Plasma Expands Under Ultrafast Beams

New insight into the behavior of ultrafast laser pulses could improve their performance in manufacturing, diagnostics and other research.

A laser's pulse at durations of 100 fs, or quadrillionths of a second, causes electrons to reach temperatures >60,000 ºC during the duration. The pulses create precise patterns in a process called cold ablation, which turns material into a plasma of charged particles.

Mechanical engineering doctoral student Wenqian Hu, who graduated this fall, works on a complex optical setup that is part of research at Purdue University to uncover details about the behavior of ultrafast laser pulses. (Image: Mark Simons/Purdue University)

Images of a laser pulse taken with a high-speed camera show tiny mushroom clouds that expand outward at speeds of 100 to 1000 times the speed of sound within less than 1 ns. However, new findings by researchers at Purdue University reveal that an earlier cloud forms immediately before the mushroom cloud, and this early plasma interferes with the laser pulses, hindering performance, said Yung Shin, a professor of mechanical engineering and director of Purdue’s Center for Laser-Based Manufacturing.

Shin said that finding a way to eliminate the interference caused by the early plasma could open up new applications in manufacturing, in materials and chemical processing, in machining and advanced sensors to monitor composition, and in chemical and atomic reactions on an unprecedented scale.

“We found the formation of early plasma has very significant bearing on the use of ultrashort pulse lasers because it partially blocks the laser beam,” Shin said. “The early plasma changes the optical properties of air, but the mechanism is still largely unknown.”

This series of high-speed images shows how plasma expands when material is exposed to ultrafast laser pulses. The Purdue researchers have discovered details that could help to harness the technology for applications in manufacturing, diagnostics and research. (Image: Yung Shin, Purdue University School of Mechanical Engineering)

The researchers studied the early plasma by tracking the movement of millions of individual atoms in it — observing how the laser beam travels in space and interacts with plasma — and by using a “laser pump probe shadowgraph,” a technique in which one laser ablates a material, producing the early plasma. Using a series of optical elements and mirrors, a second laser is then fired perpendicular to the first, which is then used to study the plasma cloud.

The work was published online Dec. 6 in Applied Physics Letters and in the September issue of Physics of Plasmas.

For more information, visit: www.purdue.edu