Search Results (5)

Metamaterials represent artificially structured materials that exhibit unusual properties, such as a negative refractive index, negative permeability and permittivity, negative cloaking by Poisson ratios and optical effects, etc., which are inaccessible in natural materials. According to recent developments, novel devices and tools based on metamaterials are attracting great interest as they offer improved performance, functionality, sensitivity, biocompatibility, complex structures, and design freedom. Leveraging numerical design approaches, such as finite element analysis and finite difference time domain methods, researchers have tailored metamaterials to meet specific requirements in various areas through a range of manufacturing techniques. These materials can be broadly classified into optical, mechanical, thermal, electromagnetic, and acoustic categories based on their properties and intended use. The choice of fabrication method depends heavily on the specific application, the desired scale, and the complexity of the metamaterial design. These manufacturing methods can be broadly divided into top-down and bottom-up approaches, while each of them has advantages and limitations and offers valuable pathways for the development of the final product. This review offers a basic overview of metamaterials, covering their fundamental principles, fabrication and characterization techniques, and current design methodologies. It also explores their diverse applications, including specific case studies in medicine, while addressing existing limitations and challenges. Finally, this review highlights future perspectives, emphasizing the need for continued innovation in fabrication and characterization to unlock the full potential of metamaterials. Full article

The design theory for electromagnetic metamaterials with negative refractive indices by using a distributed transmission-line model is introduced to the design of acoustic metamaterials, and a negative refractive index (NRI) acoustic lens is designed theoretically. Adjustments to the negative refractive indices of metamaterials have been carried out by calculations with numerical simulators in conventional design methods. As the results show, many calculations are needed to determine the shape of the unit structures and there are issues in that it is difficult to design those rigorously, meaning that limitations regarding the degree of freedom in the designs are many. On the other hand, the transmission-line model can rigorously design the unit cell structures of both the negative refractive index metamaterials and the background media with the positive refractive indices by calculations with the design formulas and modifying the error from the theory with a small calculation. In this paper, a meander acoustic waveguide unit cell structure is proposed in order to realize a structure with characteristics equivalent to the model, and the waveguide width and length for realizing an NRI acoustic lens are determined from the design formula of the model. The frequency dispersion characteristics of the proposed structure are also computed by eigenvalue analysis and the error in the waveguide length from the theoretical value is modified by a minor adjustment of the waveguide length. In addition, the NRI acoustic lens is constituted by periodically arranging the proposed unit cell structure with the calculated parameters, and the full-wave simulations are carried out to show the validity of the design theory. The results show that the designed lens operates at 2.5 kHz. Full article

Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow for the creation of negative refractive indexes; light trapping with epsilon-near-zero compounds; bandgap selection; superconductivity phenomena; non-Hermitian responses; and more generally, manipulation of the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited attention towards 1-D and 2-D metamaterials. However, the true potential of metamaterials is ultimately reached in 3-D configurations, when the degrees of freedom associated with the propagating direction are fully exploited in design. This is expected to lead to a new era in the field of metamaterials, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present, and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs, and functional manufacturing techniques. Full article

Longitudinal waves have important applications in modern scientific research and production; as a special acoustic wave, longitudinal waves have a sound transmission performance in negative refractive index materials. This paper has designed a new structure for negative refractive index materials by virtue of COMSOL software and conducted related simulation analysis. Experimental results illustrated that the structure designed had good acoustic longitudinal wave transmission performance. Besides, the effect of sound wave focusing could be achieved by a combination of existing test methods. The design proposed could break the limitation of previous structures on acoustic longitudinal wave transmission. This study is expected to have important implications for the development of solid metamaterial structures. Full article

Metamaterials, man-made composites that are scaled smaller than the wavelength, have demonstrated a huge potential for application in acoustics, allowing the production of sub-wavelength acoustic absorbers, acoustic invisibility, perfect acoustic mirrors and acoustic lenses for hyper focusing, and acoustic illusions and enabling new degrees of freedom in the control of the acoustic field. The zero, or even negative, refractive sound index of metamaterials offers possibilities for the control of acoustic patterns and sound at sub-wavelength scales. Despite the tremendous growth in research on acoustic metamaterials during the last decade, the potential of metamaterial-based technologies in aeronautics has still not been fully explored, and its utilization is still in its infancy. Thus, the principal concepts mentioned above could very well provide a means to develop devices that allow the mitigation of the impact of civil aviation noise on the community. This paper gives a review of the most relevant works on acoustic metamaterials, analyzing them for their potential applicability in aeronautics, and, in this process, identifying possible implementation areas and interesting metabehaviors. It also identifies some technical challenges and possible future directions for research with the goal of unveiling the potential of metamaterials technology in aeronautics. Full article