Focusing of ultrasound waves is critical to a number of clinical and industrial applications including biomedical and underwater imaging, nondestructive evaluation and material processing. This paper discusses the use of a novel 'add-on' gradient refractive index (GRIN) metamaterial structure made of concentric shells, to focus ultrasonic waves generated by conventional transducers. Analysis based on the Huygen's principle and numerical simulations is used to design the geometric and material properties of the proposed structure, whose working is demonstrated through experiments. Varying the shell material or thickness is shown to offer an elegant and straightforward way to tailor the focal spot inside the target material. The concentric-shell GRIN lens proposed here has a simple design, and has a potential to be used in dynamic focusing without advanced lenses or electronic steering.

超声波的聚焦对于 许多 临床和 工业应用至关重要，包括生物医学和水下成像， 无损评估和材料处理。本文讨论了 由 同心壳制成的新型“附加”梯度折射率（GRIN）超材料结构的使用， 聚焦传统换能器产生的超声波。基于惠更斯原理和数值模拟的分析被用来设计所提出结构的几何和材料特性，并通过实验证明了其工作原理。显示出改变外壳材料或厚度提供了一种优雅而直接的方式来调整目标材料内部的焦点。本文提出的同心壳GRIN透镜设计简单，无需使用高级透镜或电子转向装置，就有可能用于动态聚焦。