Femtosecond (fs) laser is an emerging technology with immense potential for precision processing, addressing limitations that constrain conventional laser-based techniques, such as low spatial resolution, uncontrollable thermal effects, and induced mechanical stress. Femtosecond lasers enable the precise processing of micro-nanostructures, including the drilling of micro-holes, fabrication of photonic crystals, construction of nano-/micro-devices, and applications in biomedicine. Laser ablation in liquids operates in intricate environments, with the liquid phase playing multiple crucial roles, including stress mitigation, enhancement of manufacturing precision, material removal, and prevention of material redeposition. A liquid environment also results in the creation of a transient high-pressure microenvironment inside gas bubbles formed by ultrafast, high-intensity laser pulses, facilitating the production of metastable material phases such as diamond, which often requires a physicochemical environment deviating from thermodynamic equilibrium.
In the context of the rapidly emerging nanotechnology society, laser-based techniques are gaining momentum in various industrial sectors, including electronics, drug delivery, and energy storage. Conventional wet chemistry methods have limitations, such as contamination, material deactivation, and difficulties in the fabrication of metastable phases. Laser-based synthesis and material processing offer a flexible and powerful approach to micro/nanofabrication, addressing challenges that limit the applicability of conventional techniques in manufacturing and contributing to the growth of nanotechnology in various applications.
Despite the immense potential of femtosecond laser ablation in liquids for nanomaterial synthesis, the intricacies of the process, intertwined with the physical and chemical reactions taking place hand-in-hand, present challenges in mechanistic understanding and controllable fabrication. A comprehensive understanding of the advantages and complexities of femtosecond laser-liquid media interactions requires advanced high spatial and temporal microscopy/spectroscopy. This review offers a critical overview of ultrafast physicochemical phenomena in laser-induced liquid-phase ablation, where the established reaction pathways and mechanisms governing the formation of micro/nanostructures are outlined and cataloged. Meanwhile, by laying out the current status of femtosecond laser liquid ablation, the limitations and future directions of the field are also discussed, leading to insights into promising future directions and significance for a broader scientific community.
Femtosecond laser ablation in a liquid covers a wide range of temporal and spatial scales and involves complex physical and chemical events. Figure 1 qualitatively illustrates the distribution of various processes, including laser propagation, focusing, and the generation of nanostructures in cavitation bubbles, where the timescales range from milliseconds to femtoseconds. Various techniques, such as time-resolved spectroscopy, shadow imaging, interference methods, and holographic detection, are used to capture ultrafast events occurring upon light-matter interactions (Table 1). To better elucidate the mechanistic details, the femtosecond laser ablation probe has shifted towards higher temporal and spatial resolutions, multi-angle observations, and continuous/single-shot probing techniques (Fig. 2). These advancements have enabled a deeper understanding of the micro/nano fabrication process, leading to improved controllability and large-scale production.
Research in transient observation of femtosecond laser ablation is crucial for understanding and controlling rapid physical and chemical dynamics during manufacturing. These studies reveal how materials interact and evolve over different time scales, offering insights into the optimization of femtosecond laser ablation products. However, such research in liquid environments is often tool-driven, with challenges posed by the liquid phase environment for high temporal and spatial resolution measurements and the characterization of complex physicochemical processes. To provide an in-depth analysis using transient observation techniques, the current study divides femtosecond laser liquid ablation into four stages based on time scales: generation and evolution of laser filament (Fig. 3), generation and evolution of solvated electrons (Fig. 4), generation and evolution of plasma (Fig. 5), and generation and evolution of cavitation bubbles (Fig. 6). These stages are underpinned by recent ultrafast studies that reveal the optical, physical, and chemical mechanisms underlying femtosecond laser ablation. Note that these processes are not mutually exclusive, with interactions and transformations occurring across both time and space alongside other concurrent physical and chemical events. Research on ultrafast laser ablation in liquid offers richer information regarding the physicochemical details and propels the controllability and progress of precision manufacturing in micro/nano science.
While femtosecond laser liquid ablation technology shows great promise in micro/nano-manufacturing, photonics, and biomedicine, understanding its intricate mechanisms is crucial for its wide application and mass production. For example, a knowledge gap persists between the early nanoparticle generation and cavitation bubble stages in the theoretical study of femtosecond-laser-induced nanostructure fabrication. Owing to the complexity of the system involved, quantitative models predicting the outcome of laser ablation are scarce, and the exploration of microstructural manufacturing mechanisms remains limited. Advanced time-resolved characterization techniques with the following capabilities are indispensable to track the evolution of physicochemical properties during femtosecond laser liquid ablation and represent future trends.
Because of the ultrafast nature of femtosecond laser liquid ablation at the micro/nanoscale, characterization techniques must offer high temporal and spatial resolutions with reasonable signal-to-noise ratios. An enhanced time resolution can reveal fundamental aspects within femtoseconds of laser excitation, whereas an improved spatial resolution can provide more accurate surface information. Owing to the limited time and space requirements of the probe, a high signal-to-noise ratio is essential for the effective capture of transient events.
Traditional techniques, which are largely pump-probe-based, presuppose consistent sample attributes before and after detection. However, in liquid-phase ablation, the fluid dynamics and external factors can disrupt data collection. Innovations such as ultrafast continuous imaging address these challenges and collect data across all delays from one laser pulse. Such progress, previously observed in femtosecond laser ablation in air, has now been incorporated into the study of liquid-phase ablation, paving the way for the real-time monitoring of femtosecond laser fabrication processes.
In the realm of multidimensional information extraction, current methods largely rely on femtosecond laser pump detection. This photon input-output system typically yields data in spectroscopy or imaging formats, contingent on materials exhibiting an optical response. However, emerging characterization tools, including photon-, X-ray-, and electron-based instruments, are unlocking the potential of techniques such as time-resolved X-rays and energy spectroscopy. These results provide real-time insights into the atomic details, valence states, and configurations during femtosecond-laser-driven liquid-phase ablation.
This review delineates the principal stages of femtosecond laser ablation in liquids and presents a comprehensive model framework. However, it is imperative to recognize persistent ambiguities within this domain. Attention is now directed towards the promise of transient observation techniques for forthcoming developments in femtosecond laser ablation in liquids. These methodologies offer profound insights that can drive future progress in this field.
