Tracking ultrafast laser ablation to tune nanoparticle films

Hank Hogan, hank.hogan@photonics.com

Ultrafast laser ablation yields two distinctly different plumes, and the behavior of both can be described well by models, researchers report. That information could be used to produce novel nanoparticle films of magnetic and semiconductor materials, researcher Salvatore Amoruso said.

“In the latter case, we are considering the variation of the optical properties of these systems for application in optical sensing, photovoltaics, and so on,” he said.

Amoruso, assistant professor of physical sciences at the University of Naples Federico II, added that ultrafast laser ablation, which involves femtosecond pulses, also has potential applications in micro-machining, laser-induced breakdown spectroscopy, nanoparticle generation and film deposition. All of these would benefit from a better understanding of the ablation process.

An ablation plume is produced by ultrafast femtosecond laser pulses hitting a silver target in high vacuum. Images courtesy of Salvatore Amoruso, University of Naples Federico II.

Amoruso and a team from Trinity College Dublin in Ireland and Tianjin University of Technology and Education in China wanted to validate a physical model. To do this, the investigators had to take a multipronged approach because ultrafast laser ablation produces two separate plumes. One is composed of atoms and expands at a rate of about 10,000 m/s. The second, which is the bulk of the material, is made up of nanoparticles. It travels 100 times slower than the first, at about 100 m/s.

A model exists that describes the atomic plume produced by nanosecond pulse ablation, a process that does not yield a nanoparticle plume. However, it had been unclear as to whether the model applies when femtosecond pulses are used.

Scanning electron micrograph of a typical nanoparticle-assembled film produced by ultrafast laser ablation.

In answering this question, the group used 250-fs laser pulses at a 527-nm wavelength, firing these at a nickel target sitting in a vacuum. After two shots, they measured the damaged area using an optical microscope, determining the amount of ablated material by applying scanning white-light interferometry to the craters produced by the laser.

To find out where that material went, they captured the ionized part of the plume on a copper plate, securing a representative sample of the atomic plume. They caught the uncharged nanoparticles on a transparent piece of plastic. Using these two methods, they measured the angular distribution of the ejected material for various laser pulse conditions. Then they mapped how the two plumes evolved and interacted.

Spectrally resolved images of ultrafast laser ablation copper plumes, with the top showing the one composed of nanoparticles (delay after laser shot, 5 μs) and the bottom of the atomic plume (delay 60 ns).

The existing model described the ionic plume well, they reported in a Journal of Applied Physics paper published online Aug. 30, 2010. It also did a good job predicting the nanoparticle plume for lower laser power. The measured results diverged from what was forecast, however, for higher laser fluence. This discrepancy they attributed to the ionic plume, saying that, at higher laser power, it influenced the early evolution of the nanoparticle plume, causing a deviation from what was predicted.

Their results also revealed details about the ablation threshold and the absorption depth, two key factors in predicting ablation results. The researchers found that the depth varied logarithmically with the laser fluence, meaning that an exponential increase in laser power led to a linear increase in ablation depth.

Maps of film thickness (in false color) from ultrafast laser ablation after low- (a) and high-fluence (b) laser shots.

Double or tailored laser pulses are being considered as a way to tune plume properties, Amoruso said. This tuning is of interest in understanding the ablation process and in using it to produce materials for various applications. Of particular appeal is the ability of the technique to generate nanoparticles of different materials in the well-controlled and very clean conditions of high vacuum.

So what does the future hold for this research? “We intend to pursue the method of ultrafast laser ablation for the deposition of nanoparticles and their assembly into a film,” Amoruso said.