Laser-Based Ultrasound Spectroscopy for Characterization of Mechanical Metamaterials

Mechanical metamaterials, possessing custom-designed 3D structures at the nano- to microscale, exhibit exceptional properties that surpass those of their constituent materials. These structures unveil a diverse mechanical property landscape. Previous studies relied primarily on contact-based methods for mechanical characterization, such as nanoindentation, extracting static properties like stiffness and strength. These techniques often necessitate laborious sample preparation or specialized sample designs. There is a need for time-resolved characterization techniques to accelerate the process of characterizing the mechanical behavior of metamaterials and broaden our understanding of their dynamic response.<br/>We present a characterization method for metamaterials that allows us to capture their effective dynamic elastic constants and linear dynamic response in a reliable and iterative manner. We fabricate architected materials using two-photon lithography with feature sizes on the order of ~1 µm and unit cells on the order of 10 µm. We employ a laser ultrasonic scheme to determine their effective elastic properties to excite elastic waves within the materials. We utilize a common-path interferometric setup to register the sample eigenvibrations. We experimentally reconstruct partial dispersion relations and attenuation properties of the 3D micro-architected materials, present their fully experimental dynamic elastic surfaces, and demonstrate the potential to use dynamic responses to identify defects in these materials. We validate this technique using a variety of 3D architectures along various crystallographic orientations. The field of mechanical metamaterials is in need of novel characterization techniques since, at this point, the design of new metamaterials has significantly outpaced efforts to construct structure-property relations. Our work addresses this need and provides a route for the accelerated data-driven discovery of materials and microdevices for dynamic applications such as protective structures, medical ultrasound, or vibration isolation.