Sub-nanosecond pump-probe analysis of metal targets during nanoparticle generation by laser ablation in liquid and air – Temporal and spatial laser pulses control towards productivity increase and nanoparticle size control

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The synthesis of nanoparticles (NPs) has become an essential technology for creating nanomaterials with adjustable physicochemical properties. Among various NP synthesis techniques, pulsed laser ablation in liquids (PLAL) is particularly remarkable due to its versatility in target materials and the resulting NP compositions. In the first phase of our project (428315411), pump-probe microscopy (PPM) experiments were conducted to examine the dynamics of PLAL, from pulse impact to cavitation bubble (CB) collapse, on a timescale ranging from ps to µs. These single pulse results were utilized to address the primary limitations of PLAL. For example, sub-ns double pulse PLAL was used to decrease NP bimodality. Additionally, the study investigated productivity-limiting factors of PLAL, such as bubble and plasma shielding, as well as the significance of the scanning strategy for scaling up PLAL. Building upon the fundamental PLAL processes established in the previous project, our objective is to investigate how fundamental mechanisms of temporal and spatial pulse shaping in PLAL affect NP size distribution and productivity, with the aim of achieving continuous high yield NP production. During the first project phase, we discovered that a second pulse delayed 600 ps in a double pulse PLAL setup can reduce NP bimodality. We plan to further explore this technique, as well as a second temporal frame at the ns scale (CB formation), where a second pulse can influence spallation layer emergence, NP growth, and coalescence. In addition to the inter-pulse delay, the peak fluence (J) has a direct impact on ablation efficiency and NP size distribution. Thus far, fluence control has been achieved by varying the laser pulse energy, but we propose modifying the spatial Gaussian beam into an elliptical profile to increase the interaction area and process the target at the most efficient J, maximizing NP production. Furthermore, the use of diffractive optical elements (DOEs) can generate multiple beams, reducing repetition rates and allowing for parallel processing with increased inter-pulse distance. DOEs represent a promising alternative to technologically limited faster scanning systems for avoiding CB shielding in PLAL and maximizing NP production. We will evaluate the proposed PLAL setups for Au and FeNi, two materials with different physicochemical properties that permit the study of fundamental mechanisms for implementing temporal and spatial pulse shaping in PLAL to optimize NP size distribution and increase production. Our final objective is to study and develop a cost-effective spatially and temporally shaped PLAL setup that provides NP size control and continuous high production rates reaching the g/h scale required for industrial applications.