Prof. Naresh Thadhani
Time-Resolved Optomechanical Sensing of Pressure Distributions During Shock-Compression Of Heterogenous Materials
Time-resolved meso-scale monitoring of the shock-compression response of heterogeneous materials, e.g., explosives, energetic/reactive materials, and particulate systems, is essential for understanding the phenomena being studied, validation of models describing the related phenomena, and the design of devices used under such extreme conditions. Computational models, while predicting characteristics of meso-scale processes, fail to capture the resulting macroscopic responses due to lack of experimental validation of phenomena occurring at such length scales. Even our ability to monitor pressure distributions across a plane is limited to planarity measurements based on wave-front arrival. We are investigating optomechanical sensors based on 1-D photonic crystal structures that generate size-tunable characteristic spectral property changes as a function of pressure. Examples of these include an Optical Micro Cavity (OMC) composed of dielectric cavity layer placed between two metal mirror layers and a Distributed Bragg Reflector (DBR) composed of dielectric stacks of alternating layers of high and low refractive index materials. Unlike other commonly utilized piezoresistive or piezoelectric pressure (stress) gauges, optomechanical multilayer structures (of nano-to-micrometer thickness) offer the potential for significant spatial sensitivity, as localized changes in multilayer physical states caused by high pressure effects, produce corresponding changes in similarly scaled spectral responses. The pressure-induced spectral responses are studied by directly subjecting the OMC and DBR structures to both homogeneous and heterogeneous (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, to correlate predictions with experimental observations. The results demonstrate highly time-resolved spectral shifts of characteristic spectral peaks to shorter wavelengths (blueshifts), with simultaneous velocimetry establishing that the blueshifts are unambiguously correlated to the laser-driven shock pressure pulse. Model predictions of the spectral response of the multilayers as a function of pressure, informed with quality empirical data, also show good quantitative match with the experimentally observed blueshifts. Experiments and simulations of spatially heterogeneous shock loading demonstrate the ability of the multilayers to resolve not only multiple pressures, but also the ability to capture the subtle features present in shock-compressed heterogeneous materials, all while maintaining nano-second level temporal resolution. The overall trends demonstrate the unique potential of the 1-D photonic crystal optomechanical sensors and their utility for capturing complex meso-scale pressure histories needed to enable new insights into the complex dynamic response of heterogeneous materials.
Bio: Naresh Thadhani is Professor and Chair of the School of Materials Science and Engineering (MSE) at Georgia Tech (GT). He joined the GT faculty in 1992, after six-years in the Center for Explosives Technology Research at New Mexico Tech, and two years as a post-doc at CalTech. He received his B.E. in 1980 from the Malaviya National Institute of Technology in Jaipur, India, M.S. from South Dakota School of Mines, and Ph.D. from New Mexico Tech, all in Metallurgical Engineering.
Professor Thadhani’s research focuses on the fundamental mechanisms of shock-induced physical, chemical, and mechanical changes under high-pressure shock-compression, and the deformation and fracture response of metals, ceramics, polymers, and composites, subjected to shock compression and high-strain-rate loading. He has led significant advancements in the understanding of shock-induced phase transformations, chemical reactions, and mechanical properties of bulk metallic glasses; design, development, and characterization of structural energetic materials, and the shock-compression response of highly heterogeneous (granular) materials through meso-scale computational simulations and experimental studies with meso-scale spatial and nano-second resolution temporal diagnostics. He has advised 15 visiting scientists/post-docs; 30 Ph.D and 14 M.S degree students; mentored 100+ undergraduate researchers; attracted research funding from federal agencies including the AFOSR, ARO, DARPA, DTRA, ONR, NSF, as well as from several national laboratories and industries; and co-edited 12 books/proceedings, published more than 300 papers in refereed journals (including several invited review articles) and conference proceedings. He has served or is serving on review boards including the National Academy of Science panel at the Army Research Laboratory (2015 and 2016), academic program review at the Universities of Texas at Austin and at El Paso and the University of Notre Dame, University of California multi-campus national lab collaborative research and training awards program review, and external advisory boards of several Materials Science and Engineering programs.
Professor Thadhani is a Fellow of ASM International and the American Physical Society.