Optics & Photonics

Mechanics of ultra-quick femtosecond laser ablation at gigahertz bursts

Gigahertz femtosecond lasers are well suited to improve and control laser machining quality and engineer the physicochemical properties of materials. Gigahertz femtosecond lasers are used by materials scientists to study the laser-material interactions, but the technique is difficult because of the ablation dynamics involved.

In a new report now published in Science Advances, Minok Park and a team of scientists in laser technologies and mechanical engineering at the University of California, Berkeley, studied the ablation dynamics of copper using gigahertz femtosecond bursts via time-resolved scattering imaging, emission imaging, and emission spectroscopy.

Researchers have combined several methods to reveal the process of gigahertz femtosecond bursts, which rapidly removed molten copper from an irradiated spot for material ejection. The process of material ejection stopped after burst irradiation due to the limited amounts of remnant matter. This provided insights into the mechanisms of complex ablation triggered via gigahertz femtosecond bursts that are employed to select optimal laser conditions in cross-cutting processes, nano- and micro-fabrication, and spectroscopy.

Gigahertz and femtosecond laser ablation
Laser ablation is the process of removing material from surfaces via the interaction of high-power lasers, with significant impact across energy harvesting and storage, biomedicine, optoelectronics, and spectroscopy. Materials scientists have achieved significant capacities to offer a direct, one-step, chemical-free pathway for material machining and ablation sampling by using ultrafast, femtosecond laser ablation. The process is suited to precisely regulate the ablation features.

In this study, Park and colleagues developed a variety of methods to examine real-time laser ablation dynamics. They studied the ablation of copper with a gigahertz femtosecond laser pulse and compared the outcomes to femtosecond pulse ablation. The combined methods resulted in the fast removal of molten liquid material while halting material removal after burst irradiation. The researchers obtained direct insights into the dynamics and dominant mechanism of gigahertz ablation with femtosecond pulses.

The ultrafast laser experiments
During the experiments, the team used an optical system to investigate the ablation mechanisms of copper with a single femtosecond laser pulse and gigahertz femtosecond bursts under atmospheric pressure. Using time-resolved scattering and emission images, the researchers visualized light-emitting and non-emitting species. They characterized the crater morphology with white light interferometry and scanning electron microscopy to ablate a pristine copper surface to a depth of 500 nm. The scientists noted the appearance of irregular, resolidified structures on the irradiated spot. The ablation efficiency of the gigahertz bursts improved manifolds compared to single-pulse irradiation.

Radiation from a single-pulse fs laser. a.u., arbitrary units, Time-resolved emission imaging, optical emission spectroscopy, and scattering imaging displaying the ablation dynamics at a fluence of 18.7 J/cm2 at various time scales. Using different ICCD gate widths of 100 ns, 200 ns, 500 ns, and 1 s, respectively, scattering pictures were captured. The Cu target surface is depicted in these photographs by the blue lines, and the images below the lines show mirror reflections from the polished Cu surface. Blue scale bars are 10 m; white scale bars are 50 m. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397

Ablation at GHz in fs with 50 pulses. At a fluence of 18.7 J/cm2 (0.37 J/cm2 per pulse, 38-ns dwell time), time-resolved emission imaging (A), optical emission spectroscopy (B), and scattering imaging (C) demonstrate the dynamics and processes of ablation. Images of the scattering occurred at intervals of 100 ns, 200 ns, 500 ns, and 1 s, respectively. The target Copper surface is depicted by the blue lines. 50 m white scale bars. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397.

Visualizing the outcome
The research team observed time-resolved images, emission spectra, and scattering images to investigate the ablation dynamics of a single-pulse femtosecond laser on a copper surface. The images revealed the ejection of two different types of particles from the substrate, including those released at different timescales: (1) after a 0–200 nanosecond delay, and (2) those ejected between 300 nanoseconds and 4 microseconds.

The researchers explored time-resolved emission imaging and spectroscopy alongside images of ablated plumes induced via gigahertz bursts composed of 50 pulses. They observed spherically shaped copper plasmas for a period of 30 nanoseconds during the experiments.

Laser-ablation dynamics.
After a time period of 200 nanoseconds, the team did not observe ejecta at the center of the laser-matter interaction zone, indicating that the target was not ablated further. This behavior distinctly differed from the dynamics of single-pulse ablation.

The team devised two contributing mechanisms to the underlying process of material ejection, including (1) the vaporization of materials at the center and (2) the ejection of liquid from the molten pool edge via fast, radially outward fluid motion to recoil pressure exerted by vaporization. While the copper nanoparticles were expelled from the edge of the molten pool, a limited amount of liquid remained frozen on the crater surface, which they verified using scanning electron microscopy.

Summary of ablation dynamics (A) R-t plots of observed ejecta induced by single-pulse fs laser and GHz bursts Experimental findings on the ablation dynamics of (B) single-pulse fs lasers and (C) GHz fs lasers. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adf6397

Comparative laser-ablation dynamics
The scientists used time-resolved emission imaging, emission spectroscopy, and scattering images of ablation, driven by gigahertz femtosecond laser bursts. When they released the scattering images at a timescale later than 300 s, the ejecta showed how the irradiation spot cooled down to inhibit material removal.

The researchers compared the two experimental conditions and further studied the early ablation dynamics of copper driven by gigahertz bursts to note distinctly different ablation dynamics of a gigahertz burst driven with 200 pulses compared to the gigahertz burst with 50 pulses. The outcomes provided direct confirmation of the different mechanisms of gigahertz-directed laser-induced ablation when compared to single-pulse irradiation.

In this way, Minok Park and colleagues observed the ablation dynamics of copper by using single femtosecond laser pulses and gigahertz bursts with 50–200 pulses via multimodal probing methods. The single-pulse femtosecond laser irradiation produced two types of particles with different ejection speeds at different timescales.

The outcomes provide insights to comprehensively understand the ablation mechanisms underlying gigahertz femtosecond bursts that are critical to exploring a variety of applications across laser processing, machining, printing, and spectroscopic diagnostics.

More information: Minok Park et al, Mechanisms of ultrafast GHz burst fs laser ablation, Science Advances (2023). DOI: 10.1126/sciadv.adf6397

Jan Kleinert et al, Ultrafast laser ablation of copper with ~GHz bursts, Laser Applications in Microelectronic and Optoelectronic Manufacturing (LAMOM) XXIII (2018). DOI: 10.1117/12.2294041

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