Rings reveal details
on ultrafast ablations
Using femtosecond laser pulses and time-resolved microscopy, researchers at the University of Essen (Essen, Germany) observed Newtonian rings in a variety of materials, an optical phenomenon that could lead to a better understanding of laser ablation and the identification of a material-independent ablation mechanism (see figure). During the last few years ultrafast-laser-based ablation has received considerable attention. The process is important for applications ranging from laser surgery to micromachining.
In essence, a laser quickly heats a material, creating a micron-scale explosion of heat and pressure. Short-pulse--usually nanosecond or less--laser ablation holds promise for micromachining because the very short bursts of energy limit the thermally induced stress to surrounding material, while producing a smaller plume of debris.
Better understanding
Klaus Sokolowski-Tinten and colleagues at the University of Essen sought to better understand the ablation process by creating movies of laser ablation, "snapping" a frame every tenth of a picosecond. Subpicosecond resolution was crucial to the experiment, Sokolowski-Tinten said, because it reduces observation errors when imaging materials with strongly inhomogeneous spatial reflectivity. The Essen grou¥focused on materials important to the electronics and optics industry, such as silicon, mercury, gold, and titanium.
Varying reflectivity is at the heart of the Newtonian rings. The optical phenomenon, which is usually observed during investigation of thin films, is basically an interference pattern created by light. In the short-pulse laser-ablation process, light reflects off the boiling, ablated material and the solid material underneath, creating the interference.
In each of the materials investigated, the researchers observed that meta stable ablating material becomes nearly transparent (very low absorption) and has a high index of refraction compared with the remaining material. Also, the ultrafast pulses create a very stee¥ablation front with a very smooth ablation plume, or density distribution of the ablating material. Changes in the rings also allowed the grou¥to determine that the speed of the ablating front is in the meters-per-second range.
Sokolowski-Tinten explained that fluence and material temperature are crucial to the formation of the Newtonian rings. "The phenomenon we observed (interference patterns) is only observed below the threshold for creation of a surface plasma," he said. They used "an old-fashioned 10-Hz, amplified, colliding-pulse-modelocked dye-laser system" at 620 nm with pulse durations of 100 fs and an energy per pulse of 2 to 5 mJ to excite the metals. Fluences were typically kept between 0.1 and 1 J/cm2. Sokowlowski-Tinten added that focus points were not as important as long as the lateral scale length is much larger than the absorption depth--typically between 10 and 100 nm in metals.
When first excited, the femtosecond pulses deposit their energy into the material. This energy thermalizes between the electronic and ionic states within just a few picoseconds, keeping the volume of the energy deposition constant. As the material reaches high temperatures (a few thousand K) and pressures (a few tens of gigapascals), ablation occurs by hydrodynamic expansion of the hot, pressurized matter into the surrounding vacuum. Under specific conditions common to ultrafast-pulse laser ablation and tightly controlled fluences, the material goes into an inhomogeneous liquid-gas state. "This is responsible for all of our observations--highly refractive transparent state, stee¥ablation front, and so on," Sokowlowski-Tinten said.
"Our results are not only nice from a fundamental physics point of view--things are nice when they are easy--the improved understanding of femtosecond laser ablation is also important to the current discussion on the potential of ultrafast laser pulses in material processing," he adds. "Our results should be applicable to any material system that could be described in the frame of such a thermal picture."
The grou¥plans to explore further the initial stage of the material expansion, particularly to see if a metal-insulator transition exists before the liquid-gas phase in the expanding liquid metal. Evidence of this phenomenon has been observed in mercury near its critical point, but is very difficult to explore in "normal" metals, Sokowlowski-Tinten said, because of the high temperatures and pressures needed. Ultrafast lasers offer a way to study these metastable metals in their brief (few picoseconds) nonequilibrium states.
R. Winn Hardin