Pulsed laser ablation studied using digital holographic interferometry

Published: 10 October 2007

Laser ablation is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. It has a lot of applications including modification of the physical or chemical microstructure of materials, microhole drilling, art conservation and thin film deposition. We use pulsed digital holographic interferometry to investigate the plume and the shock wave generated in the laser ablation process on different targets under atmospheric air pressure.

 A Q-switched Nd-YAG laser (pulse duration = 12 ns) is used both for ablating the material and for measurement (see Fig. 1). The fundamental wavelength 1064 nm is used for processing and the frequency doubled 532 nm is used for the measurement. The green light is split by a beam splitter (BS1). The reflected part is reflected by mirror M1, expanded by a negative lens (NL), collimated by a positive lens (L2) and illuminates a diffuser (D) after passing along the target. The light that passes the beam splitter BS1 is used as a reference beam (R). Digital holograms are recorded using a CCD camera with a resolution of 1280 1024 pixels, a pixel size of 6.7 µm 6.7 µm and a dynamic range of 12 bits.

Phasemap
Figure 2. A typical phase map at a time delay of 530 ns and a power density of 4.2 GW/cm2 of PCBN target.

Numerical data of the integrated refractive index field are calculated and presented as phase maps (see Fig. 2).

Reconstructed refraction index distribution.
Figure 3. The reconstructed refraction index distribution in the xy-plane at z=0.15mm.

Radon inversion is used to estimate the 3D refractive index field measured from the projection assuming rotational symmetry (see Fig. 3). The green colour in the figure represents the undisturbed air, the red colour represents the shock wave front and the blue represents the ablated plume. The shock wave front densities at different time delays are calculated from the reconstructed refractive index fields using the Gladstone-Dale equation. A comparison of the shock wave front densities calculated from the reconstructed data and that calculated using the point explosion model show a quite good agreement. The reconstructed refractive index field is used to estimate the electron number density distribution within the laser induced plasma. The electron number densities are found to be in the order of 1018 cm-3 and decay at a rate of 3x1015 electrons/cm3ns (for more information see paper 2). The results show that pulsed digital holographic interferometry is a promising technique to study the laser ablation process. More information that can be extracted from the recorded digital holograms can be found in my publications.