Researches On Novel Propagation Characteristics Of Acoustic Waves Concerning Stoneley Waves And Evanescent Waves In Artificial Structure

Recently, the distinct physics properties of phononic crystal and the new useful applications based on phononic crystal have received more and more attention. Furthermore, the concept of acoustic metamaterials based on the concept of phononic crystals is proposed. The realization of acoustic metamaterials provides significant values and potential applications, such as the integration of acoustic devices, sub-wavelength imaging with a flat lens, acoustic waves cloaking and so on. For its wavelength is shorter than the wavelength of acoustic bulk waves, acoustic surface waves play an important role in the achievements of the integration of acoustic devices and sub-wavelength imaging.In this thesis, the Finite-difference-Time-Domain (FDTD) method is used to investigate the distinct properties of acoustic surface waves in acoustic systems, such as phononic crystals slab, immersed V-groove, a steel slab with periodic array of subwavelength slits and two different square rod arrays. Thus, applications in tunable acoustic filters, waveguides for acoustic surface waves, acoustic near field microscope and classical analog of electromagnetically induced transparency in acoustics can be achieved by using these distinct properties of acoustic surface waves.Firstly, we present an analysis of acoustic resonant modes in a slab with a periodic array of holes. Our analysis shows that these resonant modes can significantly affect the transmission of externally incident acoustic waves. The spectral line shapes exhibit complex asymmetric feature. This distinct property of the resonant modes can then be used to introduce phononic crystal structures achieving tunable acoustic filters and sensors.Secondly, it is shown that the immersed V-groove is a type of topographic waveguides for acoustic surface waves, which demonstrates major features for dispersionless waveguide, subwavelength guiding and low wave velocity, just the same as the other topographic waveguides. However, the immersed V-groove confines almost all the elastic energy in the fluid domain along the bottom of the groove in contrast to the other topographic waveguides which confine elastic energy in the solid domain of the wedge tips. Therefore, the acoustic groove modes are attractive for possible applications in underwater acoustics, acoustic actuators and acoustic biosensor.Next, it was shown that a steel slab with periodic array of subwavelength slits may serve as a material of tunable refractive index (thus can be high as required) for airborne sound, and the structured slab can also support acoustic guided modes. By coupling with these guided modes, the near field of the source can be transported through the structured slab. Therefore, the structured slab is capable of serving as an acoustic near field microscope, which can transfer an image with subwavelength resolution from the front surface of the microscope to the back surface. Although our near field microscope has to be placed in the close region of the source to capture evanescent waves, the thickness of the microscope could be much larger than the wavelength of operation as long as the thickness fulfills the Fabry-Perot resonance condition at that wavelength. Hence, the acoustic near field microscope may also be regarded as an acoustic endoscope which can transport images with subwavelength resolution for a rather long distance. Moreover, it is worthy to note that, compared with the other acoustic metamaterials, the structured slab is much simpler to design and fabricate.Lastly, an acoustic structure exhibiting EIT-like phenomenon is proposed. The structure unit consists of two square rods with one rod rotate 45°respect to the other. The square rods can support surface resonant modes around the circumference of them. These resonant modes can couple to incident waves from free space and thus their losses are predominately due to radiation coupling, for the absorption in such acoustic system is neglectable. It shows that, two certain resonant modes supported by the two rods respectively have near resonant frequencies but very different displacement field distributions. For the displacement field distributions determine how the resonant mode couples to the incidence wave, the two resonant modes have very different quality factors. Consequently, after tuning the resonant frequencies of the two resonant modes to be identical, they can act as radiative mode and dark mode in the analogue respectively, and the destructive interference of them results in the EIT-like transmission response.