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 0.63λ and full length at half maximum of 2.75λ 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.

中文翻译：

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兆赫频率下超声手术刀的宽带三维聚焦

超声聚焦具有很强的穿透性和优越的生物相容性，引起了生物医学科学的极大兴趣。传统的聚焦技术通常依赖于有源换能器阵列或曲面透镜，而声学超材料最近已作为替代方法出现。然而，低于 1 MHz 的超声聚焦超表面的低工作频率和窄带宽阻碍了其在实践中的广泛应用。在这里，我们基于紧凑和被动的超表面，通过实验实现了兆赫频率的宽带超声聚焦。三维点聚焦，范围从 0.5 到 1.4 MHz，具有高空间分辨率（半高全宽 0.63 λ和半高全长 2.75 λ在 1 MHz) 和可调谐焦距 (18.5-71 mm) 中进行了演示。通过实现宽带超声手术刀，进一步丰富了聚焦模式，有效地将声能汇聚在一个尖锐的“针”区域内。此外，随着超声强度的增强，焦点区域出现显着的温升，研究了其对超声频率的依赖性，并揭示了优化的工作频率。宽带和亚波长超声点聚焦和兆赫频率的手术刀将促进在粒子操纵、无损检测、诊断超声检查和超声治疗中的应用。