Riken creates unique surface structures with GHz femtosecond bursts

31 Jan 2023

Interaction with silicon allows formation of periodic patterns rather than simple ablation.

Femtosecond laser pulses can create a variety of physical modifications to the surface of materials, potentially enhancing their physical properties.

Laser etching and ablation is an increasingly common example, such as in the University of Rochester's study of femtosecond-laser treated surfaces able to act as high-temperature absorbers for enhanced thermoelectric generation efficiency.

A project at Japan's Riken Center for Advanced Photonics (RAP) has now demonstrated how femtosecond sources acting in burst mode at GHz frequencies can create unique 2D laser-induced periodic surface structures (LIPSS) on silicon substrates.

The work builds on previous Riken studies into how trains of femtosecond laser pulses can improve the efficiency and quality of surface ablation compared with conventional single-pulse approaches. A GHz burst mode can also be attractive for ablation because it improves heat diffusion away from the processed area which improves ablation efficiency, but the Riken project investigated whether the same laser burst might also produce more sophisticated effects.

As reported in International Journal of Extreme Manufacturing, the new approach creates more complex surface nanostructures than the simpler ablation operation that has been the focus of much surface modification research. This could potentially open new routes to micro and nanofabrication.

"Formation of LIPSS is a well-known phenomenon, realized on diverse solid surfaces by irradiating the material surfaces with multiple pulses of linearly polarized lasers, even in the air," commented Riken. "The ability of the GHz burst mode to enable fabrication of 2D LIPSS will offer the possibility of the formation of more functionalized surfaces and thereby diversify the application."

New commercial applications for femtosecond laser processing

In trials on crystalline silicon, the GHz bursts were able to produce not just 1D structures perpendicular to the laser polarization, of the kind made by a laser in single-pulse mode, but also other periodic structures parallel to the polarization to create a 2D lattice pattern.

According to the Riken team, the complex action of an ultrashort pulse laser on a surface involves three distinct nonlinear absorption process, along with energy transfer, phonon excitation, and physical ablation. With a conventional single-pulse source these interactions occur between laser and static material, but with a GHz burst each pulse interacts with an excited material in relaxation from a previous pulse, which can produce quite different results.

"This is a unique physical process that cannot be realized and controlled by the conventional single-pulse mode," commented Riken, which believes that novel ablation shapes and the creation of new materials through novel bonding structural changes might be possible with the new approach.

Riken anticipates that if fabrication of 2D LIPSS offers functionalized surfaces with diverse applications, then the commercialization of the research could soon follow.

"The results may offer a new possibility of GHz burst mode for processing other than ablation, including microbonding, crystallization, polishing, two-photon polymerization, and internal optical waveguide writing, said Riken's Koji Sugioka. "We believe that GHz burst mode will open new paths to femtosecond laser processing."