UC Berkeley

Acoustic metamaterials are artificial materials designed to realize unprecedented properties such as super-resolution imaging, asymmetric wave transport, cloaking, etc. The critical material parameters considered for the design of acoustic metamaterials are mass density and bulk modulus. Previous studies have demonstrated the existence of acoustic metamaterials with negative effective mass density and/or bulk modulus. In this thesis, the realization of acoustic metamaterials with extreme materials parameters including simultaneous zero effective mass density and infinite bulk modulus as well as complex-valued parameters is discussed. Metamaterials with simultaneous zero density and infinite bulk modulus exhibit zero refractive index properties called acoustic double zero index metamaterials in which the propagating sound wave has an infinite phase velocity. This infinite phase velocity results in an infinite wavelength, such that the propagating acoustic wave has a uniform phase inside the metamaterial, which can be used for the realization of acoustic wave tunneling and collimation. Our designed acoustic double zero index metamaterial is induced by a Dirac-like cone dispersion at the Brillouin zone center associated with triply degenerated monopolar and dipolar modes. The monopolar and dipolar modes induce an infinite bulk modulus and a zero mass density, respectively. Meanwhile, the simultaneous zero density and infinite bulk modulus induce a finite acoustic impedance, which can be designed to match with the background medium and enhance the transmission through the boundaries between the metamaterial and the surroundings.

On the other hand, metamaterials with complex-valued material parameters are related to the attenuation and amplification of acoustic waves. When the loss and gain inside the metamaterial are exactly balanced, the medium is parity-time (PT) symmetric so that the wave propagation is invariant under PT operator. Such PT symmetric systems contain two phases: "exact" and "broken" phases, with exceptional points where the system exhibits unidirectional transparency marking the transition between these two phases. Even though the real parts of the material parameters are usually non-dispersive, the imaginary parts can vary significantly with changing frequencies. In this case, the PT symmetric condition is satisfied only within a narrow frequency band, while the exceptional points can appear at different frequencies. This limitation challenges the observation of the exceptional points and the realization of unidirectional transparency. A systematic method to access these exceptional points can be achieved by tuning the spacing between the loss and gain materials, which modulates the multiple scattering inside the system to realize unidirectional transparency.

In most cases, acoustic metamaterials consists of deep subwavelength resonators operating around their resonance frequencies. When these resonators are arranged in a two-dimensional surface, they form a metasurface which modulates the phase of acoustic wave and reshape the propagating wavefront. This property can be used for the design of an ultrathin acoustic carpet cloak or diffuser, as shown by the metasurface formed by 3D printed Helmholtz resonators in this dissertation. In the case where the target object contains rapidly changing profiles, the cloaking effect of the designed carpet cloak is preserved. However, the design of acoustic diffuser with rapidly changing profiles requires resonators that can independently modulate the amplitude and phase of the reflected wave.

The control of the phase of acoustic waves enables the generation of vortex beams, which carry orbital angular momentum (OAM) of sound. A topological charge is defined to characterize the amount of OAM carried by the wave. Acoustic vortex beams with different topological charges are orthogonal to each other, which form independent channels to convey information when being multiplexed. Such OAM multiplexing technique increases the information capacity and spectral efficiency for acoustic communication, which holds the potential to increase the communication speed significantly. Besides OAM, acoustic spin defined as sound waves with rotating particle velocity field can be generated. Spin dependent propagation can be observed in a metamaterial waveguide formed by periodic grooves. When a probe meta-atom with acoustic dipole resonance is placed in wave with acoustic spin, a torque induced by spin matter interaction can be measured. The difference between the OAM and spin can be observed by the motion of probe particle. A probe particle moves along a circular trajectory centered at the field center when placed in wave with OAM, while rotates about its own axis when placed in wave with spin.