Vignesh Kannan (LMS, Polytechnique)

Fabrication and characterization of Si-based microarchitected metamaterials for elastic wave guiding

Abstract: Among a wide range of functionalities studied, the control of elastic wave propagation (or “wave guiding”) through architected metamaterials is particularly exciting, albeit challenging. Recent advances in scalable computational frameworks have found conventional experimental capabilities wanting on the following fronts: (1) fabricating materials with large number of unit cells (scale separation), (2) exciting and measuring elastic waves with high spatial and temporal resolution (to sample enough points on the dispersion surface), and (3) de-coupling structural/architectural and material effects on dispersion. We overcome these limitations through a multi-step experimental paradigm involving the fabrication of micro-architected materials on a silicon wafer, and characterizing their dynamic behavior using a home-built photo-acoustic pump-probe experiment. We will begin our discussion with an overview of “conventional” experiments on truss-based architected meta-structures, driven by existing numerical models. Motivated by the inadequacies of these experiments in the context of recent scalable computations, we will delve into a discussion of our new experiment. Samples were fabricated using cleanroom micro-fabrication techniques using commercially-procured Silicon-On-Insulator (SOI) wafers – our final samples are free-standing micro-architected “thick” films with over 500’000 unit cells within a 100 mm diameter wafer. We will then bring these samples to the lab, to characterize wave propagation using a pulsed-laser source to excite the acoustic wave, and an in-house-built heterodyne interferometer to measure particle velocities with tens of nanosecond temporal and sub-nanometer spatial resolution. We will end our discussion with experimental realizations of computationally-designed spatially-graded architectures for wave guiding, and the potential of this experiment to feed computational design codes with large, automated and high-throughput data sets — invaluable for rigorous data-driven computational design of elastic wave guides.