Inflatable acoustic metasurfaces for tunable wave focusing

Acoustic metasurfaces are two-dimensional architected materials designed to enable non-trivial control of waves, with a thickness that is either thinner than or comparable to the wavelength. However, most metasurfaces today have a fixed geometry and lack the ability to tune acoustic waves on command. This limits their ability to perform multiple functions, such as beam steering and dynamic focusing. This study introduces inflatable acoustic metasurface (IAM) lenses that enable tunable focusing. The IAMs feature two-dimensional diffractive focusing patterns embedded in a membrane that can be inflated nonplanarly through hydraulic control. It is experimentally demonstrated that inflation allows continuous focal length adjustment from -2.49λ to +3.17λ. To characterize the lens performance, changes in focal characteristics, including peak pressure, full width at half-maximum, and full length at half-maximum, are tracked at different levels of inflation. Furthermore, it is shown that IAMs can correct aberrations that occur as the angle of incidence increases in conventional planar lenses. To validate this, IAMs were tested in a concave configuration at a 20° oblique incidence angle. The results of this study may be applicable to fields requiring continuous and real-time response in tunable focusing, including acoustic imaging and communication, ultrasound surgery, and neuromodulation.