Property Tailoring And Functionality Designing To The Phononic Crystal And Metamaterials

The richness of dispersion bands in phononic crystal (PC) allows the manipulation of acoustic/elastic waves at the scale of wavelength in various unprecedented ways. This gives rise to many promising applications. With the further development of these applications, the conventional ways to construct and design the dispersion bands of the PC, i.e., by changing the material and/or structure parameters, can not satisfy the every-increasing applied requirements anymore. In Chapter 2 of this thesis, we propose to introduce grade into PC to achieve new degrees of freedom in the construction and designing of the PC's dispersion bands. Introduction of grade in PC can be achieved flexibly by the appropriate gradual modifications of various parameters along various directions by various ways, greatly enhancing the wave manipulation power of PC. Using the proposed idea of graded PC, we realize a flat lens with graded negative refractive index by a two-dimensional phononic crystal. The index-grade is achieved by gradual modification of the filling fraction along the transverse direction to propagation. We demonstrate that this lens can realize the focusing of parallel incident acoustic waves. This lens is expected to bear significance in applications such as coupling or integration with various types of acoustic devices.In-band applications of PC require waves to be coupled into the PC effectively. In general, as the result of impedance mismatch, the reflection at the ends of PC structures may seriously affect the performance of PC devices. Thus, the reduction of unwanted reflection at the interfaces between a PC and a uniform media is considered to be an important and crucial problem. In Chapter 3 of this thesis, we propose to minimize the reflection at the interfaces of PC by using the concept of antireflection coating (ARC). We demonstrate by using numerical simulations that the ARC structures composed of cylindrical rods can be optimized for minimum reflection at the interfaces of a two-dimensional PC. To stress the effectiveness of the proposed method, we simulate the performance of the designed ARC structures for two typical PC devices, i.e., the point source imaging lens and the parallel incident wave focusing lens, respectively. The simulated results show that the performance of these two devices can be significantly improved by the introduction of the ARC structure.Different from PCs, acoustic metamaterials (MMs) provide the manipulation of acoustic/elastic waves at subwavelength scales. Our ability to manipulate waves has been greatly enhanced with the recent advances in MMs, due to the many unusual properties they possess. These properties can be utilized to design novel acoustic devices. In Chapter 4 of this thesis, we investigate the subwavelength imaging by acoustic metamaterial slabs. A near-field subwavelength image formed by evanescent waves is achieved by a designed metamaterial slab with negative mass density and positive modulus. A subwavelength real image is achieved by a designed metamaterial slab with simultaneously negative mass density and modulus. These results are expected to shed some lights on designing novel devices of acoustic metamaterials.The available limited properties of conventional materials have been considerably expanded with MMs, as their effective parameters can be, in principle, designed to achieve any desired values. An important issue involved with MMs is surface waves propagating along various interfaces. In Chapter 5 of this thesis, we investigate the Stoneley waves on a delicately designed MM and demonstrate the existence of an unconventional acoustic surface resonance state which behaves exactly the same as the electromagnetic surface plasmon polaritons (SPPs) on metal. Thus the mapping of electromagnetic SPP into its acoustic correspondence is realized for the first time here by the property-tailoring of MMs. Considering the great success electromagnetic SPPs have achieved in their subwavelength confinement and manipulation of light, the novel acoustic surface resonance state discovered here will surely provide promising prospects of application. This work aims to provide a paradigm for the tailorable designing of novel acoustic devices by metamaterials.