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MSE Seminar: Time-resolved Optomechcanical Sensing of Shock Pressures via Photonic Crystal Structure Speaker: Naresh Thadhani, MSE Professor & Chair, Georgia Tech University Title: TIME-RESOLVED OPTOMECHANICAL SENSING OF SHOCK PRESSURES USING 1-D PHOTONIC CRYSTAL STRUCTURES Abstract: 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 understanding of shock-initiated chemical, mechanical, and physical changes in reactive/energetic particulate materials. The highly heterogeneous structure of such materials involves changes dominated by meso-scale processes associated with interactions of shock waves with constituents of widely different physical and mechanical properties. Time-resolved sensing of localized pressure distributions associated with the interactions is essential for understanding the mechanistic processes and designing devices for use under extreme shock-compression loading conditions. Computational modeling approaches provide qualitative descriptions of meso-scale processes, but lack validation due to unavailability of time-resolved experimental methods for capturing the conditions at such length scales. To address this critical gap, we are investigating the design and application of 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 structures generate size-tunable characteristic spectral changes that can be observed in reflection either through reflectance peak (for DBRs), or minima (for OMCs). The multilayer structures also provide spatially-varying pressures, as localized changes in multilayer physical states produce corresponding changes in similarly scaled spectral responses. In this presentation, the shock-induced spectral responses obtained by directly subjecting the DBR and OMC structures to both homogeneous and heterogeneous pressure loads, using laser-driven shocks and time-resolved spectroscopy enabled by spectrograph-coupled streak camera, will be described, along with results of concurrently performed optomechanical simulations utilizing a custom multi-physics framework. The results 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 micro-scale spatial variations, is also demonstrated for particulate materials. Bio: Dr. Thadhani is Professor and Chair of the School of Materials Science and Engineering at Georgia Tech. His research focuses on the mechanisms of physical, chemical, and mechanical changes in materials, subjected to shock-compression and high-strain-rate loading. He has authored/co-authored 300+ publications in journals and conference proceedings, including several reviews and book articles. He is Editor of Springer Book Series on Shock Compression, Associate Editor of the Journal of Dynamic Deformation of Materials, and of Shock Waves: An International Journal. He has served as a consultant for several industries, and on various national academies panels, advisory boards, and academic program review committees. He is recipient of the TMS Leadership award, Fellow of ASM International and American Physical Society, and Academician of EuroMediterranean Academy of Arts and Sciences. This Event is For: Campus
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