“Time-resolved optomechanical sensing of pressure distributions during shock-compression of heterogeneous materials”
Shock-compression of materials generates unique and non-equilibrium states that allow studies in regimes not easily accessible by other methods. Most intriguing is the initiation of reactions in reactive/energetic particulate mixtures or composites, occurring via meso-scale processes dominated by shock wave interactions with constituents of widely different elastic and plastic properties. Time-resolved sensing of pressure distributions is essential for understanding the meso-scale reaction processes and designing devices for use under extreme shock-loading conditions. Various models have successfully provided qualitative descriptions of meso-scale processes in heterogeneous materials. However, it has not been possible to quantitatively capture those effects, due to lack of time-resolved experimental methods available for validation of conditions occurring at such length scales. To address this critical gap, we are investigating the design and application of novel optomechanical sensors based on a Distributed Bragg Reflector (DBR) composed of dielectric stacks alternating layers of high and low refractive index materials, and an Optical Micro Cavity (OMC) composed of dielectric cavity layer material placed between two metal-mirror layers. These 1-D photonic crystal based structures generate size tunable characteristic spectral properties that can be clearly observed in reflection either through reflectance peak (for DBRs) or minima (for OMCs). Unlike other commonly utilized piezoresistive/piezoelectric sensors, optomechanical multilayer structures also provide spatially-varying pressures, as localized changes in multilayer physical states produce corresponding changes in similarly scaled spectral responses. The shock-induced spectral responses are studied by directly subjecting the DBR and OMC structures to both homogeneous and stepped pressure loads, using laser-driven shocks and time-resolved spectroscopy enabled by spectrograph-coupled streak camera. Concurrently, optomechanical simulations utilizing a custom multi-physics framework are performed. The experiments reveal a highly time-resolved spectral response to shock compression, manifesting as wavelength shifts and intensity changes as a function of the shock pressure, which correlate well with simulations from predictive models. The ability to capture pressure distributions with micrometer-scale spatial variations is also demonstrated, and is being utilized for time-resolved spatial sensing of shock compression effects in heterogeneous particulate material forms.