Acoustic planar antireflective focusing lens with sub-diffraction-limit resolution based on metamaterials

Focusing acoustic energy is of fundamental importance for various applications. Traditional acoustic lenses are vulnerable to the backscattering that may be detrimental to the performance. Here, we propose the design of a thin planar acoustic antireflective focusing lens (AFL) based on metamaterials capable of converging the incident energy into a focus spot with the full-width at half-maximum less than $λ/2$ while preventing the backscattering energy reflecting back to the incident side. Such antireflection functionality results from the coupling of two metastructures as constituent units of the lens that modulates the wavevector of the incident wave and selects the uniformity of wavevectors, respectively, ensuring the precise focusing of the incident wave and the suppression of the backscattered wave. Performance is verified via a comparison against the conventional focusing lens, and numerical results evidence a high contrast of reflected intensity in the incident region between these two lenses surrounded by rough boundaries. Our proposed AFL with a planar profile, compact size, high focus resolution, and unique antireflection ability would open new design possibility for acoustic lens and find diverse applications in relevant fields.