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Water boosts laser ablation efficiency

22 May 2008

An ablation technique that fires nanosecond pulses onto a sample with a layer of water on its surface could aid laser processing of metals.

Placing a thin layer of water on top of a metal sample results in more efficient processing compared with dry conditions, say a team from the US and South Korea. The researchers found that firing nanosecond laser pulses onto a sample of aluminium with a layer of water on the surface increased material removal rate by up to eight times and caused minimal damage to the peripheral regions (Journal of Applied Physics 103 083101).

"We found that laser-metal processing was enhanced especially at the low ablation threshold thanks to the mechanical impact of water," Hyun Wook Kang, a researcher from American Medical Systems, US, told optics.org. "The layer of water generated high pressure impact during vaporization as well as removing laser-induced material debris to effectively deliver laser energy."

According to Kang, wet ablation could also benefit biomedical applications such as laser bone cutting, dental tissue removal, root canal treatments and local kidney tumour removal.

In the experiment, a 1 mm-thick plastic ring filled with distilled water was attached to the surface of the aluminium. Then, a Ti:sapphire laser emitting at a wavelength of 800 nm fired 1 ns pulses onto polished aluminium.

Transient pressure levels were monitored during ablation using a piezoelectric transducer, which was mounted beneath the aluminium. The mechanical impact of the laser ablation was assessed by probing the aluminium with a continuous-wave HeNe laser operating at 632.8 nm.

"Both pressure and reflectance signals were compared in terms of energy," explained Kang. "From this, we detected the change in the aluminium's surface reflectance due to bubble formation. We also evaluated ablation threshold and efficiency with a profilometer (quantitative) and scanning electron microscope (qualitative)."

A key finding was that ablation could be carried out below the plasma threshold with radiant exposures less than 2.5 J/cm2. The group believes that it is the effect of bubble formation and subsequent collapse that helped to increase the material removal rate by about eight times compared with dry ablation.

The team would like to extend its analysis to other materials with various mechanical and thermal properties. "The property discrepancy would clarify material dependence of wet laser ablation and the degree of enhanced material removal," concluded Kang. "We also want to understand the effect of multiple pulses or even shorter pulses (such as pico- and femtosecond) on metal cutting."

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