| Metamaterials(MMs)are artificial structures,typically periodic(but not necessarily so),composed of small meta-atoms that,in the bulk,behave like a continuous material with unconventional effective properties.They can perform in a broad frequency range from the electromagnetism to the acoustics.The MMs have many novel characteristics of electromagnetic/acoustic responses,and with these unusual wave properties,the MMs can achieve many unique phenomena which cannot appear in a conventional medium,such as negative refraction,high-index material,and localized field enhancement,etc.Taking advantage of these distinctive characteristics,researchers have designed many novel MM devices,such as invisible cloaks,unidirectional antennas,and broadband absorbers.In this thesis,we carry out thorough investigations on the controlling of the electromagnetic and acoustic wave propagation characteristics by exploiting the MMs,and design some special devices based on the theoretic results.As a start,we carry out a comprehensive study on the light trapping capability of a hyperbolic metamaterial slow light waveguide.By calculating the band structure,we find that there are two modes,which are forward mode and backward mode,co-exist in the waveguide.The two modes degenerate at the critical thickness and lead to the inter-modal coupling,which will cause the reduction of the group velocity.Although the pulse of the light cannot be trapped in such a structure forever,the duration that light remains trapped can be flexibly prolonged by adjusting the volume-filling ratio of the metamaterial or the metallic material dispersion.The trapping effect of one waveguide can be improved up to～35%by exploiting the optimized parameters.Not only in electromagnetism,we also investigate some innovating effects of the MMs in acoustics.By comparing with its optical counterpart,we develop the method of the acoustics surface transformation(AST),and design the ANM by exploiting a metal-fluid structure based on some natural materials.The AST method has many new features:i)No complicated mathematic calculations are needed during the whole design process.We only need to design the devices’geometrical shapes of the input and output surfaces.ii)All devices designed by AST only need one anisotropic homogeneous medium,i.e.acoustic-null material(ANM),to realize.iii)Many acoustic devices with novel functions(e.g.producing an acoustic plane wave/converged conical wave,and compressing/expanding an acoustic beam),which cannot be achieved by traditional methods,have been designed by AST.Furthermore,we design an acoustic lens of quasi-periodic hole array and fabricate it by 3D printing.The lens can generate multiple hot spots with subwavelength nature using incident soundwave at the frequency of 20 kHz.Unlike the traditional super-resolution acoustic lenses,our design works by super-oscillation,which doesn’t rely on evanescent waves.Consequently,the lens works well in the far-field.We carry out the experiments using our self-built 3D scanning platform.The experiment results fit well with the theoretic calculations with the minimum full width at half maximum of the hot spots of-7.38 mm(0.43λ).The hot spots generated show high stability of the subwavelength nature over a range of 2λ.(This work is carried out with the cooperation of Prof.Xiaobo Yin at University of Colorado,Boulder.)... |
