High-pressure physics
Shocks are stress waves associated with high pressures and high strain rates, characterized by a discontinuous jump in state variables across the shock front. Shock loading is associated with strain rates on the order of 106 to 109 s-1 and stresses exceeding tens of GPa. We delve into the examination of diverse materials subjected to extreme shock loading conditions. Our primary objective is to conduct experimental investigation of shock metamorphism on a tabletop scale, with the application goal of synthesizing exotic materials. Recently, we have successfully showcased the formation of Danburite rings within borosilicate glasses under tabletop shock loading, resulting in substantial improvements in material toughness. In my dream, we can push boundaries further, envisioning the transformation of materials into even more exotic states, particularly converting graphite into lonsdaleite (diamond with hcp crystal structure, more than 50% harder than conventional fcc diamond).
Figure. (A) The focusing shock method. The images show a laser-generated shock wave converging to a focus in a thin liquid layer sandwiched between two glass substrates. (B) to (E) show the material responses under the focusing shock loading.
Before we can fully realize our flagship setup to create Mbar shock (anticipated to be fully functional by the end of 2026), we have achieved a milestone in creating a novel shock enhancement method named “Zebra” using a free-space optical cavity to successively boost an initially weak acoustic wave from the linear to nonlinear regime directly on the surface of a solid sample. The fast nature of the technique also enables the shortening of the “shock formation length” into a sub-mm scale, effectively avoiding shock attenuation during the build-up process. This is different than classical laser-shock experiments, which are based on the absorption of a laser pulse in a planar transducer layer deposited onto the sample of interest often resulting in damage and deformation. The key idea of the methodology is to architect a train of line-shaped laser pulses carefully positioned along the propagation path of the initial stress wave, giving it a boost by every photoacoustic excitation along the way. In material science, especially in the study of phase transitions, it has been difficult to optically excite modes at low frequencies. This methodology may offer a solution for related problems, since the hereby induced shock wave with highly directional enhancement results in unique profiles of dynamic compressive or tensile strain.
Figure. Zebra technique, named after the structured laser pattern on the sample plane (A) Additive photoacoustic excitation principle (B) Enhanced SAW acoustic wave measured by a common path interferometer driven by a CW laser.
MHz vibrational spectroscopy tailored for soft matters
Polymer-based mechanical metamaterials with microscopic three-dimensional morphologies could exhibit extraordinary material properties. Specifically, their unique properties such as high stiffness/strength-to-density ratios have been demonstrated, but their response under extreme conditions (e.g., high pressures, high strain rate) remains largely unknown. To fill this knowledge gap, we combine expertise from our ZEBRA method and two-photon polymerization to develop a MHz vibrational spectroscopy tailored for metamaterials. Increasing interest in metamaterials under extreme conditions has developed due to their potential use in nuclear fusion, where their tunable shock impedance is hypothesized to facilitate shock transmission and improve the compression of a deuterium-tritium mixture. Our method will enable measurements approaching such extreme conditions, providing a route to systematically discover extreme mechanical properties and new metamaterial designs for future applications.
THz radiation
The development of methods for the generation of strong ultrafast EM waves in the terahertz (THz) range has led to a surge of progress in nonlinear THz spectroscopy and THz control of molecular and collective responses. Here, we work on a Terahertz Ring Excitation (T-REx) scheme for generating large THz fields in a LiTaO3 waveguide. The key element is a customized echelon to shape a fs laser pulse into a cone, forming concentric laser rings to successively generate THz wave in LiTaO3. The laser rings launch the THz wave in a focusing geometry, allowing larger field strengths to be obtained via superposition at the center of the rings. In addition, a variable magnification telescope is used to control the effective scan speed of the rings across the waveguide. By carefully tuning the telescope, the rings can be made to move with the generated THz waves, resulting in continuous pumping over a large area. By using the focusing effect and a velocity-matching pump beam, we achieve THz field strength up to 300 kV/cm (100 mT magnetic field in the out-of-plane direction) in thin LT waveguides.
Click on the following image for a video, showing a focusing THz wave.
