Functional Design And Research For Surface Wave/Mode Of Two-Dimensional Phononic Crystals

The manipulation of phononic crystal (PC) on the propagation of acoustic/elastic waves has caused great interest of researchers. Taking advantage of the adjustability of dispersion bands in phononic crystal, we could design a series of acoustic devices for different applications. However with the development of study, there are many new problems and challenges turn to be important:on the one hand the conventional manipulation of the dispersion bands by changing the material and/or structure parameters of phononic crystal can’t meet the ever-growing applied requirement anymore; on the other hand, it is required the periodic of PC was comparable with the wavelength of the propagation acoustic/elastic wave if we want to manipulation them, and this limitation promotes the research on the control of acoustic/elastic wave by subwavelength structure. Based on this context, this thesis contains two research points:functional design and the research for surface wave and mode of phononic crystals. On one hand, we could make some simple changes of structure based on the periodic/quasi-periodic PC to achieve the functional design; on the other hand, we made a preliminary research on the surface wave (located mode) and surface guided wave. This dissertation contains five parts in details:1. We demonstrate the focus behaviors of acoustic wave through two-dimensional 8-fold-symmetry phononic quasicrystals in experimentally. By measuring the field distributions in the image plane and the negative refraction index, we demonstrated that the same with the PC,8-fold-symmetry phononic quasicrystals also have the negative refraction effect. These properties make the phononic quasicrystals promising for application in a range of phononic devices.2. We design a two-dimensional sonic crystal with gradient negative refractive index, and a Focusing effect is experimentally observed for acoustic plane wave normally incident onto it. The gradual refractive-index is achieved by gradual modification of the lattice spacing both along the transverse and longitudinal directions. It is found that the focal length is controllable by modulation of the lattice spacing. The experiment results are in excellent agreement with theoretical calculation by a multiple scattering theory method. These features can be used widely in the design of acoustic device.3. We design a simple geometrically asymmetric steel grating structure, and a unidirectional transmission of acoustic waves is realized through it. This exotic phenomenon stems from the one-way diffraction effect induced by the different periods of the slits on the both surfaces of the sample. And the frequency range of unidirectional transmission is simply determined by the structure periods. The experimental results agree well with the theoretical simulation. This remarkable effect is expected potential applications in ultrasonic devices, such as acoustic rectifiers and acoustic diodes.4. We study the acoustic transmission through a monolayer of periodical polymethyl methacrylate (PMMA) cylinders immersed in water both experimentally and numerically. Beyond our expectation, nearly-total reflection is observed for the system consisting of two ingredients with low impedance contrast. Further investigation manifests that this extraordinary acoustic shielding mostly stems from the localized mode in individual cylinders. Two different modes’originations are different:one is the ordinary Mie scattering resonant mode, and the pressure field was good confined inside the PMMA cylinder; and the other one is related to the localized Stoneley surface waves, and the pressure was concentrated at the interface of PMMA and water. Such local modes are rooted in the complicated coupling between the longitudinal and transverse waves and are unique in acoustic systems. We consider this system can achieve the control of acoustic wave by the subwavelength structure.5. We study the surface waveguide of a monolayer of periodical steel cylinders immersed in water both experimentally and numerically. We calculated the band structure of this system through the Layer-Multiple Scattering Theory and found that there exist the surface waveguide modes even at very low fill rate (under the dispersion line of water). Excellent waveguide effect was observed through experiment. The surface waveguide supported by this unconnectedness system was un-radiation in matrixes have a good direction and effectively in a wide range of frequencies. We consider this surface waveguide originated from the discrete scatters, which can supple a new design of acoustic waveguide.