Yongxiang Hu

Nanoscale rotational dynamics has recently gathered significant interest in many fields, including ultrasensitive sensing and precision manipulation, but presents challenges for observation techniques. Here, we employ movie-mode transmission electron microscopy to investigate the rotational dynamics of laser-excited nanotriangles, achieving multiframe imaging with nanosecond and nanometer resolution. A motion reconstruction methodology was developed to facilitate accurate spatiotemporal trajectory tracking. The observed rotation dynamics of nanotriangles exhibit an obvious axis dependence, characterized by stable rotation around the first principal axis and inherent instability with respect to the second axis. This rotational stability bifurcation resulting from angular momentum redistribution aligns impeccably with the tennis racket effect, marking the first confirmation of classical rigid-body rotational theory at the nanosecond-nanometer scale. Furthermore, through the theoretical analysis of laser-induced rotation, we quantitatively assessed the adhesion state between the nanotriangle and substrate, offering an effective approach for evaluating interfacial properties. This study confirms the tennis racket effect at the nanosecond-nanometer scale and enhances the understanding of nanoscale rotational dynamics.

Background and Objective: The utilization of ultrafast lasers in bone material ablation is of paramount importance in minimally invasive surgical procedures. While ultrafast lasers are precise, achieving high efficiency and low thermal damage in bone ablation remains challenging.Methods: This study investigates the ablation behaviors of bovine bone (storage in water) under different defocus distances using a femtosecond laser under ambient air conditions.Results: The presence of bone canaliculi on the ablated surface indicates that the microstructure of the bone is well‐preserved during laser ablation. The surface morphology and ablation area far beyond the heat‐affected zone indicate that mechanical forces, rather than thermal effects from the femtosecond laser, were the primary factor in bone removal. The laser fluence and the overlap rate are pivotal in determining the ablation rate, which can be expressed in terms of cumulative laser fluence. The use of a scanning electron microscope (SEM) and a Raman spectrometer has revealed that no thermal damage occurs during laser ablation when the laser parameters are appropriately adjusted. It has been demonstrated that excessive cumulative laser fluence can result in thermal damage during continuous ablation.Conclusion: The ablation method achieves a rate of 1.2 mm³/s with no thermal damage, which is comparable to that of the Er: YAG laser. The study thus establishes a threshold for a process that removes bone material with a high ablation rate and minimal damage, using a femtosecond laser.

Multiscale parameter optimization for laser peen forming (LPF) on 6005A-T6 aluminum alloy plates was conducted through a combination of simulation and experimentation. By obtaining the optimal parameter, this study aims to explore the constrained deformation and forming laws of the integral stiffened plates. Detailed descriptions were provided regarding the dynamic response process and transient behavior of aluminum alloy plates under ultrahigh strain rates, along with an in-depth analysis of the stress evolution. The results reveal that laser beam diameter and laser beam energy can achieve large range forming, while the number of tracks facilitates the precise deformation adjustment. During the 12-track LPF process, there is an overall upward trend in deformation values accompanied by a dynamic increase in the bend curvature. After static relaxation, the deformation value recovers to 55.2% of the final bending curvature. The chord direction scanning of stiffened plates exhibits a larger bending curvature, indicating its greater forming capacity for large-sized single unfolding direction formation; whereas, the unfolding direction scanning of stiffened plates excels in achieving efficient integrated two-way forming.

Understanding laser-induced jumping has attracted great interest in nanomaterial launching and transfer but requires a high spatiotemporal resolution visualization. Here, we report a jumping dynamics of nanoplate driven by ultrafast laser-induced stress using time-resolved transmission electron microscopy. Single-shot imaging reveals a nondestructive launching of gold nanoplates in several nanoseconds after the pulsed femtosecond laser excitation. The temperature rise and acoustic vibration, derived from ultrafast electron crystallography with a picosecond time resolution, confirm the existence of a laser-induced elastic stress wave. The generation, propagation, and reflection of thermal stress waves are further clarified by atomic simulation. The nonequilibrium ultrafast laser heating produces a compressive stress wave within several picoseconds, constrained by the supporting substrate under nanoplate to provide thrust force. This compressive stress is subsequently reflected into tensile stress by the substrate, promoting the nanoplate to jump off the substrate. Furthermore, the uneven interface adhesion results in the jumping flip of nanoplates, as well as, diminished their jumping speed. This study unveils the jumping regime driven by impulsive laser-excited stress and offers understanding of light-matter interaction.

Irreversible ultrafast events are prevalent in nature, yet their capture in real time poses significant challenges. Traditional single-shot imaging technologies, which utilize a single optical pump and single delayed electron probe, offer high spatiotemporal resolution but fail to capture the entire dynamic evolutions. Here, we introduce a novel imaging method employing a single optical pump and delayed multiple electron probes. This approach, facilitated by an innovative deflector in ultrafast electron microscopy, enables the acquisition of nine frames per exposure, paving the way for statistical and quantitative analyses. We have developed an algorithm that corrects frame-by-frame distortions, realizing a cross-correlation enhancement of ∼26%. Achieving ∼12 nm and 20 ns resolution, our method allows for the comprehensive visualization of laser-induced behaviors in Au nanoparticles, including merging, jumping, and collision processes. Our results demonstrate the capability of this multiframe imaging technique to document irreversible processes across materials science and biology with unprecedented nanometer-nanosecond precision.