Ultrafast Recording Of Microstructural Alteration By Femtosecond Lasers

By Jof Enriquez,
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Scientists at the University of Rochester have developed a scattered-light-based optical imaging technique to record, for the first time, the complete process by which a material's surface is transformed by laser beams to such extent that the materials exhibit new and useful properties. Visualizing these tiny changes will allow scientists to better produce custom materials tailored to serve different purposes.

The research team led by Chunlei Guo, a full professor in The Institute of Optics at University of Rochester, has experimented on surface structuring through lasers before. In 2015, they successfully used femtosecond (equal to one quadrillionth of a second) lasers to transform metals into extremely water repellent (super-hydrophobic) materials without the need for temporary coatings. They even reversed the process to make materials that are hydrophyllic, which attract water.

“After we determined that we could drastically alter the property of a material through creating tiny structures in its surface, the next natural step was to understand how these tiny structures were formed,” Guo says in a news release. “This is very important because after you understand how they’re formed you can better control them.”

In other words, Guo and colleagues had to visualize directly the sequence of events when the material’s surface is altered by light. So, their next experiment involved recording, for the first time, "the complete temporal and spatial evolution of the femtosecond laser-induced morphological surface structural dynamics of metals from start to finish, that is, from the initial transient surface fluctuations, through melting and ablation, to the end of resolidification," they wrote in a paper published in the journal Nature.

For the imaging set-up, the scientists split a femtosecond laser pulse into two paths: a "pump beam" is aimed at the material target in order to create micro and nanostructural changes, and a "probe beam" is made to travel along a longer path to delay its arrival at the target area, where it acts as a flashbulb to illuminate the process so it can be photographed by a high-resolution, charge-coupled device (CCD) camera.

“With the scattered light pulsing at femtosecond time intervals, we can capture the very small changes at an extremely fast speed. From these images we can clearly see how the structures start to form,” says Guo. “The technique we developed is not necessarily limited to just studying the surface effects produced in my lab. The foundation we laid in this work is very important for studying ultrafast and tiny changes on a material surface.”

Guo's research at the University of Rochester was funded by the Bill & Melinda Gates Foundation and the United States Army Research Office.