Engineers have discovered details about the behavior of ultrafast laser
pulses that may lead to new applications in manufacturing, diagnostics
and other research.
Ultrafast laser pulses are used to create features and surface textures
in metals, ceramics and other materials for applications including the
manufacture of solar cells and biosensors. The lasers pulse at
durations of 100 femtoseconds, or quadrillionths of a second, and cause
electrons to reach temperatures greater than 60,000 degrees Celsius
during the pulse duration. The pulses create precise patterns in a
process called "cold ablation," which turns material into a plasma of
charged particles.
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| This
series of high-speed images shows how plasma expands when material is
exposed to ultrafast laser pulses. Purdue researchers have discovered
details that could help to harness the technology for applications in
manufacturing, diagnostics and research. (Yung Shin, Purdue University
School of Mechanical Engineering) |
Images taken with a high-speed camera show tiny mushroom clouds eerily
similar in appearance to those created in a nuclear explosion. The
clouds expand outward at speeds of 100 to 1,000 times the speed of
sound within less than one nanosecond. However, new findings 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 University's Center for Laser-Based Manufacturing.
Finding a way to eliminate the interference caused by the early plasma
could open up new applications in manufacturing, materials and chemical
processing, machining and advanced sensors to monitor composition, and
chemical and atomic reactions on an unprecedented scale, he said.
Researchers used experiments and simulations to study the phenomenon.
Research papers about the work were published online Dec. 6 in Applied
Physics Letters and in September in the journal Physics of Plasmas. The
papers were written by doctoral student Wenqian Hu, Shin and mechanical
engineering professor Galen King.
"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."
The researchers studied the early plasma by tracking the movement of
millions of individual atoms in the plasma; observing how the laser
beam travels in space and interacts with plasma; and using a "laser
pump probe shadowgraph," a technique in which one laser ablates a
material, producing the early plasma, and a second laser fired
perpendicular to the first is used to study the cloud. A series of
optical elements and mirrors is used in the shadowgraph technique.
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