Continuously tunable ultrasonic focusing by Moiré metalenses.

Tunable ultrasonic focusing holds great significance in both medicine and engineering. Recent advancements in metalenses have introduced approaches for tunable acoustic focusing, but their complex configurations and limited tuning range remain challenges. Here, acoustic Moiré metalenses (AMMs) are proposed to achieve continuously tunable ultrasonic focusing in water. Two cascading metasurfaces that can function as Moiré diffractive elements make up the AMM. By mutually rotating the metasurface, the focal point of the AMM can be continuously tuned in a large range. The focal length can be adjusted continuously from ∼14.3λto ∼50λfor the axial focusing. We further show that the well-designed AMM can achieve the continuously tunable lateral focusing, with the deflection angle of the focal point being tunable between approximately -40,40. Both simulation and experimental results confirm the excellent tunable focusing performances of the AMMs. The proposed AMMs with continuously tunable focusing capability may have potential applications in ultrasonic imaging and ultrasound treatment.