Acoustic emission control avoids fluence shifts caused by target runaway during laser synthesis of colloids

Pulsed laser ablation in liquids (PLAL) has been established as a scalable method to synthesize ligand-free nanoparticles. However, for continuous nanoparticle synthesis with high nanoparticle yield, it is mandatory to maintain a constant fluence, demanding a fixed distance between the focusing lens and target. The latter becomes a non-trivial task during continuous gram-scale nanoparticle production due to the quick removal of target material causing a diminishing productivity. Hence, to maintain a stable process, a perpetual optimization of the working distance is required. In this paper, a field programmable gate array-based measurement and control system is used for online adjustment of the working distance based on acoustic emission (AE) measurements. Ablation-characteristic acoustic frequencies were detected at base frequency correlating to the repetition rate as well as higher harmonics of the latter. The harmonic distortion of AE during laser ablation could be correlated to the location of the focal point. The amplitude intensities correlate well with the nanoparticle productivity. The optimal working distance was reproducibly adjusted in 5.79 min ± 0.41 min with a deviation of 0.33 mm ± 0.04 mm. The automated system provides the basis for temporal stability and scalability of the ablation process.