Multilayer optical structures for time-resolved meso-scale sensing of shock-compression in heterogeneous materials
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Heterogeneous materials play important roles in many different applications across a wide range of industries. Examples include engineered composites, particulate systems, and energetic materials, which all display complex meso-scale features and behaviors. This complexity leads to significant gaps in the understanding of heterogeneous materials, especially under extreme conditions such as shock-compression. A fundamental challenge in this area of research is a lack of experimental diagnostics that can provide spatially-resolved information under the demanding temporal and environmental conditions of shock loading. Multilayer optical structures, due to their unique spectral responses that can be correlated to externally induced loads, have the potential to serve as a new class of sensor for these complex materials and conditions. This work presents the theory, development, and evaluation of novel multilayer optical structures as time-resolved pressure sensors with meso-scale spatial sensitivity. Time-resolved spectroscopy of laser-driven shock-compression experiments on the multilayers demonstrated spectral shifts of the characteristic spectral peaks to shorter wavelengths (blueshifts), and simultaneous velocimetry established that these spectral shifts are unambiguously correlated to the laser-driven shock pressure. An optomechanical model was developed and used to predict the spectral response of the multilayers as a function of pressure, and when informed with quality empirical data, quantitatively matches the experimentally observed blueshift. Experiments and simulations of spatially heterogeneous shock loading demonstrate the ability of the multilayers to resolve not only multiple pressures but also to capture the subtle features present in shock-compressed heterogeneous materials, all while maintaining nano-second level temporal resolution. Overall, multilayer-based sensing is a fundamentally new time-resolved diagnostic method in the fields of high-strain-rate material behavior and shock physics. This work has provided the theoretical and empirical foundation for broad classes of different multilayer structures, and demonstrated their unique potential utility for capturing the complex meso-scale pressure histories needed to enable new insights into the dynamic response of heterogeneous materials.