On The New Effects In Phononic Crystals And Extraordinary Transmission By Subwavelength Structured Plates

Phononic crystals (PCs) can efficiently modulate the acoustic waves/elastic waves both in time and space domain.In the past decades, the researches on PCs have been attracted much attention. Based on the PCs, many of acoustic devices that can not be realized in the conventional acoustics have been designed and realized. As the researches on PCs is unceasingly deepening, ones gradually transfer from studying the band gaps of conventional PCs and their physical mechanism, to focusing the properties of conventional PCs and the new acoustic applications by utilizing the PCs. Recently, as the metallic subwavelength structure based on the SPPs (Surface Plasmon Polaritons) and optical metamaterials have been deeply studied, the acoustic surface waves and subwavelength acoustic metamaterials also becomes gradually the research focus. This dissertation applies a variety of methods that are commonly used to theoretically study the PCs, in combination with the ultrasonic transmission experimental setup immersed in water, mainly focuses on, in the conventional PCs the rather important acoustic applications and some acoustic analogue of quantum phenomena occurring in electronic crystals; as well as the acoustic waves in periodically subwavelength structured solid/rigid plate. The main works involved in this dissertation are as follows:1. Acoustic subwavelength imaging and far-field focus in phononic crystalsWe relatively comprehensively discuss the negative refraction and the approaches to induce negative refraction in PCs, as well as the principle of acoustic subwavelength imaging. (a) We study that the PCs with large flat part of EFCs, can not only transport the propagating modes of point source, but also transport the evanescent modes, hence break through the diffraction limit. In combination with the Fabry-Perot of the PC slab, we obtain efficiently the subwavelenth image. (b) By adding a modulation onto the PC surface with negative refraction, the acoustic surface waves can be created on its surface. In virtue of the acoustic surface waves, the imaging resolution of the PC lens based on the negative refraction can be great improved, and break through the diffraction limit. (c) By introducing the surface grating onto a PC slab with gap, we obtain the negative refraction in the originally forbidden region, and based on the new negative refraction mechanism realize the far-field focus.2. Guiding acoustic waves and directional effect of phononic crystalsBy introducing an orientation angle gradient of the rectangular rods in a perfect base phononic crystal, and connecting with the self-collimated effect, we design a graded phononic crystal that can mold the flow of acoustic wave but is distinct from the conventional phononic crystal waveguide. The propagation path of the acoustic wave beams inside the GPC can be controlled at will by the structure designing of the GPC. Based on the small-magnitude negative refraction of PC, we realize the broadband acoustic directional beam with a point source placed outside PC slab.3. Acoustic Bloch oscillation and resonant Zener tunneling in Phononic crystalsBy creating periodically arrayed cavities with decreasing gradient in width along one direction in the two-dimensional phononic crystal, we observe the acoustic Bloch oscillation both in theory and in experiment. By introducing an increasing gradient in the width of elementary cell in the one-dimensional superlattice, we demonstrated the resonant Zener tunneling for a critical gradient.4. Some exotic acoustic phenomena in the periodically subwavelength structured thin solid plateIn virtue of the intrinsic surface modes in the thin solid plate, we observe three extraordinary effects and analyze the corresponding physical essence:(1) we observe the extraordinary total reflection of thin epoxy plate with translational subwavelength cuts both in theory and in experiment. This extraordinary phenomenon is caused by the resonant response of the pieces partitioned by the cuts. The standing wave modes of the symmetric coupled Stoneley waves in the pieces account for the resonant responses. (2) By closely attaching the heavy gratings onto a thin epoxy plate, we obtain negative dynamic response. Different from the known localized resonant mass, this negative mass response stems from the collective excitation of coupled Stoneley waves in the plate, hence is a new physical mechanism to obtain NDM response. (3) We observe both experimentally and theoretically the enhanced acoustic transmission (EAT) through a periodically-structured hard plate without any opening. The resonant wavelength is much larger than any characteristic lengths of the microstructure and hence precludes the correlation with the known mechanisms responsible for EAT phenomena, i.e., coherent diffraction and Fabry-Perot-like wave-guiding resonance. The exotic wave phenomenon is attributed to the structure-induced excitation of intrinsic non-leaky Lamb modes in uniform plates.5. Acoustic enhanced transmission and collimation through periodically structured perfect rigid plateWe study the physical mechanism of acoustic enhanced transmission and collimation through periodically structured perfect rigid plate in two cases:with and without designer surface waves (DSWs). In the situation without DSWs, the coherent dynamic diffraction of periodic ridges is responsible for the effects. For the case with DSWs, by adding an extra larger-period array of narrow ridges onto the plate, the DSWs are transferred into radiative waves. As a result, they lead to these extraordinary phenomena.