Characterization of the Phononic Landscape of Natural Nacre from Abalone Shells

Natural design and fabrication strategies have long served as a source of inspiration for novel materials with enhanced properties. Less investigated is the prospect of leveraging the complexity of readily available, naturally occurring micro-/nanostructures as platforms for investigating functional materials. In the field of phononics, exploiting structural biocomposites is gaining traction; but finding natural phononic structures that interact with ultra- and hypersonic acoustic waves remains an open quest. In this context, the phononic behavior of natural Nacre, a biocomposite often looked at for inspiration due to its superlattice-like architecture of alternating organic and inorganic phases, is here characterized. To such end, a combination of non-contact pump-probe laser ultrasonics techniques and Brillouin spectroscopy are employed to interrogate Nacre's hierarchical structure at the micro- and nanoscale and measure its phononic dispersion behavior in the MHz and GHz range. It is found that for wavelengths longer than the brick-and-mortar characteristic length, Nacre behaves as a dispersionless medium with effective transversely isotropic properties; but as the wavelengths become comparable to its structural periodicity an involved phononic spectrum arises which challenges the notion of a perfectly periodic, high mechanical-contrast biocomposite.