Bragg Interference Research Articles

Axisymmetric reflectors in wave energy converter arrays: Harnessing scattering to increase energy extraction

We elucidate the effect of non-extracting reflectors on the performance of wave energy converter (WEC) arrays. We consider an infinitely periodic row of converters parallel to an infinitely periodic row of discrete axisymmetric reflectors (C-R arrays), and we study how the spatial configuration affects energy extraction. Using a multiple-scattering algorithm for linear wave-array interactions, we conduct a series of simulations of C-R arrays for a range of spatial configuration parameters, wavenumbers, and wave incident angles. We find that C-R arrays can significantly increase energy extraction compared to a WEC array by itself. We offer a simplified theoretical model, based on the far-field response of periodic rows in isolation, which shows that the large increases in energy extraction result from the constructive Bragg and Laue interferences caused by wave interactions with the reflector row. For the pertinent case of incident waves of the WEC-resonant frequency, we find that optimized C-R arrays can achieve energy extraction gains of O(500%). Remarkably, the optimal C-R array extracts more energy than two rows of converters of optimal configuration even though the C-R array consists of only half as many WECs.

Extreme Spatial Dispersion in Nonlocally Resonant Elastic Metamaterials.

To date, the vast majority of architected materials have leveraged two physical principles to control wave behavior, namely, Bragg interference and local resonances. Here, we describe a third path: structures that accommodate a finite number of delocalized zero-energy modes, leading to anomalous dispersion cones that nucleate from extreme spatial dispersion at 0Hz. We explain how to design such zero-energy modes in the context of elasticity and show that many of the landmark wave properties of metamaterials can also be induced at an extremely subwavelength scale by the associated anomalous cones, without suffering from the same bandwidth limitations. We then validate our theory through a combination of simulations and experiments. Finally, we present an inverse design method to produce anomalous cones at desired locations in k space.

Seabed Observations at the New England Mud Patch: Reflection and Scattering Measurements and Direct Geoacoustic Information

Seabed reflection and scattering measurements were conducted at the New England Mud Patch to better understand the acoustic properties of fine-grained (muddy) sediments. The measurement philosophy and the measurements themselves are summarized. In addition, geoacoustic information accessed directly from the data in the time and frequency domains is presented. The main result is the existence of an angle of intromission. This observation proves that the mud sound speed is less than that of the water and yields a sediment sound speed ratio 0.9865 with outer bounds {0.985 0.989}. Another result is the observation of strong scattered arrivals from within the mud volume at/near normal incidence but not at low grazing angles. These are likely due to anisotropic sediment heterogeneities with a large horizontal to vertical scale. Evidence is also presented for a highly heterogeneous mud–sand horizon with lateral variability down to scales of order meters. Finally, the reflection measurements successfully capture Bragg interference patterns. Their importance is their substantial geoacoustic information content, which can be accessed by several inversion methods.

Complete redshift photonic bandgap and dual-wavelength polarization selection in periodic multilayer structure containing hyperbolic metamaterial

According to the Bragg interference mechanism, photonic bandgaps (PBGs) in conventional all-dielectric periodic multilayer structures shift towards shorter wavelengths (blueshift) as the incident angle increases under both TM and TE polarizations. Hence, the polarization dependence of PBGs in conventional all-dielectric periodic multilayer structures is too weak to realize polarization selection in a broad angle range. Based on the anomalous angle-dependent property of the propagating phase within hyperbolic metamaterial (HMM), we design a complete redshift PBG under TM polarization whose two band edges shift towards longer wavelengths as the incident angle increases in a periodic multilayer structure composed of alternating dielectric and HMM layers. However, the PBG under TE polarization is still blueshifted. As a result, around the long-wavelength and the short-wavelength band edges, efficient polarization selection can be achieved in a broad angle range. This dual-wavelength wide-angle polarization selector based on the complete redshift PBG might possess applications in liquid crystal displays and read–write magneto-optical data storage devices.

Multiple Wavelengths Texture Measurement Using Angle Dispersive Neutron Diffraction at WOMBAT

In contrast to conventional angle dispersive neutron diffractometers with a single-tube detector or a small-size linear position-sensitive detector, the WOMBAT diffractometer of the Australian Nuclear Science and Technology Organisation (ANSTO) is equipped with a large-area curved position-sensitive detector, spanning 120° for the scattering angle 2θ and 15° for the azimuth η, respectively. Here, WOMBAT was employed in establishing a texture measurement environment for complex textured samples, through measuring neutron diffractograms at two selected wavelengths on a typical reference sample of martensite–austenite multilayered steel sheet. All neutron patterns were simultaneously Rietveld analyzed using the software, Materials Analysis Using Diffraction (MAUD). The shorter wavelength (λ1 = 1.54 Å, k1 = 4.08 Å−1) enabled collecting the martensite reflections α-110, α-200, α-211, α-220, α-310, and α-222, as well as the austenite peaks γ-111, γ-200, γ-220, γ-311, γ-222, and γ-331 simultaneously, by pre-setting the detector range to 2Θ = 30~150°. The longer wavelength (λ2 = 2.41 Å, k2 = 2.61 Å−1) enabled separating the overlapping strong martensite α-110 and austenite γ-111 Laue–Bragg interferences more reliably. Moreover, the detector panel division along the vertical direction has a good stereographic coverage in the azimuthal angle η,. Such a combination of multiple-wavelength neutron diffraction combined with simultaneous Rietveld texture analysis was confirmed as being very valuable for realizing high precision measurements for complex textured samples at an orientation distribution graticule of 5°, and in a much shorter beam time than the conventional angle dispersive method.

Spatially Shaping Waves to Penetrate Deep inside a Forbidden Gap.

It is well known that waves with frequencies within the forbidden gap inside a crystal are transported only over a limited distance-the Bragg length-before being reflected by Bragg interference. Here, we demonstrate how to send waves much deeper into crystals in an exemplary study of light in two-dimensional silicon photonic crystals. By spatially shaping the wave fronts, the internal energy density-probed via the laterally scattered intensity-is enhanced at a tunable distance away from the front surface. The intensity is up to 100× enhanced compared to random wave fronts, and extends as far as 8× the Bragg length, which agrees with an extended mesoscopic model. We thus report a novel control knob for mesoscopic wave transport that pertains to any kind of waves.

Subwavelength elastic topological negative refraction in ternary locally resonant phononic crystals

Negative refraction of elastic waves has attracted significant research interest due to its potential prospects in acoustic imaging, communication, and structural health monitoring. However, previous implementations based on Bragg interference suffer from lattice-scale size restrictions or defect-sensitive characteristics, hindering their application in compact subwavelength systems. Here, we propose a new design of ternary valley-Hall phononic crystals (PCs) with subwavelength topological negative refraction. By introducing the subwavelength resonators into a complex lattice similar to graphene allotrope, the valley polarized edge modes induced by locally resonant state are obtained, and the defect-immune topological negative refraction of the edge states outcoupling into free space is established. Furthermore, the total mode conversion from longitudinal to transverse waves is demonstrated by the wave mode separation and the equifrequency curves (EFCs) analysis on the negative refraction. Finally, utilizing the valley-selective excitation, a topological elastic valley filter is designed by coupling the valley-projected edge states with bulk valley polarizations. The research provides new possibilities for robust control of isolated transverse elastic waves in the deep subwavelength regime that can be employed for designing compact communication devices.

Omnidirectional narrow-band ultraviolet filtering based on one-dimensional defective photonic crystal containing hyperbolic metamaterial

As a kind of Fabry-Perot cavities, one-dimensional photonic crystals (1DPCs) with defect layers have been widely utilized for narrow-band filtering. However, according to the Bragg interference condition, photonic band gaps (PBGs) in conventional all-dielectric 1DPCs shift towards shorter wavelengths (blueshift) as the incident angle increases. Therefore, transmittance peaks in all-dielectric one-dimensional defective PCs also shift along with PBGs as the incident angle increases. Recently, the emergence of angle-insensitive PBGs in 1DPCs containing hyperbolic metamaterials (HMMs) provides us a possibility to realize omnidirectional narrow-band filtering. In this paper, we design an angle-insensitive ultraviolet PBG in a 1DPC containing HMM, where HMM layer is mimicked by a subwavelength dielectric-metal multilayer. By introducing a defect layer into the 1DPC and adjusting the dielectric thickness within the 1DPC, we achieve omnidirectional narrow-band ultraviolet filtering with angle-insensitive property. This omnidirectional narrow-band filtering might possess applications in digital display, organic solar cell and wavelength division demultiplexing.

Anchor Loss Reduction of Lamb Wave Resonator by Pillar-Based Phononic Crystal.

Energy leakage via anchors in substrate plates impairs the quality factor (Q) in microelectromechanical system (MEMS) resonators. Most phononic crystals (PnCs) require complicated fabrication conditions and have difficulty generating a narrow bandgap at high frequency. This paper demonstrates a pillar-based PnC slab with broad bandgaps in the ultra high frequency (UHF) range. Due to Bragg interference and local resonances, the proposed PnC structure creates notably wide bandgaps and shows great advantages in the high frequency, large electromechanical coupling coefficient () thin film aluminum nitride (AlN) lamb wave resonator (LWR). The dispersion relations and the transmission loss of the PnC structure are presented. To optimize the bandgap, the influence of the material mechanical properties, lattice type, pillar height and pillar radius are explored. These parameters are also available to adjust the center frequency of the bandgap to meet the desirable operating frequency. Resonators with uniform beam anchors and PnC slab anchors are characterized. The results illustrate that the Q of the resonator improves from 1551 to 2384, and the mechanical energy leakage via the anchors is significantly decreased using the proposed PnC slab anchors. Moreover, employment of the PNC slab anchors has little influence on resonant frequency and induces no spurious modes. Pillar-based PnCs are promising in suppressing the anchor loss and further improving the Q of the resonators.

Ultra-large omnidirectional photonic band gaps in one-dimensional ternary photonic crystals composed of plasma, dielectric and hyperbolic metamaterial

Abstract Owing to the Bragg interference mechanism, photonic band gap (PBG) in all-dielectric one-dimensional photonic crystal (PC) is strongly sensitive to the incident angle, which limits the width of omnidirectional photonic band gap (OPBG). Recently, researchers realized a kind of special PBG called angle-insensitive PBG in one-dimensional binary PC composed of alternating dielectric and hyperbolic metamaterial (HMM) layers. Different from the conventional strongly angle-insensitive PBG, this kind of PBG is angle-insensitive and naturally omnidirectional, providing us an efficient way to achieve ultra-large OPBG. In this paper, by periodically introducing plasma layers into one-dimensional binary PC composed of alternating dielectric and HMM layers with angle-insensitive PBG, we find that the PBG can be efficiently broadened. Although the PBG becomes slightly angle-sensitive since the introduction of plasma layers breaks the phase variation compensation condition, the width of OPBG can still be efficiently broadened. Under the proper design, we achieve an ultra-large OPBG with the width up to 1321 nm (68% relative bandwidth) in the one-dimensional ternary PC composed of alternating plasma, dielectric and HMM layers at near-infrared wavelengths. The whole one-dimensional ternary PC is composed of only three kinds of materials: indium tin oxide (ITO), silicon (Si) and silicon carbide (SiC), which can be fabricated by the atomic layer deposition (ALD) method under the current experimental conditions. This ultra-large OPBG in simple one-dimensional structure could be utilized to design ultra-broadband omnidirectional optical mirror and narrowband optical filter with ultra-large omnidirectional stopband.

Broadband omnidirectional near-infrared reflector based on an angle-insensitive photonic band gap.

Owing to the Bragg interference mechanism, band gap in all-dielectric one-dimensional photonic crystal (1DPC) is strongly sensitive to the incident angle, which limits the width of omnidirectional band gap. Based on the angle-insensitive Bragg band gap in 1DPC composed of alternating dielectric and hyperbolic metamaterial layers, we design a broadband omnidirectional near-infrared mirror with the bandwidth of 943 nm (52.3% relative bandwidth) without using the conventional cascade method. In addition, we show that the angle-insensitive property of the band gap is robust against the layer thickness disturbance. This multilayer-based broadband omnidirectional reflector will possess potential applications for omnidirectional filters within broad stopbands and photonic crystal fibers with broad operating bandwidths.

Annular Structural Colors from Bowl‐Like Shriveled Photonic Microshells of Cholesteric Liquid Crystals

AbstractStructural colors obtained by self‐organized microstructures have found a variety of applications, such as decorations, displays, and encryption microarrays, and have been widely investigated for the excellent stability and anisotropic optical responses upon external stimuli. Here, the fabrication of a variety of bowl‐like shriveled microshells using helical cholesteric liquid crystals (CLCs) by glass‐capillary‐based microfluidic techniques is reported. Interestingly, the internal and selective Bragg interferences of the radially arranged helical CLC structure enable the color‐correlative optical responses, resulting in the generation of annular color rings with variable reflection wavelengths and geometries by tuning the pitch and adjusting the incubation parameters. In addition, the manipulation of shriveled photonic microshells is demonstrated under a magnetic field. Benefiting from the low‐cost fabrication of optical interference microstructures and the controllable generation of structural color, it is expected that this class of shriveled photonic CLC microshells might pioneer a novel route for future photonic applications and beyond.

Reflectivity of three-dimensional GaAs photonic band-gap crystals of finite thickness

We study the optical reflectivity of three-dimensional (3D) photonic band gap crystals with increasing thickness. The crystals consist of GaAs plates with nanorod arrays that are assembled by an advanced stacking method into high-quality 3D woodpile structures. We observe intense and broad reflectivity peak with stop bands that correspond to a broad gap in the photonic band structures. The maximum reflectivity quickly reaches high values even for a few crystal layers. Remarkably, the bandwidth of the stop bands hardly decreases with increasing crystal thickness, in good agreement with FDTD simulations. This behavior differs remarkably from the large changes observed earlier in weakly interacting 3D photonic crystals. The nearly constant bandwidth and high reflectivity are rationalized by multiple Bragg interference that occurs in strongly interacting photonic band gap crystals, whereby the incident light scatters from multiple reciprocal lattice vectors simultaneously, in particular from oblique ones that are parallel to a longer crystal dimension and thus experience hardly any finite size effects. Our new insights have favorable consequences for the application of 3D photonic band gap crystals, notably since even thin structures reveal the full band gap functionality, including devices that shield quantum bits from vacuum fluctuations.

Subwavelength Acoustic Valley-Hall Topological Insulators Using Soda Cans Honeycomb Lattices.

Topological valley-contrasting physics has attracted great attention in exploring the use of the valley degree of freedom as a promising carrier of information. Recently, this concept has been extended to acoustic systems to obtain nonbackscattering sound propagations. However, previous demonstrations are limited by the cut-off frequency of 2D waveguides and lattice-scale size restrictions since the topological edge states originate from Bragg interference. Here we engineer topologically valley-projected edge states in the form of spoof surface acoustic waves that confine along the surface of a subwavelength honeycomb lattice composed of 330-mL soda cans. The inversion symmetry is broken through injecting a certain amount of water into one of the two cans in each unit cell, which gaps the Dirac cone and ultimately leads to the topological valley-Hall phase transition. Dual-frequency ranges of the valley-projected edge states below the sound line are observed, which originate from the first-order and second-order resonances, respectively. These results have the potential to enable promising routes to design integrated acoustic devices based on valley-contrasting physics.

Magnonic band gap and mode hybridization in continuous permalloy films induced by vertical dynamic coupling with an array of permalloy ellipses

We investigate magnonic band structure in thin homogeneous permalloy film decorated with periodic array of elliptically shaped permalloy dots and separated by non-magnetic Pt spacer. We demonstrated experimentally formation of the magnonic band structure for Damon-Eshbach wave propagating in permalloy film with the band gap opened at the Brillouin zone border and band splitting at smaller wavenumbers, due to the Bragg interference and interaction of propagating wave of the continuous film with a standing resonant mode of the nano-ellipses, respectively. The shape anisotropy of the permalloy nanodots allows to control the spin wave dynamics through the switch between two states of the magnetization with respect to the underneath film magnetization, thus enabling magnonic band structure reprogrammability. With numerical analysis we show, that predominant role in formation of the magnonic band structure is played by a vertical dynamic coupling between propagating wave in the film and magnetization oscillations in the nanodots.

Spoof Plasmonics: From Metamaterial Concept to Topological Description.

Advances in metamaterials have offered the opportunity of engineering electromagnetic properties beyond the limits of natural materials. A typical example is "spoof" surface plasmon polaritons (SPPs), which mimic features of SPPs without penetrating into metal, but only with periodic corrugations on metal surfaces. They hold considerable promise in device applications from microwaves to the far infrared, where real SPP modes do not exist. The original spoof SPP concept is derived from the description of corrugated surfaces by a metamaterial that hosts an effective plasma frequency. Later, studies have attempted to describe spoof SPP modes with the band structure by strictly solving Maxwell's equations, which can possess band gaps from polaritonic anticrossing principle or Bragg interference. More recently, as inspired by the development of topological framework in condensed matter physics, the topological description of spoof SPPs is used to propose topologically protected waveguiding phenomena. Here, the developments of spoof SPPs from both practical and fundamental perspectives are reviewed.

Subwavelength Localized Modes for Acoustic Waves in Bubbly Crystals with a Defect

The ability to control wave propagation is of fundamental interest in many areas of physics. Photonic and phononic crystals have proved very useful for this purpose but, because they are based on Bragg interference, these artificial media require structures with large dimensions. In [Ammari et al., Subwavelength phononic bandgap opening in bubbly media, J. Diff. Eq., 263 (2017), 5610--5629], it has been proved that a subwavelength bandgap opening occurs in bubbly phononic crystals. To demonstrate the opening of a subwavelength phononic bandgap, a periodic arrangement of bubbles is considered and their subwavelength Minnaert resonance is exploited. In this paper, this subwavelength bandgap is used to demonstrate cavities, very similar to those obtained in photonic and phononic crystals, albeit of deeply subwavelength dimensions. The key idea is to perturb the size of a single bubble inside the crystal, thus creating a defect. The goal is then to analytically and numerically show that this crystal has a localized eigenmode close to the defect bubble.

Design and Modeling of Highly Sensitive Lossy Mode Resonance-Based Fiber-Optic Pressure Sensor

This paper for the first time presents design and theoretical model for a fiber-optic pressure sensor based on a highly sensitive phenomenon of lossy mode resonance (LMR). Unlike transmission-based sensors, the proposed LMR-based pressure sensor works in reflection mode where sensing signal is measured in terms of the retro-reflection spectra. A detailed analysis of zinc oxide (ZnO) coated reflection-based LMR pressure sensor has been carried out. Also to improve sensitivity of the proposed sensor, an additional film of halfnium dioxide (HfO2) is considered, and the combined effect of ZnO and oxide thickness ratio on the sensitivity has been investigated. Maximum sensitivity of the proposed pressure sensor is $2~\mu \text {m}/\text {MPa}$ , which is higher than the previously reported popular fiber Bragg grating, interference and surface plasmon resonance-based pressure sensors. Additional feature of the proposed sensor is that one can achieve a tunable sensitivity and operating range only by changing the thickness ratio of ZnO and oxide overlay.

Soda Cans Metamaterial: A Subwavelength-Scaled Phononic Crystal

Photonic or phononic crystals and metamaterials, due to their very different typical spatial scales—wavelength and deep subwavelength—and underlying physical mechanisms—Bragg interferences or local resonances—, are often considered to be very different composite media. As such, while the former are commonly used to manipulate and control waves at the scale of the unit cell, i.e., wavelength, the latter are usually considered for their effective properties. Yet we have shown in the last few years that under some approximations, metamaterials can be used as photonic or phononic crystals, with the great advantage that they are much more compact. In this review, we will concentrate on metamaterials made out of soda cans, that is, Helmholtz resonators of deep subwavelength dimensions. We will first show that their properties can be understood, likewise phononic crystals, as resulting from interferences only, through multiple scattering effects and Fano interferences. Then, we will demonstrate that below the resonance frequency of its unit cell, a soda can metamaterial supports a band of subwavelength varying modes, which can be excited coherently using time reversal, in order to beat the diffraction limit from the far field. Above this frequency, the metamaterial supports a band gap, which we will use to demonstrate cavities and waveguides, very similar to those obtained in phononic crystals, albeit of deep subwavelength dimensions. We will finally show that multiple scattering can be taken advantage of in these metamaterials, by correctly structuring them. This allows to turn a metamaterial with a single negative effective property into a negative index metamaterial, which refracts waves negatively, hence acting as a superlens.

Quantum well effect based on hybridization bandgap in deep subwavelength coupled meta-atoms

In this paper, quantum well (QW) effect in a hybridization bandgap (HBG) structure via hiring deep subwavelength coupled meta-atoms is investigated. Subwavelength zero-index-metamaterial-based resonators acting as meta-atoms are side-coupled to a microstrip, forming the HBG structure. Both numerical and microwave experimental results confirm that, through properly hiring another set of meta-atoms, band mismatch between two HBGs can be introduced resulting in the HBG QW effect. Compared with the conventional QW structure based on Bragg interferences in photonic crystal, the device length of the proposed HBG QW structure can be reduced to only 1/4, demonstrating well the deep subwavelength property. Therefore, the above features make our design of HBG QW structures suitable to be utilized as multi-channel filters or multiplexers in microwave and optical communication system.