Compact metamaterial plates for acoustic wave steering and focusing across an ultra-broad frequency bandwidth in MHz imaging applications

• Meta-plates and meta-multi-card designed by time-domain design method. • Acoustic wave manipulation across an ultra-broad frequency bandwidth of 3-20 MHz. • A 1.5-mm-thick meta-plates and meta-multi-card are thinner than a coin. • Demonstration of non-destructive testing with excellent detection of small defects. Metamaterials offer unique possibilities in cloaking, focusing, steering, holography, absorption and free the designer from the constraints of traditional materials. However, existing acoustic metamaterial structures for wave-field shaping are limited to operating within a specific and relatively narrow frequency range, limiting their applications to those that require wide frequency bandwidth signals, such as medical and non-destructive imaging. In this paper, we propose a time-domain design method that leads to a metamaterial structure capable of steering and focusing ultrasonic waves in the range 3-20 MHz. This design is fabricated as 1.5-mm-thick metamaterial plates (meta-plates), composed of silicone and resin layers, to achieve acoustic wave steering and focusing across this ultra-broad frequency bandwidth. The performance of the meta-plates is demonstrated on metallic specimens containing defects, demonstrating their effectiveness for non-destructive testing applications. Hence, this paper brings the concept of broadband metamaterial-based devices into practice, paving the way for their broader use in MHz imaging applications.