A Spectroscopic and Theoretical Photo-thermal approach for Analysing Laser Ablated Structures
We investigated experimentally and theoretically the micro-ablation process of metallic (Al) targets by using nanosecond laser pulses at 532.0 nm wavelength in atmospheric air. We analysed experimentally the ablation rate and the temperature of the ablation plasma as a function of laser fluence and pulse number. The fluence was varied between 3 and 3000 J/cm2 by changing the laser pulse energy, whereas the pulse number was varied with an increment that ensures high accuracy of the optical microscopy measurements on the ablated structures and also an approximate constant ablation rate during multi-pulse irradiation. The microscopy data indicate that the ablation rate and the crater depth increase approximately linearly with the 1/3 power of the fluence. For a given fluence, the crater depth increases strongly, approximately linearly during the first 30 pulses and much slowly during the next pulses. The variation of the crater depth and ablation rate with pulse number was further addressed by spectrometric analysis of the ablation-plasmas produced at high fluences on the metallic target. We found direct connection between the emission lines intensities and the crater depth, whereas the plasma electron-temperature varies similarly to the ablation rate when increasing the pulse number. The ablation threshold fluence and the ablation rate in the low and medium fluence regime (up to several tens of J/cm2) were also addressed theoretically within the frame of a photo-thermal model which accounts for the material heating, melting and evaporation upon irradiation with the nanosecond laser pulses. The thermal and optical properties of the target material are considered dependent on the aggregation state of the material (solid or liquid) during the laser irradiation. The theoretical and experimental results are in good agreement indicating the validity of the theoretical model for low and medium laser fluences.
Keywords: Nd:YAG laser, aluminium, laser ablation, ablation plasma spectroscopy, photo-thermal modelling