Investigating material modification in a heretofore unexplored laser regime

Very short laser pulses have promise in materials processing applications, particularly for etching patterns onto substrates used in photonics. However, the critical power for self-focusing can lead to instabilities, limiting potential applications. McMillen et al. explored laser-material interactions in the laser regime of ultrashort pulses beyond this critical power limit, which have been mostly overlooked until now.

The authors exposed fused silica to low-energy, sub-30 femtosecond focused laser pulses. Observing the resultant microstructures, they found a variety of stable, defined bulk modifications in the silica. Their findings contradict the popular belief that this regime is unsuited for material modifications.

“Some of these stable bulk modifications had never been seen before,” said author Yves Bellouard.

One modification they observed was densification in the fused silica. Higher levels of densification in fused silica increase the refractive index contrast between laser-affected zones and their surroundings, which could ultimately help develop new functionalities and devices for photonics and allow more compact photonics circuits to be produced.

The authors also found that densification of the silica by ultrashort laser pulses was an order of magnitude greater than previously reported with longer pulses. This finding suggests that shorter pulse durations may make materials more dense.

“This work is important as it shows that the exposure regime of ultrashort pulses beyond the critical power for self-focusing should be explored further as it leads to new morphologies with highly densified materials,” Bellouard said.

Next, the authors will study modifications caused by a laser in this regime for an assortment of glass and crystal materials.

Source: “Laser-induced densification of fused silica using spatially overlapping sub-30 fs pulses,” by Ben McMillen, Oliver Uteza, Raphaël Clady, Nicolas Sanner, Marc Sentis, Farhang Nabiei, Ya Cheng, and Yves Bellouard, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0011317 .