Yun Kai

Mechanical failure is a marked limitation for plastics used in structural, protective and coating applications. In particular, perforation under high-rate deformation is difficult to mitigate through conventional molecular design^1,2. Cross-linking is widely used to improve the thermal and chemical stability of polymers, yet under mechanical deformation, it typically renders materials more brittle, limiting impact resistance and functional lifetime³. Overcoming this fundamental trade-off between stability and toughness remains a central challenge. Here we demonstrate that embedding a small fraction of force-sensitive mechanophores as cross-links into common polymers fundamentally reverses this trade-off, producing materials with substantially enhanced ballistic energy dissipation. At strain rates exceeding 10⁷ s^-1, we show that mechanophore-cross-linked networks absorb up to about 115% more energy than conventional thermosets and surpass even their uncross-linked thermoplastic counterparts. We attribute this behaviour to a force- and adiabatic-heating-driven local thermoset-to-thermoplastic transition, in which selective mechanophore scission facilitates viscoplastic deformation at the impact site while preserving network integrity in the surrounding regions. We demonstrate the generality of this strategy in both glassy polystyrene and rubbery styrene-butadiene-styrene triblock copolymers. These results establish mechanophore cross-linking as a design principle for converting commodity polymers into impact-resilient materials and open directions at the intersection of polymer mechanochemistry and extreme-strain-rate material behaviour.

We present a tabletop method to investigate how tailored laser excitation can enhance shock wave generation in water. By delivering two spatiotemporally separated laser pulses focused into a confined liquid layer, we identify an optimized excitation condition that maximizes the resulting shock pressure. Remarkably, dual-pulse laser excitation leads to an overall shock pressure that far exceeds the sum of the shock pressures generated by the individual pulses. This nonlinear amplification reveals a previously unexploited laser-matter interaction regime that overcomes saturation limits in laser-driven shock generation.

Silica-based glasses have found numerous applications in every field of human endeavor. Understanding their mechanical behavior under high strain rates is essential for the use of these materials in extreme environments. We report on a highly unusual fracture behavior observed in borosilicate glass facilitated by stress-induced molecular rearrangements, allowing the glass to withstand tensile stresses up to 11 GPa. Converging surface acoustic waves (SAW) with controlled amplitude are generated optically and used to investigate the high-strain-rate (10 ⁸ s ⁻¹ ) fracture behavior of borosilicate glass. Above a tensile stress threshold of 6 GPa, fracture of the glass surface is observed, characterized by ejection of material and radial cracking. Unexpectedly, upon further increase of the SAW stress, a second threshold of 8 GPa tensile stress is observed above which the fracture probability dramatically decreases. Raman spectra and nanoindentation measurements of shocked samples indicate significant changes in the topology and coordination numbers of silicon and boron atoms in the amorphous network. These results suggest the ability of thresholded atomic rearrangements to serve as an intrinsic high-strain-rate toughening mechanism for enhanced fracture toughness in amorphous borosilicate glass under dynamic strain conditions.

The quasi-static properties of micro-architected (meta)materials have been extensively studied over the past decade, but their dynamic responses, especially in acoustic metamaterials with engineered wave propagation behavior, represent a new frontier. However, challenges in miniaturizing and characterizing acoustic metamaterials in high-frequency (megahertz) regimes have hindered progress toward experimentally implementing ultrasonic-wave control. Here, we present an inertia design framework based on positioning microspheres to tune responses of 3D microscale metamaterials. We demonstrate tunable quasi-static stiffness by up to 75% and dynamic longitudinal-wave velocities by up to 25% while maintaining identical material density. Using noncontact laser-based dynamic experiments of tunable elastodynamic properties and numerical demonstrations of spatio-temporal ultrasound wave propagation, we explore the tunable static and elastodynamic property relation. This design framework expands the quasi-static and dynamic metamaterial property space through simple geometric changes, enabling facile design and fabrication of metamaterials for applications in medical ultrasound and analog computing.

The development of methods for the generation of strong ultrafast electromagnetic pulses in the terahertz (THz) spectral range has led to a surge of progress in nonlinear THz spectroscopy and THz control of molecular and collective responses. For spectroscopy in the 1-THz range, the submillimeter wavelengths and associated large spot sizes, large optical elements, and short distances between final focusing elements and samples can lead to cumbersome experimental setups that are incompatible with some sample environments. Here, we introduce a novel terahertz ring excitation (TREx) optical pumping geometry to generate superposing, focusing fields in planar THz waveguides made out of the electro-optic material lithium tantalate. High THz fields, >175 kV/cm, are generated and measured optically with no free-space THz propagation. The field level achieved by pumping with a sequence of concentric rings of excitation light exceeds by about 20× the result of a single cylindrically focused line of pump light that has been used routinely in previous work. The technique opens new prospects for compact waveguide-based linear and nonlinear THz spectroscopy and signal processing.

Mechanical metamaterials at the microscale exhibit exotic static properties owing to their engineered building blocks^1,2,3,4, but their dynamic properties have remained substantially less explored. Their design principles can target frequency-dependent properties^5,6,7 and resilience under high-strain-rate deformation^8,9, making them versatile materials for applications in lightweight impact resistance^10,11,12, acoustic waveguiding^7,13 or vibration damping^14,15. However, accessing dynamic properties at small scales has remained a challenge owing to low-throughput and destructive characterization^8,16,17 or lack of existing testing protocols. Here we demonstrate a high-throughput, non-contact framework that uses MHz-wave-propagation signatures within a metamaterial to non-destructively extract dynamic linear properties, omnidirectional elastic information, damping properties and defect quantification. Using rod-like tessellations of microscopic metamaterials, we report up to 94% direction-dependent and rate-dependent dynamic stiffening at strain rates approaching 10² s⁻¹, as well as damping properties three times higher than their constituent materials. We also show that frequency shifts in the vibrational response allow for characterization of invisible defects within the metamaterials and that selective probing allows for the construction of experimental elastic surfaces, which were previously only possible computationally. Our work provides a route for accelerated data-driven discovery of materials and microdevices for dynamic applications such as protective structures, medical ultrasound or vibration isolation.

Achieving high repeatability and efficiency in laser-induced strong shock wave excitation remains a significant technical challenge, as evidenced by the extensive efforts undertaken at large-scale national laboratories to optimize the compression of light element pellets. In this study, we propose and model a novel optical design for generating strong shocks at a tabletop scale. Our approach leverages the spatial and temporal shaping of multiple laser pulses to form concentric laser rings on condensed matter samples. Each laser ring initiates a two-dimensional focusing shock wave that overlaps and converges with preceding shock waves at a central point within the ring. We present preliminary experimental results for a single ring configuration. To enable high-power laser focusing at the micron scale, we demonstrate experimentally the feasibility of employing dielectric metasurfaces with exceptional damage threshold, experimentally determined to be 1.1 J/cm², as replacements for conventional optics. These metasurfaces enable the creation of pristine, high-fluence laser rings essential for launching stable shock waves in materials. Herein, we showcase results obtained using a water sample, achieving shock pressures in the gigapascal (GPa) range. Our findings provide a promising pathway towards the application of laser-induced strong shock compression in condensed matter at the microscale.