Super-resolution ultrasonic imaging of stiffness variations on a microscale active metasurface

Abstract

Surface acoustic waves propagating over an immersed membrane metasurface, such as an array of capacitive micromachined ultrasonic transducers, can be leveraged to achieve subwavelength focusing and imaging. This is demonstrated numerically and experimentally utilizing a time reversal method on a 2D membrane array at MHz frequencies. The focusing region is a dense metasurface of CMUT membranes with 6.5 MHz resonance frequency that supports a wave field that is evanescent normal to the metasurface and capable of super-resolution along the metasurface. Electrostatically actuated membranes, spatially separate from the focusing region, are used to generate the focused wave field. Subwavelength focusing is demonstrated on the metasurface with a resolution of a single membrane resonator or λ/5. Similar techniques allow for super-resolution imaging of a subwavelength defect or change in the medium of the focusing region. A subwavelength sized imaging target, obtained by altering the stiffness of a single membrane by 1.2%, is shown to be properly imaged with subwavelength resolution. These results pave the way for practical implementation of ultrasonic super-resolution imaging systems using metasurfaces.