Acoustic metalens with broadband high transmission under consideration of visco-thermal losses and elastic deformation

We propose an acoustic metalens with broadband high transmission. The lens is a planar gradient-index lens consisting of cuboid-shaped unit structures with different refractive indices from 1.07 to 1.98. Each unit structure has a symmetric coiled channel whose cross-sectional area is gradually varying. The cross-sectional area of channels is determined for the unit structures to have high transmittance in a broad frequency range. The transmittance of the unit structures and focusing capability of the designed metalens are investigated with consideration of visco-thermal losses and elastic deformation of the structures. The transmittance of the unit structure is higher than 0.8 in a target frequency range of 2.5-5.5 kHz and the acoustic intensity is significantly amplified by the lens in the frequency range.