Explore the vast range of titles available.

In elastic wave systems, combining the powerful concepts of resonance and spatial grading within structured surface arrays enable resonant metasurfaces to exhibit broadband wave trapping, mode conversion from surface (Rayleigh) waves to bulk (shear) waves, and spatial frequency selection. Devices built around these concepts allow for precise control of surface waves, often with structures that are subwavelength, and utilise Rainbow trapping that separates the signal spatially by frequency. Rainbow trapping yields large amplifications of displacement at the resonator positions where each frequency component accumulates. We investigate whether this amplification, and the associated control, can be used to create energy harvesting devices; the potential advantages and disadvantages of using graded resonant devices as energy harvesters is considered. We concentrate upon elastic plate models for which the A0 mode dominates, and take advantage of the large displacement amplitudes in graded resonant arrays of rods, to design innovative metasurfaces that trap waves for enhanced piezoelectric energy harvesting. Numerical simulation allows us to identify the advantages of such graded metasurface devices and quantify its efficiency, we also develop accurate models of the phenomena and extend our analysis to that of an elastic half-space and Rayleigh surface waves.

Acoustic rainbow trappers, based on frequency selective structures with graded geometries and/or properties, can filter mechanical waves spectrally and spatially to reduce noise and interference in receivers. These structures are especially useful as passive, always-on sensors in applications such as structural health monitoring. For devices that face space and weight constraints, such as microelectromechanical systems (MEMS) transducers and artificial cochleae, the rainbow trapping structures must be compact as well. To address this requirement, we investigated the frequency selection properties of a space-saving design consisting of Helmholtz resonators arranged at sub-wavelength intervals along a cochlear-inspired spiral tube. The height of the Helmholtz resonators was varied gradually, which induced bandgap formation at different frequencies along the length of the spiral tube. Numerical simulations and experimental measurements of acoustic wave propagation through the structure showed that frequencies in the range of 1–10 kHz were transmitted to different extents along the spiral tube. These rainbow trapping results were achieved with a footprint that was up to 70 times smaller than the previous structures operating at similar bandwidths, and the channels are 2.5 times of the previous structures operating at similar bandwidths.

Energy amplification in square-lattice arrays of C-shaped low-frequency resonators, where the resonator radii are graded with distance, is investigated in the two-dimensional linear acoustics setting for both infinite (in one dimension) and finite arrays. Large amplifications of the incident energy are shown in certain array locations. The phenomenon is analysed using: (i) band diagrams for doubly-periodic arrays; (ii) numerical simulations for infinite and finite arrays; and (iii) eigenvalue analysis of transfer matrices operating over individual columns of the array. It is shown that the locations of the large amplifications are predicted by propagation cut-offs in the modes associated with the transfer-matrix eigenvalues. For the infinite array, the eigenvalues form a countable set, and for the low frequencies considered, only a single propagating mode exists for a given incident wave, which cuts off within the array, leading to predictive capabilities for the amplification location. For the finite array, it is shown that (in addition to a continuous spectrum of modes) multiple discrete propagating modes can be excited, with the grading generating new modes, as well as cutting others off, leading to complicated amplification patterns. The numerical simulations reveal that the largest amplifications are achieved for a single row array, with amplifications an order of magnitude smaller for the corresponding infinite array. This article is part of the theme issue ‘Modelling of dynamic phenomena and localization in structured media (part 1)’.

Wireless sensors are vital for real-time condition monitoring of rotating machinery. Traditionally, these sensors depend on batteries, a solution that is neither eco-friendly nor cost-effective due to high maintenance. Vibration energy harvesting has emerged as a promising alternative for powering these sensors. Nevertheless, current energy harvesters commonly disregard high-frequency energy, which is weak but contains abundant condition-monitoring information. Moreover, the destructive interference between multiple vibration sources further attenuates the high-frequency energy. To address these limitations, this paper proposed a novel energy harvesting method based on a symmetrical gradient metamaterial beam (SGMB). The SGMB structure is designed to have multiple bands to enhance the high-frequency energy and diminish the destructive interference of flexural waves from two vibration sources. Multiple piezoelectric patches are integrated into SGMB to convert the dynamic stress into harvestable electrical power, enabling multi-band dual-source energy harvesting. Based on the rainbow trapping mechanism, the SGMB was first designed and optimized for desired frequency bands. Subsequently, the band characteristics and piezoelectric output performance were adjusted and validated through finite element simulation. Finally, experimental evaluations were conducted to validate the performance of the designed metamaterial. The results demonstrate that the SGMB provides multiple enhanced bands within the range from 1000 Hz to 3500 Hz and improves the energy harvesting efficiency by a factor over 100, which represents a breakthrough in developing self-powered and self-sensing wireless sensors.

Multiwavelength nanophotonic devices are essential components in photonic integrated circuits for on-chip all-optical signal processing applications with enhanced data-storage capacity and increased bandwidth. Wavelength demultiplexing is a widely used photonic processing function, required for photonic signal processing on compact silicon chip designs. In this paper, we study optical rainbow trapping in a line defect photonic waveguide, and propose to use that effect for demultiplexing the light at visible spectrum. A defect waveguide is created in graded-index photonic crystal (GRIN PC) structure (the structure is composed of Si- PC cylinders having gradually varying radii along one dimension) as main channel, which allows the rainbow-trapping along the waveguiding channel. Vertical (side) channels are introduced to the system for efficient wavelength demultiplexing with low crosstalks and high coupling efficiencies thanks to the fact that incident light is localized at different positions along the main channel for certain operating visible wavelengths of The corresponding inter-channel cross-talks of is obtained at these incident wavelengths, which strengthens the idea of using such a GRIN PC structure for efficient wavelength demultiplexing applications. The studied multichannel demultiplexing device is relatively compact with a footprint of , which could be fabricated via simple lithography processes on a single Si- wafer. As far as authors’ knowledge, this is the first time in literature that GRIN PC waveguide is implemented for multichannel wavelength demultiplexing function. The proposed GRIN PC design could be implemented for color sensitive photo-detection, absorption enhancement and optical spectroscopy.

The concept of “rainbow trapping” has generated considerable interest in wave propagation and energy harvesting, offering new possibilities for diverse and efficient acoustic wave operations. In this work, we investigate a dual-channel topological rainbow trapping device implemented within an underwater two-dimensional phononic crystal based on synthetic dimension. The topological edge states with different frequencies are separated and trapped at different spatial locations. Acoustic waves propagate simultaneously along two boundaries due to the degeneracy of the edge states. In particular, the propagation of a dual-channel topological rainbow is also realized by using a bend design. This work contributes to the advancement of multi-channel devices in synthetic space and provides a reference for the design of highly efficient underwater acoustic devices.

Acoustic sensing technology has attracted significant attention across various fields, including mechanical fault early warning and wireless communication, due to its high information density and advantages in remote wireless applications. However, environmental noise reduces the signal-to-noise ratio (SNR) in traditional acoustic systems. In response, this article proposes a novel Z-shaped channel gradient metamaterial (ZCGM) that leverages strong wave compression effects coupled with effective medium theory to detect weak signals in complex environments. The properties of the designed metamaterials were verified by theoretical derivation and finite element simulation of the model. Compared to conventional linear gradient acoustic metamaterials (GAMs), ZCGM demonstrates significantly superior performance in acoustic enhancement, with a lower capture frequency. Furthermore, the structure exhibits flexible tunability in its profile. In addition, the center frequency of each actual air gap is determined in this paper based on the swept frequency signal test. Based on this center frequency, a preset specific harmonic acoustic signal is used as an emission source to simulate the actual application scenario, and experiments are constructed and conducted to verify the performance of the designed metamaterials. The results consistently show that ZCGM has distinct advantages and promising application prospects in the detection, enhancement, and localization of weak acoustic signals.

Accompanied by the rise of plasmonic materials beyond those based on noble metals and the development of advanced materials processing techniques, it is important to understand the plasmonic behavior of materials with large-scale inhomogeneity (such as gradient permittivity materials) because they cannot be modeled simply as scatterers. In this paper, we theoretically analyze the excitation and propagation of surface plasmon polaritons (SPPs) on a planar interface between a homogeneous dielectric and a material with a gradient of negative permittivity. We demonstrate the following: (i) free-space propagating waves and surface waves can be coupled by a gradient negative-permittivity material and (ii) the coupling can be enhanced if the material permittivity variation is suitably designed. This theory is then verified by numerical simulations. A direct application of this theory, ‘rainbow trapping’, is also proposed, considering a realistic design based on doped indium antimonide. This theory may lead to various applications, such as ultracompact spectroscopy and dynamically controllable generation of SPPs. Plasmonics: rainbow trap A convenient way to couple free-space light waves to surface plasmon polaritons is demonstrated on an interface with a permittivity gradient. Surface plasmon polaritons — electromagnetic waves that propagate at interfaces between dielectrics and metals — are promising for many applications in photonics. Now, Xi Wang of the University of California, Berkeley, and colleagues have theoretically analyzed how surface plasmon polaritons are excited and move on a non-structured interface between a dielectric material and a gradient negative permittivity material. Simulations with indium antimonide (InSb) as the gradient negative permittivity material suggest that ‘rainbow trapping’, in which multiple wavelengths of light can be simultaneously trapped at different locations, may occur at such an interface. This approach is potentially useful for creating plasmonic devices for applications such as ultracompact spectroscopy.

Metasurfaces have provided a promising approach to enhance the nonlinearity at subwavelength scale, but usually suffer from a narrow bandwidth as imposed by sharp resonant features. Here, we counterintuitively report a broadband, enhanced second-harmonic generation, in nanopatterned hyperbolic metamaterials. The nanopatterning allows the direct access of the mode with large momentum, rendering the rainbow light trapping, i.e. slow light in a broad frequency, and thus enhancing the local field intensity for boosted nonlinear light-matter interactions. For a proof-of-concept demonstration, we fabricated a nanostructured Au/ZnO multilayer, and enhanced second harmonic generation can be observed within the visible wavelength range (400-650 nm). The enhancement factor is over 50 within the wavelength range of 470-650 nm, and a maximum conversion efficiency of 1.13×10 −6 is obtained with a pump power of only 8.80 mW. Our results herein offer an effective and robust approach towards the broadband metasurface-based nonlinear devices for various important technologies. Though metamaterials enhance nonlinear light-matter interactions due to their resonant features, these materials typically show a narrow spectral bandwidth. Here, the authors report broadband enhanced second-harmonic generation in patterned multilayer hyperbolic metamaterial arrays.

Important distinctions are made between two related wave control mechanisms that act to spatially separate frequency components; these so-called rainbow mechanisms either slow or reverse guided waves propagating along a graded line array. We demonstrate an important nuance distinguishing rainbow reflection from genuine rainbow trapping and show the implications of this distinction for energy harvesting designs, through inspection of the interaction time between slowed zero group velocity waves and the array. The difference between these related mechanisms is highlighted using a design methodology, applied to flexural waves on mass loaded thin Kirchhoff-Love elastic plates, and emphasised through simulations for energy harvesting in the setting of elasticity, by elastic metasurfaces of graded line arrays of resonant rods atop a beam. The delineation of these two effects, reflection and trapping, allows us to characterise the behaviour of forced line array systems and predict their capabilities for trapping, conversion and focussing of energy.