On the Pulsed Laser Ablation of Metals and Semiconductors

A comparison of effects across disparate experimental regimes through the study of pulsed laser ablation over several orders of magnitude in pulse duration, fluence, and material properties.

September 1, 2022

Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio

Experimental setup and representative spatial beam profile

Laser ablation is an incredibly active field, including research in fundamental physics (e.g. non-equilibrium thermodynamics), industrial (e.g. laser peening) and medical (e.g. laser dentistry) applications, and even security and defense applications (e.g. chemical detection and laser lethality). Over such a broad range of applications with varying degrees of precision required, it is often difficult to find unifying trends that allow researchers and practitioners to understand and compare effects across disparate physical regimes. Furthermore, differences in research goals, experimental apparatus, and theoretical approaches create barriers to establishing a broad physical intuition capable of translating results from study to study.

To that end, the fundamental goal of this research is to build from a broad set of experimental and theoretical data to a narrower set of scaling relations, heuristics, and trends that provide a roadmap for understanding laser ablation across relevant regimes. Specifically, the objective is to quantify the effects of ablation including crater morphology, ablation efficiency, and plume spectral emissions across metals and semiconductors using pulse durations spanning picoseconds to microseconds and fluences from ones to thousands of J/cm².

The laser ablation problem can be broken down into three independent groups of variables describing the laser, material, and environmental conditions. Key laser parameters are wavelength, pulse duration, pulse energy, and spot size. The primary material variables are the bulk optical and thermochemical properties. Environmental conditions of interest are the sample temperature, the interface conditions, and the background pressure above the sample.

For this research, the primary wavelength of interest is 1064 nm. This is due to the large body of literature available at this wavelength and the plethora of affordable optical equipment. While there is substantial literature on ultraviolet (UV) laser ablation as well, the results at 1064 nm are more easily translatable to longer wavelengths. This is because UV laser ablation involves significant photoionization of the plume, whereas the effect is much reduced at 1064 nm. Thus, interpreting results at longer wavelengths is more straightforward due to the laser-plume interaction consisting of mostly the same physical processes.

Another reason is that mass removal is primarily thermal at longer wavelengths, where a photon is absorbed by an electronic state in the material that couples to a lattice vibration, thereby raising the macroscopic temperature. Ultraviolet ablation often involves direct ejection of surface electrons (i.e. the photoelectric effect) that remove mass by Coulombic attraction with positive ions left behind.

The primary pulse durations of interest in the present study are tens of picoseconds (ps) to hundreds of microseconds (μs). Similar to the choice of wavelength, this is to narrow down the space to include only the processes which are “similar” enough to be able to meaningfully compare. While the goal of this research is to create ways to interpret ablation effects across these wildly different regimes, some lines must be drawn in order to derive tools simple enough to be of actual use. For that reason, femtosecond (fs) or ultrashort laser ablation is not included in this effort.

Below the tens of ps, the laser-material interaction is entirely different, and the laser-plume interaction is completely absent. Furthermore, there is extensive published research on continuous wave (CW) laser ablation, especially in the laser welding and laser lethality communities. No attempt is made to include CW effects in the heuristics developed here; there is enough physics in the decades of pulse duration research between 100 ps and 100 μs. Pulse energy and spot size are mainly combined to create fluence, but ablation effects do not just depend on fluence. The fluence needs to be applied fast enough for mass removal to occur, and the spot size really does effect ablation, even if the fluence is the same.

This work was performed by Todd A. Van Woerkom for the Air Force Institute of Technology. For more information, download the Technical Support Package (free white paper) at mobilityengineeringtech.com/tsp under the Optics, Photonics & Lasers category. AFIT-0008