180\mathrm{fs}$ laser pulse. The simulation ends at $5\mathrm{ns}$. The electron-ion thermalization moment (+) is indicated in the figure for each trajectory and corresponds to a delay of $12\mathrm{ps}$ with respect to the beginning of the laser pulse. Note the “trapped” liquid regions at 27 and $41\mathrm{nm}$ depth. The incident fluence value is $1\mathrm{J}∕{\mathrm{cm}}^2$. The solid-liquid (S-L; melting), the liquid-gas (L-G; binodal), and the onset of instabilities (Sp; spinodal) boundaries given by the EOS, as well as the position of the critical point (CP) are also depicted in the figure. Due to the uncertainty related to the passage via the instability region, the thermodynamic trajectories are not shown below the spinodal envelope. (b) Blow-up of the “trapped” region emphasizing the 27 and $41\mathrm{nm}$ trajectories. 56 and $58\mathrm{nm}$ are also shown.

Figure 11

(Color online) Thermodynamic trajectories of several simulation cells in the excited Al region in a $(\rho -T)$ diagram for a $15\mathrm{ps}$ laser pulse. The simulation ends at $5\mathrm{ns}$. The electron-ion thermalization moment is indicated in the figure and corresponds to the end of the laser pulse, at $45\mathrm{ps}$. The incident fluence value is $1\mathrm{J}∕{\mathrm{cm}}^2$. The solid-liquid (S-L; melting), the liquid-gas (L-G; binodal), and the onset of instabilities (Sp; spinodal) boundaries given by the EOS, as well as the position of the critical point (CP) are also depicted in the figure. Due to the uncertainty related to the passage via the instability region, the thermodynamic trajectories are not shown below the spinodal envelope.

Figure 12

(Color online) (a) The ratio between the maximum surface temperature achieved by LP and SP sequences at different input fluences. (b) The relative liquid content (confined $\text{liquid}∕\text{gas}+\text{confined}$ liquid) of the plume for SP and LP sequences at different input fluences. Only the confined liquid droplets are taken into consideration. The corresponding experimental fluence ranges at $3F_{\mathit{th}}$ and $5F_{\mathit{th}}$ are marked in both figures.