Helmholtz resonant cavity based metasurface for ultrasonic focusing

As a new method of acoustic focusing, metasurfaces have the advantage of achieving high-resolution focusing with compact and planar geometry in a relatively broad frequency band. Among these, the Helmholtz resonator cavity based metasurface has been widely utilized due to its superior performance. However, the research on this metamaterial has focused on the audible frequency band and it remains a challenge to apply this structure to the ultrasonic frequency band for biomedical applications. One reason is that the ultrasonic metasurfaces typically require complex and deep subwavelength microstructures, which is a great challenge to the current state-of-the-art fabrication techniques, and the other reason is that transferring metasurfaces with the conventional metal structure in air to those in water induces a significant transverse wave effect. In this study, we first designed a Helmholtz resonant cavity based metasurface working at 1.5 MHz according to the generalized Snell law, which is the frequency employed in biomedical applications. The resonant cavity unit was made of resin and air, which suppressed the transverse wave effect greatly. The makings and sparse distribution of the unit enabled the easy fabrication of the metasurface by 3D printing. Then, the focusing characteristics were investigated through numerical simulation and good focusing results were achieved, although the unit structure did not meet full phase coverage. Finally, the metasurface was fabricated, and the focusing was demonstrated experimentally. This work paves a way for the application of Helmholtz resonant cavity based metasurfaces in the biomedical ultrasound field.