Wavelength dependence of femtosecond laser ablation of thin gold films
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The demand for efficient laser processing of materials is as strong as ever and this requires a deep understanding of the fundamental laser-material interactions. There has been a recent upsurge in the number of studies into the evolution of nanoparticles produced during laser ablation, originally to improve debris management, but more recently to utilise these nanoparticles in various applications. This study explores methods to improve the control of the nanoparticles produced and in turn gleans insight into the wavelength dependence of laser ablation mechanisms of thin gold (Au) films. This experimental study has identified a fundamental difference in the interaction of a femtosecond laser pulse with thin Au films depending on the laser wavelength. A matrix of deposition rates, temperatures and substrates were used to generate a set of Au films (10-90 nm thickness) with a range of grain microstructures that were characterised using Atomic Force Microscopy (AFM). By accounting for the optical properties of the films, it was possible to examine the laser ablation mechanisms at three laser wavelengths, using the same absorbed fluence. A femtosecond laser (500 fs) was used to ablate each thin film at three laser wavelengths; 343 nm (UV), 515 nm (Green) and 1030 nm (IR). The ablation mechanisms at each wavelength were explored by monitoring the ejection of electrons, ions and nanoparticles from the film using a selection of equipment including a Langmuir probe, Scanning Electron Microscopy (SEM) and schlieren imaging. The first key result was that laser ablation at UV and Green wavelengths resulted in the same linear relationship between the volume of the grain and the volume of the nanoparticles. Significantly, no relationship at all was observed when the film was ablated at the IR wavelength. This wavelength dependence was explained using the complementary computational study carried out by Lin and Zhigilei who identified the significance of non-equilibrium dynamics on the response of noble metals to laser pulses. Photons with hv > 1.9 eV exceed the interband transition threshold (ITT) of Au for the excitation of 5d(10) electrons to the Fermi level. According to Fermi's Golden rule, the higher density of states associated with the d-band electrons compared to the s-band results in a greater absorption strength and heat capacity. Therefore if the ITT is exceeded, higher electron temperatures are reached, resulting in a substantial decrease in the electron-phonon coupling time. The electron-phonon coupling occurs on shorter time-scales than the mechanical relaxation of the material and thermoelastic ablation of the Au film ensues. In this study, the three laser wavelengths of UV, Green and IR had photon energies of 3.62 eV, 2.41 eV and 1.21 eV, respectively. Therefore laser absorption of either UV or Green pulses exceeded the ITT and lead to thermoelastic ablation of the film into the grains. Since the ITT was not achieved with IR photons, ablation proceeded over a longer timescale, whereby melting of the film occurred and any relationship between the grains and nanoparticles was effectively lost. This fundamental material response of Au to femtosecond laser pulses of different laser wavelengths was also demonstrated in the electronic emission, which was measured using a Langmuir probe under vacuum conditions. A substantially higher number of emitted electrons were detected following UV or Green laser pulses compared to IR, as well as the notable addition of thermionic emitted electrons. The number of detected charges was then used to calculate the contribution of Rayleigh-charge instabilities to the break up of grains into smaller nanoparticles at the UV and Green wavelengths. Finally, the development of a schlieren imaging setup facilitated the real-time visualisation of the expansion of the nanoparticle plume at each wavelength and once again demonstrated the significance of the photon energy on the laser ablation mechanism and nanoparticle generation.
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