Stiffness tuning of a functional-switchable active coding elastic metasurface

Recently, a new type of metasurface namely coding metasurface has emerged as a new technology in manipulating waves. Based on its R-bit, a coding metasurface utilizes 2^R of coding units to achieve real-time wave controls which makes the operation of a metasurface easier. Although coding metasurfaces have simpler configurations than conventional metasurfaces, it is yet unavoidable to have structural reconfiguration of metasurface in order to perform different functionalities in single or different frequencies. To overcome this drawback, a functionally switchable 3-bit active coding elastic metasurface comprised of stacked piezoelectric patches shunted with negative capacitance circuit is proposed to manipulate elastic longitudinal waves. By tuning the negative capacitance in the circuitry element, arbitrary tuning of stiffness can be attained which leads to the coverage of full 2π phase range without modifying the structure of metasurface. In addition, analytical model and numerical simulations are developed to verify capability of the proposed coding metasurface for covering full 2π phase shift besides having high transmission at the same time. By utilizing the 8 fundamental coding units with different phase response, the proposed metasurface is designed and simulated based on a finite element approach to illustrate switchable functionalities such as arbitrary refraction and wave focusing, which can be realized through proper tuning of the negative capacitance, in broadband frequencies.