A field experiment on surface wave isolation by partially embedded periodic pile barriers

The bandgap characteristics of periodic structures show a great application prospect in seismic isolation and vibration mitigation. A T-shaped partially embedded periodic pile barrier is designed and studied based on the Bloch’s theorem. The proposed structure is fabricated and tested in field experiment to validate the simulation and the isolation performance. Good agreements are observed between the measured and the corresponding simulated results which present effective isolation for surface waves. The influences of the embedded length in the soil and the arrangement of the pile are investigated. The results show that the embedded length is the key parameter affecting the vibration attenuation. The smaller the embedded length, the lower the frequency of attenuation zone, and thanks to the embedded length; the proposed barrier exhibits better performances than the fully embedded and non-embedded barriers in maximum bandgap width, tunability, and feasibility in engineering. Gradient distribution of the embedded length leads to a wider frequency range of attenuation.

A Gaussian Mixture Variational Autoencoder-Based Approach for Designing Phononic Bandgap Metamaterials

Phononic bandgap metamaterials, which consist of periodic cellular structures, are capable of absorbing energy within a certain frequency range. Designing metamaterials that trap waves across a wide wave frequency range is still a challenging task. In this study, we proposed a deep feature learning-based framework to design cellular metamaterial structures considering two design objectives: bandgap width and stiffness. A Gaussian mixture variational autoencoder (GM-VAE) is employed to extract structural features and a Gaussian Process (GP) model is employed to enable property-driven structure optimization. By comparing the GM-VAE and a regular variational autoencoder (VAE), we demonstrate that (i) GM-VAE has the advantage of learning capability, and (ii) GM-VAE discovers a more diversified design set (in terms of the distribution in the performance space) in the unsupervised learning-based generative design. Two supervised learning strategies, building independent single-response GP models for each output and building an all-in-one multi-response GP model for all outputs, are employed and compared to establish the relationship between the latent features and the properties of interest. Multi-objective design optimization is conducted to obtain the Pareto frontier with respect to bandgap width and stiffness. The effectiveness of the proposed design framework is validated by comparing the performances of newly discovered designs with existing designs. The caveats to designing phonic bandgap metamaterials are summarized.

Design approach of perforated labyrinth-based acoustic metasurface for selective acoustic levitation manipulation

This paper proposes a metasurface design approach with perforated labyrinthine path coil structure to manipulate the acoustic transmission with inexpensive materials. The medium in the labyrinthine path coils in this design is air, but not limited to air. A systematic approach has been proposed for the unit cell design of acoustic metamaterials with adjustable resonance peak frequencies and bandgap width. The theory demonstrates that the length of pipe segments determines resonance peak frequencies and the cross-sectional area ratio adjusts the bandgap width. The proposed design approach uses an equivalent pipe circuit based analytical model to design the high transmission (high pass) and high reflection (low pass) unit cell. The simulation and experiment has been performed to evaluate the validity of the theory. Although there exists some assumptions in the theory, the theory still has enough accuracy to guide the metasurface design illustrated by the simulation and experiment results.

Investigating Fill Factor Effect on Brillouin Zone of Metamaterial-based 2D Photonic Crystal

: Fill factor in the negative permittivity materials are tailored within physically permissible limit to characterize the Brillouin zone for two-dimensional crystal under propagation of both s and p-polarized waves. Two lowermost bandgaps are computed along with corresponding midband frequencies, where plane wave expansion method is invoked for computational purpose. Rectangular geometrical shape is considered for the simulation, and all the results are calculated inside the ‘Γ’ point and ‘X’ point of first Brillouin zone. Simulated findings depict monotonous variations of both bandgap width as well as midband frequency for higher negative permittivity materials, when magnitude of fill factor is comparatively low, for both TE (Transverse Electric)and TM (Transverse Magnetic)mode of propagations. Lower negative permittivity leads to random fluctuations which makes it unsuitable for photonic component design. Multiple forbidden regions may be observed for some specific artificial materials which can be utilized in antenna or multi-channel filter design in higher THz region. Aims: The present paper aims to compute the shape of the first Brillouin zone from the fill factor for a two-dimensional photonic crystal structure. Background: EBG (Electromagnetic Band gap) of a photonic crystal plays a major role in determining its candidature for optical applications, which is critically controlled by fill factor. Therefore, it is significant to investigate the effect of F.F on the wave propagation characteristics of 2D PhC(Two-dimensional photonic crystal). Objective: Investigation of metamaterial based photonic crystal structure for electromagnetic bandgap analysis in the desired spectrum of interest as a function of fill factor inside the first Brillouin zone Method:Maxwell’s equations are solved using plane wave propagation method to solve the problem, and simulation is carried out in MATLAB® software. Result: Both the first and second photonic bandgaps are simultaneously computed with variation of refractive index differences of the constituent materials as well as with the fill factors. Results are extremely significant about the formation of narrowband and wideband filters on certain material combinations and structural designs. Conclusion: Better tenability is observed for metamaterial structure compared to conventional positive index materials, and fill factor has a great role in shaping the Brillouin zone and corresponding bandgap width.

Катодолюминесценция слоев TiO-=SUB=-2-=/SUB=-, полученных методом молекулярного наслаивания

Use of the method of local cathodoluminescence in Si–TiO_2 and Si–SiO_2–TiO_2 structures helps to elucidate the nature of centers influencing the operation of memristors. These measurements showed that electroforming leads to the appearance of luminescence in a 250–400 nm wavelength range in the external part of TiO_2 layer characterized by high concentration of defects. This observation leads to a conclusion that a sharp interface is formed between dielectric layers of the structure, provides estimation of the absorption coefficient of TiO_2 layer, and allows its bandgap width (~3.3 eV) to be evaluated for the oxide layers formed by the given technology.

Analyses of multi-bandgap property of a locally resonant plate composed of periodic resonant subsystems

A locally resonant (LR) plate made up of a thin plate attached with different types of resonators is analyzed in this paper. Each periodic element may consist of one or more spring-mass resonators attached onto one and the same surface of the plate lattice. The correctness of theoretical plane wave expansion (PWE) method adopted in this paper is validated through the comparisons with the classical theory and finite element method (FEM). When composing the LR plate system with two types of periodic resonant subsystems, there will appear two complete bandgaps, while other additional resonators may cause mainly directional gaps, calculated theoretically and numerically. From the comparisons of band-structure curves between a two-resonator-per-unit-element (TR-UE) system and both corresponding one-resonator-per-unit-element (OR-UE) systems, the bandgap width of the TR-UE system are not stacking effects of two OR-UE systems due to resonance interaction of different types of resonators. Moreover, via the deformation contours by FEM, the correspondence between the vibration modes of subsystems and the bandgap frequencies is demonstrated. The finite plate with limited resonators of two periodic types of parameters is modeled to show visually how flexural waves propagate within/without the bandgaps. Further, by adjusting the damping characteristic of both types of resonators, vibration attenuation band can be broadened widely.

Broadening Bandgap Width of Piezoelectric Metamaterial by Introducing Cavity

In this research, a semi-analytical model of the adaptive piezoelectric metamaterial, built upon continuum mechanics characterization, was formulated and analyzed to reveal the fundamental features of bandgap with respect to unit-cell parameters under transverse wave. A new mechanism to broaden the bandgap width, was then introduced through geometric cavity synthesis. It was demonstrated that the cavities incorporated into the host structure of the piezoelectric metamaterial can increase the electro-mechanical coupling of the system, which effectively yields broadened bandgap width. Case studies were performed to demonstrate the enhanced performance of the new design, as well as the tunability. Compared with the conventional piezoelectric metamaterial, the metamaterial with cavity synthesis can increase the bandgap width from 45 Hz to 126.7 Hz.