Realization of an acoustic metalens exhibiting broadband high transmission

Acoustic metamaterials and acoustic metasurfaces have recently been exploited to design lenses that have extraordinary sound focusing capabilities. Among these so-called metalenses, many thin and planar ones are composed of space-coiled structures, which are widely employed structures as unit cells for the lenses. Although it is known that visco-thermal losses in narrow channels and elastic deformation of thin structures severely affect the power transmission coefficients and refractive indices of the structures, the losses are often neglected and the structures are assumed to be rigid. In this study, we realize a planar acoustic lens with broadband high transmission by taking into account both the visco-thermal losses and elastic deformation in the design procedure. Experiments verify that the lens has a broad bandwidth of 2.5–5.5 kHz and focuses sound waves into a narrow region with a full width at half maximum of 0.56λ within the bandwidth, amplifying the intensity by a factor of almost six at the center frequency of 4 kHz.