Research provides insight into ultrafast laser pulse behaviour

Insights into the behaviour of ultrafast laser pulses could improve their performance in manufacturing, diagnostics and research.

Using high-speed analysis, researchers at Purdue University observed the appearance of plasma clouds that interfere with the action of the laser. Mitigating this effect could both improve the performance of existing techniques and even open up entirely new applications, according project lead Prof Yung Shin.

According to Purdue, 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 100 femtoseconds and cause electrons to reach temperatures of more than 60,000ºC during the pulse duration.

Previous work using high-speed cameras has shown the formation of tiny mushroom clouds at the laser-surface interface. The clouds expand outward at speeds of 100 to 1,000 times the speed of sound within less than one nanosecond.

However, the findings by the Purdue team reveal that an earlier cloud forms immediately before the mushroom cloud, and this early plasma interferes with the laser pulses — hindering performance.

The team studied the early plasma by tracking the movement of millions of individual atoms in the plasma. It also observed how the laser beam travels in space and interacts with plasma 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.

’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.’