Broadband Three-Dimensional Focusing for an Ultrasound Scalpel at Megahertz Frequencies

Ultrasound focusing, with strong penetrability and superior biocompatibility, has attracted substantial interest in biomedical science. Conventional focusing techniques generally rely on an active transducer array or curved lens, and acoustic metamaterials have recently emerged as an alternative approach. However, the low operating frequency of an ultrasound focusing metasurface below 1 MHz and the narrow bandwidth hinder its broad applications in practice. Here, we experimentally realize broadband ultrasound focusing at megahertz frequencies based on a compact and passive metasurface. Three-dimensional spot focusing, ranging from 0.5 to 1.4 MHz, with high spatial resolution (Full width at half maximum of 0.63\ensuremath{\lambda} and full length at half maximum of 2.75\ensuremath{\lambda} at 1 MHz) and tunable focal length (18.5--71 mm), is demonstrated. The focusing pattern is further enriched by realizing a broadband ultrasound scalpel, which effectively converges sound energy within a sharp ``needle'' area. Additionally, accompanied by an enhancement of ultrasound intensity, a prominent temperature rise appears in the focal region, the dependence of which on the ultrasound frequency is investigated and the optimized operating frequency is revealed. Broadband and subwavelength ultrasound spot focusing and a scalpel at megahertz frequencies would promote applications in particle manipulation, nondestructive detection, diagnostic sonography, and ultrasound therapy.