Periodic Holes Research Articles

Reconfigurable waveguides defined by selective fluid filling in two-dimensional phononic metaplates

We investigate two-dimensional phononic metaplates consisting of a periodic array of cups on a thin epoxy plate that is perforated with periodic cross holes. The cups are individually filled with water or remain empty, in view of creating reconfigurable phononic waveguides. Phononic band gaps exist for empty or filled epoxy cups, leading to waveguides defined with either positive or negative contrast. Straight and 90° bent waveguides are considered experimentally. Lamb waves are excited by a piezoelectric patch glued onto the metaplate and are imaged using a scanning laser vibrometer. Experimental results are compared to a three-dimensional finite element model of fluid–structure interaction. Passing and forbidden frequency ranges are identified for positive and negative contrast, and confined propagation is observed along the waveguides. Significantly, the propagation of acoustoelastic waves in the 90° bent waveguides is observed experimentally. Reconfigurability and reusability are thus realized based on the coupling of elastic waves in the solid and acoustic waves in the fluid. The results show plenty of potentiality for the practical design of multiplexed and programmable acoustic devices implemented with reconfigurable waveguides printed on demand in a phononic metaplate.

Microwave Sensor Based on Microstrip Line Photonic Band Gap (PBG) Structure

A hybrid integration of planar microstrip line and Photonic Band Gap (PBG) structure is proposed for liquid material complex permittivity characterization. The periodic non-through holes drilled in the substrate to generate the bandgap protect also the microstrip from direct contact with liquid under test (LUT). Based on this configuration, two versions are developed to address the measurement constraint. The first sensor exploits the frequency shift of the bandgap center due to the change in the permittivity of the LUT filled in the holes. The second sensor exploits the behavior of the bandgap as a reflector to construct a resonant structure sensitive to the variation in LUT permittivity. The dimensions of the planar structures are optimized to achieve high precision and discrimination capability. The different empirical expressions describing the complex permittivity with the measured parameters for the two sensors were carried out. For experimental validation, prototypes are used to characterize different commercial oils. The frequency shift related to a change of 1 in LUT permittivity corresponds to 300 MHz around 8.3 GHz. The resonant-mode sensor spans a permittivity range from 1 to 9 with a precision better than 6.6 %. The proposed low-cost sensors are simple and reusable, satisfying the requirement of industrial applications.

Optical Analysis of Hole Patterned Ag Nanoparticle Structure.

A structure with periodic sub-wavelength nanohole patterns interacts with incident light and causes extraordinary optical transmission (EOT), with metal nanoparticles leading to localized surface plasmon resonance (LSPR) phenomena. To explore the effects of metal nanoparticles (NPs), optical analysis is performed for metal NP layers with periodic hole patterns. Investigation of Ag NP arrangements and comparisons with metal film structures are presented. Ag NP structures with different hole configuration are explored. Also, the effects of increasing light incident angle are investigated for metal NP structures where EOT peak at 460 nm wavelength is observed. Moreover, electric field distributions at each transmittance peak wavelengths and optical noise are analyzed. As a result, optical characteristics of metal NP structures are obtained and differences in resonance at each wavelength are highlighted.

Multi-sided B-spline surfaces over curved, multi-connected domains

We propose a new surface representation, the Generalized B-spline (GBS) patch, that combines ribbon interpolants given in B-spline form. A GBS patch can connect to tensor-product B-spline surfaces with arbitrary Gm continuity. It supports ribbons not only along the perimeter loop, but also around holes in the interior of the patches.This is a follow-up paper of a recent publication (Várady et al., 2020) that described multi-sided Bézier surfaces over curved multi-sided domains. While the fundamental concept is retained, several new details have been elaborated. The weighting functions are modified to be products of B-spline and Bernstein basis functions, multiplied by rational terms. A new local parameterization method is introduced using harmonic functions, that handles periodic hole loops, as well. Interior shape control is adapted to the B-spline representation of the ribbons. Several examples illustrate the capabilities of the proposed scheme. Our implementation is based on a computationally efficient discretization.

Tunable characteristics of low-frequency bandgaps in two-dimensional multivibrator phononic crystal plates under prestrain

In view of the influence of variability of low-frequency noise frequency on noise prevention in real life, we present a novel two-dimensional tunable phononic crystal plate which is consisted of lead columns deposited in a silicone rubber plate with periodic holes and calculate its bandgap characteristics by finite element method. The low-frequency bandgap mechanism of the designed model is discussed simultaneously. Accordingly, the influence of geometric parameters of the phononic crystal plate on the bandgap characteristics is analyzed and the bandgap adjustability under prestretch strain is further studied. Results show that the new designed phononic crystal plate has lower bandgap starting frequency and wider bandwidth than the traditional single-sided structure, which is due to the coupling between the resonance mode of the scatterer and the long traveling wave in the matrix with the introduction of periodic holes. Applying prestretch strain to the matrix can realize active realtime control of low-frequency bandgap under slight deformation and broaden the low-frequency bandgap, which can be explained as the multiple bands tend to be flattened due to the localization degree of unit cell vibration increases with the rise of prestrain. The presented structure improves the realtime adjustability of sound isolation and vibration reduction frequency for phononic crystal in complex acoustic vibration environments.

Suppression of Walker breakdown in nanowires with periodic holes

The application based on current-driven domain wall (DW) motion mainly depends on the effective control of the moving velocity of the DW and its structural stability. In this paper, we show that a single square hole has a certain suppression effect on Walker breakdown (WB), and by introducing periodic square holes and improving the relevant design, a suppression effect on WB can be achieved in a limited time. As a result, the domain wall remains structurally stable while moving at a very high velocity. The size and position of the holes have an important influence on energy dissipation (ΔE) which plays a crucial role in the suppression of WB. We find that the ΔE strongly relies on the hole size (a), and theΔE has a polynomial relationship with the critical distance. When the current density is the critical value of JWB, the square hole size a = 16 nm and the distance between adjacent holes l = 1500 nm, WB can be suppressed perfectly, and the magnetic domain wall can keep moving at high speed for a long time.

Broad band gaps for flexural wave manipulation in plates with embedded periodic strip acoustic black holes

Acoustic black holes (ABHs) achieved by thinning structural thickness following a power-law profile have shown fantastic applications in vibration and noise suppression, energy harvesting, and wave manipulation. For the latter, many passband properties exemplified by self-collimation, focusing, and bi-refraction, have been reported on metamaterial plates with embedded circular ABHs in periodic arrangement. To date, however, band gaps (BGs), being the main feature for metamaterials, have not yet been observed in ABH plates. In this paper, we propose a new class of phononic crystal consisting in two crossed strip ABHs to open broad and complete BGs in plates, based on the dual benefit of local resonance and Bragg scattering effect. The dispersion curves of infinite strip ABHs are recovered by means of the Gaussian expansion method, then validated by a finite element model. The influences of ABH radius, ABH order and residual thickness, on the GBs have been carried out by parametric studies. The results show that the BGs can be modulated to very low frequencies via adjusting the ABH parameters, while maintaining the broadband characteristics. Finally, the wave propagation properties including isolation, guiding, and shielding, in plates with finite periodic arrays have been characterized, indicating that a small number of ABHs are very efficient to quarantine flexural waves. Experimental samples have been fabricated and tested, showing very close results compared to the BGs predicted by the GEM. Even though the proposed strip ABHs seems fragile in strength, peripheral frame can be adopted to partially alleviate this problem. The proposed PCs possess a great potential to broaden ABH applications in vibration control and wave manipulation in plates based on the highlighted BG feature.

Complete sub-wavelength flexural wave band gaps in plates with periodic acoustic black holes

Acoustic Black Hole (ABH) effect shows promise for vibration control, but mainly limited to a relatively high frequency range. Though achievable in 1D periodic ABH structures, complete sub-wavelength band gaps (BGs) have not yet been realized in 2D configuration. Capitalizing on the unique wave propagation characteristics of the ABH, we propose a new type of plates containing periodically arranged double-layer ABH cells which offer complete and omnidirectional BGs. The phenomena originate from the combined effects of the ABH-specific local resonances and Bragg scattering, which are made possible through a dual process: a proper channeling of the wave propagation path and an impaired coupling between the ABH-induced local resonances and the global vibration of the unit cells. The former is warranted by a proper structural tailoring of the unit cells and the latter by the dynamics of the double-layer ABH design. It is shown that the BGs can be tuned through adjusting ABH parameters. Meanwhile, attaching the centers of the double ABH branches with a connecting cylinder can further broaden and lower the frequencies of the BGs as a result of the enhanced Bragg scattering. It is also demonstrated numerically and experimentally that remarkable vibration attenuation and energy insulation can be achieved in a plate with only a small number of ABH cells, thus pointing at the possibility of achieving sub-wavelength vibration control in structures with reasonable dimensions.

Ultrahigh hydrogen storage capacity of holey graphyne

Holey graphyne (HGY), a novel two-dimensional (2D) single-crystalline carbon allotrope, was recently synthesized by Castro–Stephens coupling reaction. The naturally existing uniform periodic holes in the 2D carbon–carbon network demonstrate its promising potential in energy storage. Herein, we conduct density functional theory (DFT) calculation and ab initio molecular dynamics simulations (AIMD) to predict the H storage properties of a single-layer HGY sheet modified by Li metal atoms. The DFT calculations demonstrate that Li atoms can bind strongly to the HGY sheet without forming clusters, and each Li atom can anchor four H2 molecules with an average adsorption energy of about −0.22 eV/H2. The largest H storage capacity of the doped HGY sheet can reach as high as 12.8 wt%, showing that the Li/HGY complex is an ideal H storage material at ambient conditions. In addition, we investigate the polarization mechanism of the storage media and find that the polarization originates from the electric field induced by both the ionic Li atoms and the weak polarized H2 molecules. Finally, the desorption mechanism of the adsorbed H2 molecules is thoroughly investigated using a kinetic AIMD method.

Electron energy loss spectroscopy on freestanding perforated gold films

We report on a combined far- and near-field study of surface plasmon polaritons on freestanding perforated gold films. The samples are fabricated by focused ion beam milling of {a periodic hole array into} a carbon membrane followed by thermal evaporation of gold and plasma ashing of the carbon film. Optical transmission spectra show a series of characteristic features, which can be attributed to the excitation of surface plasmon modes via the periodic nanohole array. The corresponding near-field distributions are mapped by electron energy-loss spectroscopy. Besides the optically bright surface plasmon modes, we observe in the near-field an additional dark plasmon mode, which is absent in the normal incidence far-field spectra. Our experimental results are in good agreement with numerical computations based on a discontinuous Galerkin time-domain method.

Complex band structure and evanescent Bloch wave propagation of periodic nested acoustic black hole phononic structure

In this study, the periodic nested acoustic black hole phononic structure is presented, and different complex band structures and corresponding evanescent Bloch wave propagation are discussed. Vibration transmissions and modes indicated that the corresponding regions of the bandgaps were due to the coupling effect between the Fano-like resonance and Bragg scattering, which also presented the negative mass and stiffness characteristics. Moreover, strain tensors at interface and central sections of the unit cells were used to evaluate the structural instability and failure. Contrast models proved the superiority of the periodic nested acoustic black hole phononic structure for evanescent Bloch wave control and vibration experiments verified its effectiveness. The relevant conclusions can be used in the vibration reduction design of lightweight structure and enrich the relevant research experience on acoustic black hole.

Highly Sensitive Graphene-Au Coated Plasmon Resonance PCF Sensor.

This paper presents a graphene-Au coated photonic crystal fiber (PCF) sensor in the visible regime. Designing a side-polish D-shaped plane over the PCF’s defect of the periodic air holes can effectively enhance the evanescent field. Graphene on gold can enhance the sensor’s sensitivity because it can stably adsorb biomolecules and increase the propagation constant of the surface plasmon polariton (SPP). Using the finite element method (FEM), we demonstrated that the sensing performance is greatly improved by optimizing the PCF’s geometric structural parameter. The proposed PCF sensor exhibited high performance with a maximum wavelength sensitivity of 4200 nm/RIU, maximum amplitude sensitivity of 450 RIU−1, and refractive index resolution of 2.3 × 10−5 RIU in the sensing range 1.32–1.41. This research provides a potential application for the design a new generation of highly sensitive biosensors.

Buckling Behaviors of Elastomers With Periodic Elliptical Holes Under Negative Pressure Activation

Buckling Behaviors of Elastomers With Periodic Elliptical Holes Under Negative Pressure Activation

Mechanical characterization of axially compressed metallic foams containing periodic rectangular holes with FEM analysis and analytical considerations

This work presents an approach to evaluate axially compressed metallic foams with the presence of holes on the structure further than foam voids. Adding holes, the total material weight is decreased but the stiffness is not linearly reduced. The stress–strain characterization is investigated and presented considering the number, size, and thickness of rectangular holes. The characterization uses representative volume elements to decompose the foam into fundamental portions. Symmetries, boundary conditions and homogenization principles are combined in a formulation in terms of parameters describing the holes, resulting in a parametric characterization. Examples are presented where the influence of holes is analyzed.

Heat diffusion imaging: In-plane thermal conductivity measurement of thin films in a broad temperature range.

This work combines the principles of the heat spreader method and the imaging capability of the thermoreflectance measurements to measure the in-plane thermal conductivity of thin films without the requirement of film suspension or multiple thermometer deposition. We refer to this hybrid technique as heat diffusion imaging. The thermoreflectance imaging system provides a temperature distribution map across the film surface. The in-plane thermal conductivity can be extracted from the temperature decay profile. By coupling the system with a cryostat, we were able to conduct measurements from 40 K to 400 K. Silicon thin film samples with and without periodic holes were measured and compared with in-plane time-domain thermoreflectance measurements and literature data as validation for heat diffusion imaging.

Flexural wave energy harvesting by multi-mode elastic metamaterial cavities

In this paper, we design the elastic metamaterial cavities with multiple cavity modes for harvesting the flexural wave energy. The metamaterial plate is formed by drilling periodic holes in a thin aluminum plate, which exhibits a flexural wave mode band-gap. The metamaterial L1–L4 cavities are designed. The flexural waves are highly confined inside these metamaterial cavities due to the existence of the band-gap. The designed cavities can be utilized to confine and convert the mechanical wave energy to electricity for more than one cavity mode. To experimentally validate the multi-mode cavity design, a metamaterial L3 cavity is fabricated. The three cavity modes of the L3 cavity are observed experimentally, which agrees well with the numerical prediction. Three piezoelectric discs are attached in the metamaterial L3 cavity as the energy harvesters. The electrical outputs are measured to evaluate the performance of the metamaterial energy harvester. Compared to the reference harvester on a bare plate, the average power outputs are amplified by the metamaterial L3 cavity at three resonant frequencies. For the optimal resistance load, more than 40 times of the power amplification is achieved by the L3 cavity energy harvester. As prior energy harvesters based on a metamaterial cavity are usually efficient only for a single frequency, the multi-mode metamaterial cavity design can effectively widen the frequency bandwidth of the energy harvester.

Glide-Symmetric Metallic Structures With Elliptical Holes for Lens Compression

In this article, we study the wave propagation in a metallic parallel-plate structure with glide-symmetric elliptical holes. To perform this study, we derived a mode-matching technique based on the generalized Floquet theorem for glide-symmetric structures. This mode-matching technique benefits from a lower computational cost since it takes advantage of the glide symmetry in the structure. It also provides physical insight into the specific properties of Floquet modes propagating in these specific structures. With our analysis, we demonstrate that glide-symmetric structures with periodic elliptical holes exhibit an anisotropic refractive index over a wide range of frequencies. The equivalent refractive index can be controlled by tuning the dimensions of the holes. Finally, by combining the anisotropy related to the elliptical holes and transformation optics, a Maxwell fish-eye (MFL) lens with a 33.33% size compression is designed. This lens operates in a wideband frequency range from 2.5 to 10 GHz.

Metastructure-inspired ultraviolet and blue light filter

Currently, many studies are being conducted to apply optical metamaterials to the transmittance control of visible light and near infrared wavelengths. In this study, a metastructure was fabricated with periodic metal holes that can reduce the transmittance of only ultraviolet (UV) and blue light in the wavelength band from UV to visible light. The connection structure of the metal-hole-patterns, with a hole size of tens to hundreds of μm, and the crystallinity of the metal could be controlled through the three-step process of polystyrene (PS) nanosphere array formation, using the air–water interface method, and size adjustment of the PS nanosphere, through the etching process, and through the adjustment of the metal film thickness. The glass substrate, with an Al-hole-patterned metastructure (an Al-hole-pattern size of ∼245 nm and an Al thickness of ∼60 nm), showed a low transmittance of 33% and 28% in the wavelength bands of 350 nm and 470 nm, respectively, and a high transmittance of 67% and 57% in the ranges of 540 nm and 670 nm, respectively, owing to the metacharacteristics manifested in periodically connected metal patterning structures. This Al-hole-patterned metastructure is expected to be applicable to various optical devices that require a reduction in the transmittance of only the UV and blue light wavelength band.

Demonstration of mid-infrared slow light one-dimensional photonic crystal ring resonator with high-order photonic bandgap.

Integrated mid-infrared sensing offers opportunities for the compact, selective, label-free and non-invasive detection of the absorption fingerprints of many chemical compounds, which is of great scientific and technological importance. To achieve high sensitivity, the key is to boost the interaction between light and analytes. So far, approaches like leveraging the slow light effect, increasing optical path length and enhancing the electric field confinement (f) in the analyte are envisaged. Here, we experimentally investigate a slow light one-dimensional photonic crystal ring resonator operating at high-order photonic bandgap (PBG) in mid-infrared range, which features both strong field confinement in analyte and slow light effect. And the optical path length can also be improved by the resoantor compared with waveguide structure. The characteristics of the first- and second-order bandgap edges are studied by changing the number of patterned periodical holes while keeping other parameters unchanged to confine the bands in the measurement range of our setup between 3.64 and 4.0 µm. Temperature sensitivity of different modes is also experimentally studied, which helps to understand the field confinement. Compared to the fundamental PBG edge modes, the second PBG edge modes show a higher field confinement in the analyte and a comparable group index, leading to larger light-matter interaction. Our work could be used for the design of ultra-sensitive integrated mid-infrared sensors, which have widespread applications including environment monitoring, biosensing and chemical analysis.

Effects of Unshielded Air Holes Periodically Perforated in Substrate Integrated Waveguides

In this work, directly perforated substrate integrated waveguides (SIWs) in the form of periodic air holes are studied with a focus on the electromagnetic shielding effect by examining its bandpass and stopband filtering characteristics. Two different configurations of the air-filled holes are considered for this electromagnetic bandgap structure, which are called shielded and unshielded cases. In both the scenarios, the relationship between the center frequency of the bandgap and the structural material and geometric parameters is established and used for the bandgap investigation. In addition, the impact that the shielding of the air-filled holes yields on the loss of the structure is systematically examined. Finally, equivalent circuits for both the cases are formulated and experimentally verified up to 30 GHz.