Reconfigurable large-scale bulk metamaterials for broadband ultrasonics

Recent research has focused on developing smart metamaterials with programmable acoustic properties, but past designs have only demonstrated the ability to tune small samples composed of at most few dozen cells. Their reconfiguration mechanisms are not scalable to larger structures and yield lossy designs unsuitable for ultrasound applications of interest today such as ultrasound imaging. This work introduces a method to create ultrasonic metamaterials programmable on very large macroscopic scales. Remarkably, the method can impose desired, inhomogeneous acoustic properties to bulk metamaterials composed of arbitrarily large collections of unit cells using only one controller. The concept is experimentally demonstrated by designing a low loss ultrasonic beam forming and steering device that has very high directivity, is programmable, and has the bandwidth requirement necessary for ubiquitous ultrasound applications. The device consists of two patterned plates composed of thousands of unit cells tuned to form the prescribed inhomogeneous refractive index profiles needed for beamforming and dynamic steering. The geometry of the cells is collectively modified by rotating one of the plates about one of its ends. The technique to reconfigure broadband bulk metamaterials at ultrasound frequencies will enable unprecedented wave control on a large scale and with very low power consumption. (paper)