Systematic design and experimental realization of multiplexed acoustic double-zero-index metamaterials

An intriguing property of double-zero-index metamaterials (DZIMs) is that waves propagate through them without phase variation, which has received significant attention in wave-front and dispersion engineering. Recently, Dirac-like cone dispersion has been successfully exploited to realize a DZIM that operates only at a fixed frequency, while realizing DZIMs with multiple separated Dirac-like cones remains challenging. Here, we propose the first realization of a multiplexed acoustic double-zero-index metamaterial (MADM) capable of realizing three separated Dirac-like points at the Brillouin zone center for airborne sound. The resulting device is formed by introducing a series of blind holes with different heights, which are periodically arranged on the upper and lower parts of the waveguide. The underlying mechanism is that the proposed MADM possesses three Dirac-like cones derived from an accidental degeneracy of monopolar and dipolar resonances, which can be mapped to a DZIM with zero effective compressibility and mass density. We present an experimental realization of the designed MADM with three separated Dirac-like points, and demonstrate the functionality of the resulting device by means of extraordinary wave tunneling and acoustic focusing. We anticipate that our mechanism will offer the potential for designing multiplexed acoustic double-zero-index devices and may have substantial and profound applications in a variety of fields, such as ultrasonic therapy and acoustic communication.