Pulsed Laser Ablation in Liquid (PLAL) is a cutting-edge technique for nanoparticle synthesis. The technique can be understood as the irradiation with a laser source of a material immersed in a liquid. The versatility of the technique due to the possibility to change the liquid and irradiated material represents a critical advantage. Besides, the nanoparticles can also be tailored by adding specific ions and dopants, allowing precise control over the nanoparticles' composition and surface chemistry. This control is essential for catalysis, or nanomedicine applications where purity, composition, and nanoparticle surface groups directly influence the performance.

Despite the advantages of PLAL, controlling nanoparticle size distribution remains a challenge. Traditional approaches involve the employment of surfactants, which can reduce PLAL's purity and surface activity. Recent strategies to control nanoparticle size without adding extra substances involve photonic approaches to modify the laser-material interaction. During PLAL, a critical element are the formed cavitation bubbles due to the interaction of the laser with the liquid and target. These bubbles are key elements in the nanoparticle formation process, since they act as micro-reactors for nanoparticle synthesis. Hence, controlling the bubble formation by adjusting laser parameters such as intensity, duration, and distance between beams is a promising approach for nanoparticle size control. The effect of bubble engineering, especially the lateral interactions between bubbles, on nanoparticle size during PLAL has yet to be explored.

Therefore, in this contribution lead by Farbod Riahi, PhD student, Dr. Carlos Doñate Buendía research group leader, and Prof. Dr. Bilal Gökce, head of Materials Science and Additive Manufacturing at the University of Wuppertal, an investigation is reported on how the spatial separation between synchronous double laser pulses affect the formation and dynamics of the cavitation bubbles and consequently, the properties of the nanoparticles produced by PLAL. The relevant research results are published in Photonics Research, Volume. 11, Issue 12, 2023 (Farbod Riahi, Alexander Bußmann, Carlos Doñate-Buendia, Stefan Adami, Nicolaus A. Adams, Stephan Barcikowski, Bilal Gökce. Characterizing bubble interaction effects in synchronous-double-pulse laser ablation for enhanced nanoparticle synthesis[J]. Photonics Research, 2023, 11(12): 2054)

Exploring the dynamics of synchronous spatially separated bubble pairs (Fig. 1a) induced by double laser pulses during laser ablation in water of gold (Au) and yttrium aluminum garnet (YAG) was performed by a developed coaxial time-resolved diffused shadowgraphy imaging in order to observe the merging and collapse of these bubbles from multiple perspectives with ns temporal resolution (Fig. 1b). The results confirm that the lateral separation of these bubbles significantly influence their interaction. One of the key findings of this research is the notable impact of double bubble interactions on the size distribution of synthesized nanoparticles. When bubbles are positioned at a specific lateral distance of twice the maximum cavitation bubble size, a significant increase in particle size of more than 3 times is observed, a phenomenon that is consistent across both materials explored, Au, and YAG (Fig. 1c).

The increase of the resulting nanoparticle size is explained by a modified interaction between bubbles for that specific lateral separation, including symmetry breaking, shrinking, and collapse dynamics (Fig. 1d). Specifically, for a distance between bubbles of twice of their maximum size, new bubble interaction dynamics as the formation of inter-bubble jets before the collapse and high speed bubbles' movement are observed (Fig. 1e). Thanks to these observations from the developed shadowgraphy setup, allow to associate the three-fold nanoparticle size increase (Fig. 1f) to the unique bubble interaction dynamics.

Fig. 1 (a) Laser induced bubble pairs with maximum bubble height normalized spacing. (b) Schematic of the side- and top-view imaging geometry. (c) STEM images of Au and YAG nanoparticles synthesized for different inter-pulse distances. (d) The shadowgraphs exhibit the temporal evolution of the cavitation bubble dynamics. (e) Inter-bubble distance (IBD) measurements at ∆x=460 µm, are plotted against the attraction velocity of the individual bubbles toward each other. (f) Impact of normalized lateral interpulse distance on YAG and Au nanoparticle size.

Manipulating bubble interactions in PLAL is confirmed as a potential method to control nanoparticle size distribution without additional substances. This investigation represents a significant step in understanding how cavitation bubbles can be engineered to influence nanoparticle synthesis, opening new paths for material processing and nanoparticle design.

This study paves the way towards further exploring the effects of spatial and even temporal control of bubble pairs towards modifying their interaction and establishing paths for PLAL size controlled nanoparticle synthesis. Future cavitation bubble control through temporal and spatial inter-pulse control is essential towards nanoparticle size control to customize the size of the produced colloids, as required for energy, biomedicine, or catalysis applications among many others.