[Paper Review] Damage threshold evaluation of thin metallic films exposed to femtosecond laser pulses: the role of material thickness

George D. Tsibidis, Dimitris Mansour+1 more|arXiv (Cornell University)|May 11, 2022

Laser Material Processing Techniques61 references35 citations

TL;DR

This study presents a multiscale theoretical model to predict the damage threshold of thin metallic films under femtosecond laser irradiation, accounting for thickness-dependent optical and thermal responses. The model, validated with experimental data on Ni, Cr, and Au, reveals a linear decrease in damage threshold with decreasing film thickness up to the optical penetration depth, followed by saturation toward bulk-like behavior.

ABSTRACT

The employment of femtosecond pulsed lasers has received significant attention due to its capability to facilitate fabrication of precise patterns at the micro- and nano- lengths scales. A key issue for efficient material processing is the accurate determination of the damage threshold that is associated with the laser peak fluence at which minimal damage occurs on the surface of the irradiated solid. Despite a wealth of previous reports that focused on the evaluation of the laser conditions that lead to the onset of damage, the investigation of both the optical and thermal response of thin films of sizes comparable to the optical penetration depth is still an unexplored area. In this report, a detailed theoretical analysis of the impact of various parameters such as the photon energies and material thickness on the damage threshold for various metals (Au, Ag, Cu, Al, Ni, Ti, Cr, Stainless Steel) is investigated. A multiscale physical model is used that correlates the energy absorption, electron excitation, relaxation processes and minimal surface modification which leads to the onset of material damage. The satisfactory agreement of the theoretical model with some experimental results indicates that the damage threshold evaluation method could represent a systematic approach towards designing efficient laser-based fabrication systems and optimizing the processing outcome for various applications.

Motivation & Objective

• To develop a theoretical framework that predicts the damage threshold of thin metallic films under ultrashort laser pulses.
• To investigate how material thickness influences the optical absorption and thermal response of metals during femtosecond laser irradiation.
• To correlate thickness-dependent energy absorption and electron-lattice relaxation dynamics with the onset of surface damage.
• To validate the model against experimental data for Ni, Cr, and Au, ensuring predictive accuracy.
• To provide a systematic tool for optimizing laser processing parameters in nanofabrication and thin-film applications.

Proposed method

• Employment of a 1D Two-Temperature Model (TTM) to simulate electron and lattice temperature dynamics in thin metal films on dielectric substrates.
• Incorporation of a multiple reflection algorithm to compute thickness-dependent dielectric functions and optical properties.
• Use of a Drude-Lorentz model with temperature-dependent relaxation times to account for transient optical behavior during laser pulses.
• Application of a melting-point-based thermal criterion to define the damage threshold as the fluence causing lattice temperature to reach the melting point.
• Simulation of laser irradiation at 515 nm and 1026 nm with 170 fs pulse duration to assess wavelength dependence.
• Inclusion of electron-phonon coupling and heat conduction terms in the TTM equations to model energy transfer and thermal relaxation.

Experimental results

Research questions

• RQ1 How does the thickness of a thin metallic film affect its damage threshold under femtosecond laser irradiation?
• RQ2 What is the role of optical penetration depth in determining the absorbed energy and subsequent thermal response in thin films?
• RQ3 How do variations in photon energy (515 nm vs. 1026 nm) influence the thickness-dependent damage threshold?
• RQ4 To what extent do electron localization and reduced electron diffusion in thin films alter the damage threshold compared to bulk materials?
• RQ5 Can a theoretical model incorporating thickness-dependent optical properties and TTM accurately predict experimental damage thresholds?

Key findings

• The damage threshold decreases linearly with decreasing film thickness up to approximately the optical penetration depth.
• For film thicknesses beyond the optical penetration depth, the damage threshold asymptotically approaches the bulk material value.
• The model shows good agreement with experimental data for Ni, Cr, and Au, validating its predictive capability.
• Thinner films exhibit enhanced energy localization due to suppressed electron diffusion, leading to lower damage thresholds.
• The optical properties of thin films deviate significantly from bulk values when thickness is comparable to the optical penetration depth, affecting absorption and damage onset.
• The model predicts that materials with distinct electron distributions (e.g., noble vs. transition metals) exhibit different thickness-dependent damage thresholds due to variations in electron-phonon coupling and optical response.