Many questions regarding dynamic materials could be answered by using time-resolved ultra-fast imaging techniques to characterize the physical and chemical behavior of materials in extreme conditions and their evolution on the nanosecond scale. In this work, we perform multi-frame phase-contrast imaging (PCI) of micro-voids in low density polymers under laser-driven shock compression. At the Matter in Extreme Conditions (MEC) Instrument at the Linac Coherent Light Source (LCLS), we used a train of four x-ray free electron laser (XFEL) pulses to probe the evolution of the samples. To visualize the void and shock wave interaction, we deployed the Icarus V2 detector to record up to four XFEL pulses, separated by 1-3 nanoseconds. In this work, we image elastic waves interacting with the micro-voids at a pressure of several GPa. Monitoring how the material’s heterogeneities, like micro-voids, dictate its response to a compressive wave is important for benchmarking the performances of inertial confinement fusion energy materials. For the first time in a single sample, we have combined an ultrafast x-ray framing camera and four XFEL pulse train to create an ultrafast movie of micro-void evolution under laser-driven shock compression. Eventually, we hope this technique will resolve the material density as it evolves dynamically under laser shock compression.
Ultrafast dynamic ellipsometry, a technique that probes a sample with chirped laser pulses at two angles and with two
orthogonal polarizations, was used to measure the effective refractive index across the ablation region of a Si(111) wafer
exposed to a 100 fs ablation pulse. The resulting refractive index data show a significant increase in the extinction
coefficient, indicative of the melting of silicon.
Ellipsometry is a very useful optical technique to probe the complex index of refraction of a material. We perform
dynamic ellipsometry using ultrafast lasers to probe the complex index dynamics during passage of compressional
shock waves through materials of interest. When used to measure passage of a shock wave through dielectric materials,
dynamic ellipsometry provides a direct measure of the equation of state (except temperature). In addition, the changes in
complex index can be used to measure phase transformations and their kinetics. Using two CCD cameras and two
Wollaston prisms, ellipsometric data at two incidence angles and two polarizations can be obtained simultaneously.
Adding a spectrograph in front of each CCD camera and using chirped probe pulses and frequency domain
interferometry provides a few hundred ps of ellipsometric data from a single shock event.
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