Fabrication and measurement of a hybrid resonant gradient index metasurface at 40kHz

Abstract

Over the past decade, gradient index metasurfaces(GIMs) have been voraciously studied for the numerous wave control capabilities that they facilitate. In this regard, a hybrid structure consisting of shunted Helmholtz resonators and a straight channel is often chosen as building blocks of the metasurface. Prior research, however, has primarily focused on GIMs that operate in the audible frequency range, due to the difficulties in fabricating such intricate structures at the millimeter and sub-millimeter scales, for ultrasonic applications. In this paper, we design, fabricate and experimentally realize a gradient index metasurface for airborne ultrasound at 40kHz. The fabrication of such a GIM is made possible by projection micro-stereolithography, an emerging additive manufacturing technique capable of micro-scale, high aspect-ratio features over a wide area. Simulations were first conducted to verify the metasurface design. Experiments were subsequently performed to corroborate the simulations and theory. The challenges faced by the thermoviscous effects, their usefulness in certain applications and optimal designs for minimal dissipation are discussed.