Overcoming Thermoviscous Effects: Design and Fabrication of Microacoustic Metagratings for Anomalous Refraction at Ultra-High Frequencies

Bianisotropic acoustic metagratings have been proposed as a solution for passive acoustic wavefront manipulation, particularly for flat acoustic lenses and ultra-high frequency ultrasound imaging. To address the challenges associated with airborne ultrasound at MHz frequencies, we have extended the concept of acoustic metagratings to a microscopic scale. However, this has led to an increase in thermoviscous effects, resulting in reduced efficiency and a drop in frequency of the transmission peak. To address these issues, we have developed several optimized microacoustic metagratings capable of refracting a normally incident wave at 2 MHz towards -35°, taking the thermoviscous effects into account in the modeling approach. Furthermore, we fabricated these designs using two-photon lithography to successfully demonstrate the feasibility of using microacoustic metagratings for anomalous refraction at ultra-high frequencies in acoustic experiments. Thanks to precise three-dimensional structuring by two-photon lithography, it was possible to create complicated metagrating geometries with accurate shape representation. In addition, this fabrication technique was adapted for the fabrication of metagratings and metamaterials directly onto MEMS devices such as acoustic transducers, resulting in a compact and integrated design. This article encapsulates the key findings from Ref. [1], while also presenting new measurements of a two-dimensional radiation pattern for the most effective metagrating design.