The pulse duration dependence and inter pulse separation of pulse sequences influence on the energy specific ablation volume are still today open questions in laser material processing. The complete timescales for the processes involved in laser ablation can be visualized from the initial pulse absorption to the material removal occurring on a microsecond time scale by using an pump-probe microscopy method. Here we present time-resolved and energetic studies of the surface dynamics of the laser ablation processes on aluminium, copper and stainless steel bulk materials, which were analysed for pulse durations ranging from 500 fs to 20 ps, while keeping all other laser parameters constant. Our results indicate that the ablation process should be initiated by pulse durations shorter than the mechanical relaxation time of 3 ps and remain uninterrupted until the final state is reached after about 1 µs.
Ultrashort lasers have gained widespread use for both scientific and industrial applications due to their highly efficient and precise material ablation properties. In order to optimise the interaction between the ultrafast laser source and the target an in-depth understanding of the optical and ablation dynamics is required. Here we present a study of the complete ablation dynamics ablation properties of the three most relevant metals (Copper, Aluminium and Stainless Steel) after ultrashort laser pulses for the first time. This is achieved through a temporal analysis of the change in the optical properties after laser irradiation using pump-probe ellipsometry and pump-probe microscopy. The complex refractive index change in the first 50 ps after laser irradiation is analysed with a 1 ps resolution using pump-probe ellipsometry. The results show a large decrease in the extinction coefficient k for all the analysed metals in the first few ps after the pulse impact. This indicates an early stage decrease in the material density due to unloading of the pressure buildup generated by the stress confinement state in the metal skin depth. This pressure buildup and density decrease results in phase change and material motion at time scales from 100 ps to 1 ns, which can be visualised with pump-probe microscopy. Depending on the metal, ablation mechanisms such as spallation and phase explosion can be visualised and followed into the equilibrium state at about 10 µs. The effects of the early stage dynamics can be used to describe ablation efficiency trends observed for double and pulse bursts of various inter-pulse delay times.
Ultra-short pulsed lasers offer a great potential in precise and efficient material processing. Experimental and theoretical studies on efficiency of laser material processing from metals have demonstrated a high degree of dependency on the laser pulse duration. Within these studies, the investigation of the transient energy deposition in material takes a great significance for the thermal and mechanical material response after laser irradiation. The scope of this study was to investigate the ultra-fast energy deposition in a copper metal during the irradiation with a 680 fs ultra-short laser pulse at a 1056 nm and a 528 nm wavelength. For this purpose, a numerical analysis of the laser-matter interaction was performed by using the optical Drude critical point (DCP) model and thermal two temperature model fully coupled with thermoelasticity theory (2T-TE). The DCP model was incorporated into the 2T-TE to simulate the ultra-fast laser energy deposition and optical material response of copper. For comparison with experimental data a pump probe ellipsometry set-up was used. The pumpprobe ellipsometry set-up combines the high temporal resolution of pump-probe technique and ellipsometric measurements of optical indices. It was found numerically that a dynamic change on re ectivity and optical penetration depth at 1056 nm is mainly induced by a rapid temperature increase. In contrast at the irradiated wavelength of 528 nm the laser pulse absorption is mainly described by the interplay of the the interband excitation and intraband heating of conduction band electrons The time resolved simulation of optical indices (n, k) confirms the temporal experimental observation of refractive index and extinction coefficient within the first 10 ps from pump-probe ellipsometry set-up.
Ultrashort pulsed laser sources offer new possibilities in precise and efficient material processing. Deep understanding of the fundamental laser-material interaction aspects is of great importance. We report on pump-probe reflectometric investigations of the ablation process on molybdenum over the complete temporal process range from the pulse impact to the final steady state. The ablation process can roughly be separated in three sections. In the first tens of picoseconds mainly the optical material properties are changed without significant material motion. Between 50 ps and a few ns the irradiated material is bulging in a spallation or phase explosion process. The actual ablation by material ejection is observed at delay times greater than 20 ns. The transient reflectivity during and in the first tens of ps after the laser irradiation in conjunction with the transient absorption influences decisively the laser-matter interaction for example when working with longer pulse durations or double pulse sequences. Direct measurements of the absorption properties by ultrafast time-resolved ellipsometry at fluences close to the ablation threshold fluence are missing to date. In this paper, pump-probe ellipsometric measurements on molybdenum – complementing the pump-probe reflectometric measurements – are presented showing ultrafast changes of the complex refractive index N = n – ik including additional information on the absorption. The imaginary part k is reduced already after 10 ps by 50% representing an increase of the optical penetration depth by a reduction of the material density. These extensive investigations pave the road towards a better understanding of pulse duration dependent laser ablation efficiency, double or burst mode laser ablation and lattice modifications in the first ps after the laser pulse impact.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.