# ProfessorRichardCraster

Faculty of Natural Sciences

Dean of the Faculty of Natural Sciences

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### Contact

+44 (0)20 7594 8554r.craster

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### Assistant

Ms Mary Scott +44 (0)20 7594 5477

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### Location

3.05Faculty BuildingSouth Kensington Campus

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## Publications

Publication Type
Year
to

324 results found

Aguzzi G, Kanellopoulos C, Wiltshaw R, Craster RV, Chatzi EN, Colombi Aet al., 2022, Octet lattice-based plate for elastic wave control (vol 12, 1088, 2022), SCIENTIFIC REPORTS, Vol: 12, ISSN: 2045-2322

Journal article

Chaplain GJ, De Ponti JM, Craster RV, 2022, Elastic orbital angular momentum, Physical Review Letters, Vol: 128, ISSN: 0031-9007

We identify that flexural guided elastic waves in elastic pipes carry a well-defined orbital angular momentum associated with the compressional dilatational potential. This enables the transfer of elastic orbital angular momentum, that we numerically demonstrate, through the coupling of the compressional potential in a pipe to the acoustic pressure field in a surrounding fluid in contact with the pipe.

Journal article

Aguzzi G, Kanellopoulos C, Wiltshaw R, Craster RV, Chatzi EN, Colombi Aet al., 2022, Octet lattice-based plate for elastic wave control, Scientific Reports, Vol: 12, ISSN: 2045-2322

Motivated by the importance of lattice structures in multiple fields, we numerically investigate the propagation of flexural waves in a thin reticulated plate augmented with two classes of metastructures for wave mitigation and guiding, namely metabarriers and metalenses. The cellular architecture of this plate invokes the well-known octet topology, while the metadevices rely on novel customized octets either comprising spherical masses added to the midpoint of their struts or variable node thickness. We numerically determine the dispersion curves of a doubly-periodic array of octets, which produce a broad bandgap whose underlying physics is elucidated and leveraged as a design paradigm, allowing the construction of a metabarrier effective for inhibiting the transmission of waves. More sophisticated effects emerge upon parametric analyses of the added masses and node thickness, leading to graded designs that spatially filter waves through an enlarged bandgap via rainbow trapping. Additionally, Luneburg and Maxwell metalenses are realized using the spatial modulation of the tuning parameters and numerically tested. Wavefronts impinging on these structures are progressively curved within the inhomogeneous media and steered toward a focal point. Our results yield new perspectives for the use of octet-like lattices, paving the way for promising applications in vibration isolation and energy focusing.

Journal article

Chaplain GJ, Craster R, Cole N, Hibbins AP, Starkey TAet al., 2021, Underwater focusing of sound by Umklapp diffraction, Physical Review Applied, Vol: 21, Pages: 1-11, ISSN: 2331-7019

Scholte modes that are localized between a submerged axisymmetric structured elastic plate and surrounding fluid can undergo mode conversion via Umklapp diffraction into radiative modes; this radiative response is verified by experiments that show focusing of underwater sound across a broad range of frequencies. The diffracted beams, that form a cone, are engineered to exist at a desired spatial position, associated with an abrupt change in the patterning of the plate. These structures take the form of grooves present only on one side of the plate, yet the focusing phenomena is achieved on both sides, even as viewed from the flat surface.

Journal article

Palmer SJ, Ignatov Y, Craster RV, Makwana Met al., 2021, Asymptotically exact photonic approximations of chiral symmetric topological tight-binding models, New Journal of Physics, ISSN: 1367-2630

Topological photonic edge states, protected by chiral symmetry, are attractive for guiding wave energy as they can allow for more robust guiding and greater control of light than alternatives; however, for photonics, chiral symmetry is often broken by long-range interactions. We look to overcome this difficulty by exploiting the topology of networks, consisting of voids and narrow connecting channels, formed by the spaces between closely spaced perfect conductors. In the limit of low frequencies and narrow channels, these void-channel systems have a direct mapping to analogous discrete mass-spring systems in an asymptotically rigorous manner and therefore only have short-range interactions. We demonstrate that topological tight-binding models that are protected by chiral symmetries, such as the SSH model and square-root semimetals, are reproduced for these void-channel networks with appropriate boundary conditions. We anticipate, moving forward, that this paper provides a basis from which to explore continuum photonic topological systems, in an asymptotically exact manner, through the lens of a simplified tight-binding model.

Journal article

Ungureanu B, Tournat V, Craster RV, Guenneau Set al., 2021, Theory and experiments for seismic waves propagating within an array of clamped inclusions in a soft matrix, Pages: 438-441

We validate the concept of large scale clamped metamaterials with surface seismic waves propagating in a tank filled with a soft soil (granular medium). When cylindrical inclusions are clamped to the bottom of the tank, any incident wave on the network of inclusions should be reflected in the low frequency regime according to homogenization theory for singularly perturbed problems. However, the singular perturbation theory which is based on the analysis of the spectral properties of the Navier operator for a non-dissipative isotropic periodic elastic linear medium will be affected by the fact that the medium is granular. Our experimental setup should be a proof of concept of a seismic shield at the meter scale.

Conference paper

Makwana M, Wiltshaw R, Craster RV, Guenneau Set al., 2021, Topological guidance in novel photonic crystal fibers, Pages: 144-147

We discuss the recently proposed hybrid topological-photonic guidance of light that translates concepts from the field of topological matter to that of photonic crystal fiber arrays. We focus our attention on S-polarized obliquely propagating electromagnetic waves guided by square lattice topological systems along an array of infinitely conducting fibers. The theory utilises perfectly periodic arrays that, in frequency space, have gapped Dirac cones producing band gaps demarcated by pronounced valleys locally imbued with a nonzero local topological quantity. These broken symmetry-induced stop-bands allow for localised guidance of electromagnetic edge-waves along the crystal fiber axis. Finite element simulations demonstrate the effectiveness of the proposed designs for localising energy in finite arrays in a robust manner. Potential applications of our results reside in the generation of topological photonic and phononic crystal fibers for long-haul communications.

Conference paper

Huidobro et al, 2021, Correction for Huidobro et al., Fresnel drag in space–time-modulated metamaterials, Proceedings of the National Academy of Sciences, Vol: 118, Pages: 1-1, ISSN: 0027-8424

Journal article

Putley HJ, Chaplain GJ, Rakotoarimanga-Andrianjaka H, Maling B, Craster RVet al., 2021, Whispering-Bloch elastic circuits, Wave Motion, Vol: 105, Pages: 1-19, ISSN: 0165-2125

We investigate structured arrays and rings in elasticity to design elastic platonic circuitsthat utilise resonant phenomena. Creating ring resonators, and understanding theircoupling to input and output arrays, allows for the development of platonic circuitsincluding add–drop filters (ADFs) and coupled resonator elastic arrays (CREAs), andhence we envisage integrated platonic devices. Structured rings of point-masses placedatop a thin elastic plate lead to highly confined quasi-modes that leak energy; theleakage being quantified by the limiting quality factor Q. The conditions of resonanceare deduced using highly accurate numerical simulations based on a Green’s functionapproach to solve the dispersion relation associated with the structured ring resonators.The sharp resonances that emerge are then used to filter and direct wave energy basedon input frequency, and this is illustrated through analogy with devices used in optics,e.g. integrated photonic circuits. The potential applications of the elastic devices includeelastic delay lines and passive energy harvesters.

Journal article

Balmforth NJ, Craster R, Hewitt DR, 2021, Building on Oldroyd's viscoplastic legacy: Perspectives and new developments, Journal of Non-Newtonian Fluid Mechanics, Vol: 294, ISSN: 0377-0257

The decade following the second world war heralded the publication of a collection of important papers on non-Newtonian fluid mechanics; Oldroyd’s work featured heavily in this collection. Not only did these articles establish important results, but Oldroyd’s style and methods set the scene for subsequent work in the area, exploiting mathematical analysis to formulate problems, establish results and guide further research. While Oldroyd’s name will forever be linked with the study of elastic fluids, the purpose of the present paper is to offer a modern perspective on a number of Oldroyd’s papers on viscoplastic fluids from 1947–1951 [1], [2], [3], [4], [5], [6], [7], [8]. Along the way, we sprinkle in a brief review of some of the subsequent developments stemming from Oldroyd’s advances, together with a few new results guided by his work. Following the approach of most of Oldroyd’s original papers, we focus on unidirectional flow down conduits. In an Appendix, we complement this discussion with a lubrication analysis, extending, clarifying and correcting the important original analysis of Walton and Bittleston (1991) [9]; although lubrication theory was not directly utilized by Oldroyd, the methodology aligns with his philosophy of using asymptotic and analytical approaches.

Journal article

Laforge N, Wiltshaw R, Craster RV, Laude V, Martínez JAI, Dupont G, Guenneau S, Kadic M, Makwana MPet al., 2021, Acoustic topological circuitry in square and rectangular phononic crystals, Physical Review Applied, Vol: 15, Pages: 1-13, ISSN: 2331-7019

We systematically engineer a series of square and rectangular phononiccrystals to create experimental realisations of complex topological phononiccircuits. The exotic topological transport observed is wholly reliant upon theunderlying structure which must belong to either a square or rectangularlattice system and not to any hexagonal-based structure. The phononic systemchosen consists of a periodic array of square steel bars which partitionsacoustic waves in water over a broadband range of frequencies (~0.5 MHz). Anultrasonic transducer launches an acoustic pulse which propagates along adomain wall, before encountering a nodal point, from which the acoustic signalpartitions towards three exit ports. Numerical simulations are performed toclearly illustrate the highly resolved edge states as well as corroborate ourexperimental findings. To achieve complete control over the flow of energy,power division and redirection devices are required. The tunability afforded byour designs, in conjunction with the topological robustness of the modes, willresult in their assimilation into acoustical devices.

Journal article

Quadrelli DE, Craster R, Kadic M, Braghin Fet al., 2021, Elastic wave near-cloaking, Extreme Mechanics Letters, Vol: 44, Pages: 1-8, ISSN: 2352-4316

Cloaking elastic waves has, in contrast to the cloaking of electromagnetic waves, remained a fundamental challenge: the latter successfully uses the invariance of Maxwell’s equations, from which the field of transformational optics has emerged, whereas the elastic Navier equations are not invariant under coordinate transformations. Our aim is to overcome this challenge, at least in practical terms, and thereby unlock applications in mechanics, ultrasound, vibration mitigation, non-destructive evaluation and elastic wave control. We achieve near-cloaking by recognizing that, despite the lack of invariance, a decoupling into a system of form invariant potential equations together with a quantifiable approximation, can be used effectively in many cases to control the flow of elastodynamic waves. Here, in particular we focus on the efficiency and practicability of the proposed near-cloaking which is illustrated using carpet cloaks to hide surface defects from incoming compressional and shear in-plane waves and from surface elastic Rayleigh waves.

Journal article

Chaplain GJ, Craster RV, 2021, Surface corrugated laminates as elastic grating couplers: Splitting of SV- and P-waves by selective diffraction, Journal of Applied Physics, Vol: 129, Pages: 1-10, ISSN: 0021-8979

The phenomenon of selective diffraction is extended to in-plane elastic waves, and we design surface corrugated periodic laminates that incorporate crystal momentum transfer, which, due to the rich physics embedded within the vector elastic system, results in frequency, angle, and wave-type selective diffraction. The resulting devices are elastic grating couplers, with additional capabilities as compared to analogous scalar electromagnetic couplers, in that the elastic couplers possess the ability to split and independently redirect, through selective negative refraction, the two body waves present in the vector elastic system: P- (compressional) and SV- (shear-vertical) elastic waves. The design paradigm, and interpretation, is aided by obtaining isofrequency contours via a non-dimensionalized transfer matrix method.

Journal article

Batchvarov A, Kahouadji L, Constante-Amores CR, Norões Gonçalves GF, Shin S, Chergui J, Juric D, Craster RV, Matar OKet al., 2021, Three-dimensional dynamics of falling films in the presence of insoluble surfactants, Journal of Fluid Mechanics, Vol: 906, Pages: A16-1-A16-13, ISSN: 0022-1120

We study the effect of insoluble surfactants on the wave dynamics of vertically falling liquid films. We use three-dimensional numerical simulations and employ a hybrid interface-tracking/level-set method, taking into account Marangoni stresses induced by gradients of interfacial surfactant concentration. Our numerical predictions for the evolution of the surfactant-free, three-dimensional wave topology are validated against the experimental work of Park & Nosoko (AIChE J., vol. 49, 2003, pp. 2715–2727). The addition of surfactants is found to influence significantly the development of horseshoe-shaped waves. At low Marangoni numbers, we show that the wave fronts exhibit spanwise oscillations before eventually acquiring a quasi-two-dimensional shape. In addition, the presence of Marangoni stresses is found to suppress the peaks of the travelling waves and preceding capillary wave structures. At high Marangoni numbers, a near-complete rigidification of the interface is observed.

Journal article

Haslinger SG, Lowe MJS, Craster R, Huthwaite P, Shi Fet al., 2021, Prediction of reflection amplitudes for ultrasonic inspection of rough planar defects, Insight, Vol: 63, Pages: 28-36, ISSN: 2156-485X

The characteristics of planar defects (no loss of material volume) that arise during industrial plant operation are difficult to predict in detail, yet these can affect the performance of non-destructive testing (NDT) used to manage plant structural integrity. Inspection modelling is increasingly used to design and assess ultrasonic inspections of such plant items. While modelling of smooth planar defects is relatively mature and validated, issues have remained in the treatment of rough planar defect species. The qualification of ultrasonic inspections for such defects is presently very conservative, owing to the uncertainty of the amplitudes of rough surface reflections. Pragmatic solutions include the addition of large sensitivity thresholds and more frequent inspection intervals, which require more plant downtime. In this article, an alternative approach has been developed by the authors to predict the expected surface reflection from a rough defect using a theoretical statistical model. Given only the frequency, angle of incidence and two statistical parameter values used to characterise the defects, the expected reflection amplitude is obtained rapidly for any scattering angle and size of defect, for both compression and shear waves. The method is applicable for inspections of isotropic media that feature surface reflections such as pulse-echo or pitch-catch, rather than for tip signal-dependent techniques such as time-of-flight diffraction. The potential impact for inspection qualification is significant, with the new model predicting increases of up to 20 dB in signal amplitude in comparison with models presently used in industry. All mode conversions are included and rigorous validations using numerical and experimental methods have been performed. The model has been instrumental in obtaining new statistically significant results related to the effect of tilt; the expected pulse-echo backscattered amplitude for very rough planar defects is independent of til

Journal article

Craster R, Diatta A, Guenneau S, Hutridurga Het al., 2021, On near-cloaking for linear elasticity, Multiscale Modeling & Simulation, Vol: 19, Pages: 633-664, ISSN: 1540-3459

We make precise some results on the cloaking of displacement fields in linear elasticity. In the spirit of transformation media theory, the transformed governing equations in Cosseratand Willis frameworks are shown to be equivalent to certain high-contrast small defect problems forthe usual Navier equations. We discuss near-cloaking for elasticity systems via a regularized transform and perform numerical experiments to illustrate our near-cloaking results. We also study thesharpness of the estimates from [H. Ammari, H. Kang, K. Kim, and H. Lee, J. Differential Equations,254 (2013), pp. 4446--4464], wherein the convergence of the solutions to the transmission problems isinvestigated, when the Lam\'e parameters in the inclusion tend to extreme values. Both soft and hardinclusion limits are studied and we also touch upon the finite frequency case. Finally, we propose anapproximate isotropic cloak algorithm for a symmetrized Cosserat cloak.

Journal article

Wiltshaw R, Craster R, Makwana M, 2020, Asymptotic approximations for Bloch waves and topological mode steering in a planar array of Neumann scatterers, Wave Motion, Vol: 99, ISSN: 0165-2125

We study the canonical problem of wave scattering by periodic arrays, either of infinite or finite extent, of Neumann scatterers in the plane; the characteristic lengthscale of the scatterers is considered small relative to the lattice period. We utilise the method of matched asymptotic expansions, together with Fourier series representations, to create an efficient and accurate numerical approach for finding the dispersion curves associated with Floquet-Bloch waves through an infinite array of scatterers. The approach also lends itself to direct scattering problems for finite arrays and we illustrate the flexibility of these asymptotic representations on some topical examples from topological wave physics.

Journal article

Tang K, Makwana MP, Craster RV, Sebbah Pet al., 2020, Observations of symmetry induced topological mode steering in a reconfigurable elastic plate, Physical Review B, Vol: 102, ISSN: 2469-9969

We experimentally investigate the valley-Hall effect for interfacial edge states, highlighting the importance of the modal patterns between geometrically distinct regions within a structured elastic plate. These experiments, for vibration, are at a scale where detailed measurements are taken throughout the system and not just at the input/output ports; this exposes the coupling between geometrically distinct modes that underlie the differences between wave transport around gentle and sharp bends.

Journal article

Chaplain GJ, Ponti JMD, Aguzzi G, Colombi A, Craster RVet al., 2020, Topological rainbow trapping for elastic energy harvesting in graded Su-Schrieffer-Heeger systems, Applied Physics Letters, Vol: 14, Pages: 054035 – 1-054035 – 15, ISSN: 0003-6951

We amalgamate two fundamental designs from distinct areas of wave control in physics, and place them in the setting of elasticity. Graded elastic metasurfaces, so-called metawedges, are combined with the now classical Su-Schrieffer-Heeger (SSH) model from the field of topological insulators. The resulting structures form one-dimensional graded-SSH-metawedges that support multiple, simultaneous, topologically protected edge states. These robust, enhanced localised modes are leveraged for applications in elastic energy harvesting using the piezoelectric effect. The designs we develop are first motivated by applying the SSH model to mass-loaded Kirchhoff-Love thin elastic plates. We then extend these ideas to using graded resonant rods, and create SSH models, coupled to elastic beams and full elastic half-spaces.

Journal article

De Ponti JM, Colombi A, Riva E, Ardito R, Braghin F, Corigliano A, Craster RVet al., 2020, Experimental investigation of amplification, via a mechanical delay-line, in a rainbow-based metamaterial for energy harvesting, Applied Physics Letters, Vol: 117, Pages: 143902-1-143902-6, ISSN: 0003-6951

We experimentally demonstrate that a rainbow-based metamaterial, created by a graded array of resonant rods attached to an elastic beam, operates as a mechanical delay-line by slowing down surface elastic waves to take advantage of wave interaction with resonance. Experiments demonstrate that the rainbow effect reduces the amplitude of the propagating wave in the host structure. At the same time, it dramatically increases both the period of interaction between the waves and the resonators and the wavefield amplitude in the rod endowed with the harvester. Increased energy is thus fed into the resonators over time: we show the enhanced energy harvesting capabilities of this system.

Journal article

Makwana M, Wiltshaw R, Guenneau S, Craster Ret al., 2020, Hybrid topological guiding mechanisms for photonic crystal fibers, Optics Express, Vol: 28, Pages: 30871-30888, ISSN: 1094-4087

We create hybrid topological-photonic localisation of light by introducing concepts from the field of topological matter to that of photonic crystal fiber arrays. S-polarized obliquely propagating electromagnetic waves are guided by hexagonal, and square, lattice topological systems along an array of infinitely conducting fibers. The theory utilises perfectly periodic arrays that, in frequency space, have gapped Dirac cones producing band gaps demarcated by pronounced valleys locally imbued with a nonzero local topological quantity. These broken symmetry-induced stop-bands allow for localised guidance of electromagnetic edge-waves along the crystal fiber axis. Finite element simulations, complemented by asymptotic techniques, demonstrate the effectiveness of the proposed designs for localising energy in finite arrays in a robust manner.

Journal article

Proctor M, Xiao X, Craster RV, Maier SA, Giannini V, Arroyo Huidobro Pet al., 2020, Near- and far-field excitation of topological plasmonic metasurfaces, Photonics, Vol: 7, ISSN: 2304-6732

The breathing honeycomb lattice hosts a topologically non-trivial bulk phase due to the crystalline-symmetry of the system. Pseudospin-dependent edge states, which emerge at the interface between trivial and non-trivial regions, can be used for the directional propagation of energy. Using the plasmonic metasurface as an example system, we probe these states in the near- and far-field using a semi-analytical model. We provide the conditions under which directionality was observed and show that it is source position dependent. By probing with circularly-polarised magnetic dipoles out of the plane, we first characterise modes along the interface in terms of the enhancement of source emissions due to the metasurface. We then excite from the far-field with non-zero orbital angular momentum beams. The position-dependent directionality holds true for all classical wave systems with a breathing honeycomb lattice. Our results show that a metasurfac,e in combination with a chiral two-dimensional material, could be used to guide light effectively on the nanoscale.

Journal article

Batchvarov A, Kahouadji L, Magnini M, Constante-Amores CR, Shin S, Chergui J, Juric D, Craster RV, Matar OKet al., 2020, Effect of surfactant on elongated bubbles in capillary tubes at high Reynolds number, Physical Review Fluids, Vol: 5, Pages: 093605 – 1-093605 – 21, ISSN: 2469-990X

The effect of surfactants on the tail and film dynamics of elongated gas bubbles propagating through circular capillary tubes is investigated by means of an extensive three-dimensional numerical study using a hybrid front-tracking/level-set method. The focus is on the visco-inertial regime, which occurs when the Reynolds number of the flow is much larger than unity. Under these conditions, “clean” bubbles exhibit interface undulations in the proximity of the tail, with an amplitude that increases with the Reynolds number. We perform a systematic analysis of the impact of a wide range of surfactant properties, including elasticity, bulk surfactant concentration, solubility, and diffusivity, on the bubble and flow dynamics in the presence of inertial effects. The results show that the introduction of surfactants is effective in suppressing the tail undulations as they tend to accumulate near the bubble tail. Here large Marangoni stresses are generated, which lead to a local “rigidification” of the bubble. This effect becomes more pronounced for larger surfactant elasticities and adsorption depths. At reduced surfactant solubility, a thicker rigid film region forms at the bubble rear, where a Couette film flow is established, while undulations still appear at the trailing edge of the downstream “clean” film region. In such conditions, the bubble length becomes an influential parameter, with short bubbles becoming completely rigid.

Journal article

Varma TV, Ungureanu B, Sarkar S, Craster R, Guenneau S, Brule Set al., 2020, The influence of structure geometry and material on seismic metamaterial performance, Publisher: arXiv

Diverting, and controlling, elastic vibrations impacting upon infrastructureis a major challenge for seismic hazard mitigation, and for the reduction ofmachine noise and vehicle vibration in the urban environment. Seismicmetamaterials (SMs), with their inherent ability to manipulate wavepropagation, provide a key route for overcoming the technological hurdlesinvolved in this challenge. Engineering the structure of the SM serves as abasis to tune and enhance its functionality, and inspired by split rings,swiss-rolls, notch-shaped and labyrinthine designs of elementary cells inelectromagnetic and mechanical metamaterials, we investigate altering thestructure geometries of SMs with the aim of creating large bandgaps\textcolor{black}{in a subwavelength regime}. We show that square stiffinclusions, perform better in comparison to circular ones, whilst keeping thesame filling fraction. En route to enhancing the bandgap, we have also studiedthe performance of SMs with different constituent materials; we find that steelcolumns, as inclusions, show large bandgaps, however, the columns are too largefor steel to be a feasible material in practical or financial terms.Non-reinforced concrete would be preferable for industry level scaling up ofthe technology because, concrete is cost-effective, easy to cast directly atthe construction site and easy to provide arbitrary geometry of the structure.As a part of this study, we show that concrete columns can also be designed toexhibit bandgaps if we cast them within a soft soil coating surrounding theprotected area for various civil structures like a bridge, building, oilpipelines etc.

Working paper

Wang Y-T, Craster R, 2020, Designing hyper-thin acoustic metasurfaces with membrane resonators, Publisher: arXiv

We design extremely-thin acoustic metasurfaces, providing a versatileplatform for the manipulation of reflected pressure fields, that areconstructed from mass loads and stretched membranes fixed to a periodic rigidframework. These metasurfaces demonstrate deeply subwavelength control and canhave thicknesses an order of magnitude less than those based around Helmholtzresonators. Each sub-unit of the metasurface is resonant at a frequency tunedgeometrically, this tunability provides phase control and using a set of finelytuned membrane resonators we create a phase-grating metasurface. This surfaceis designed to exhibit all-angle negative reflections with the ratio ofwavelength, $\lambda$, to thickness, $h$, of $\lambda/h\approx 23.1$, and tocreate a flat mirror using the phase profile of an elliptic reflecting mirror.A further important acoustic application is to sound diffusers and we proceedto design a deeply subwavelength membrane-based meta-diffuser that can be twoorders of magnitude thinner than the operating wavelength, i.e. thickness$\approx\lambda/102$. This paves the way for developing advanced acousticmetasurfaces with applicability to functional acoustic devices in sound-relatedindustries.

Working paper

Chaplain GJ, Ponti JMD, Colombi A, Fuentes-Dominguez R, Dryburg P, Pieris D, Smith RJ, Clare A, Clark M, Craster RVet al., 2020, Tailored elastic surface to body wave Umklapp conversion, Nature Communications, Vol: 11, ISSN: 2041-1723

Elastic waves guided along surfaces dominate applications in geophysics, ultrasonic inspection, mechanical vibration, and surface acoustic wave devices; precise manipulation of surface Rayleigh waves and their coupling with polarised body waves presents a challenge that offers to unlock the flexibility in wave transport required for efficient energy harvesting and vibration mitigation devices. We design elastic metasurfaces, consisting of a graded array of rod resonators attached to an elastic substrate that, together with critical insight from Umklapp scattering in phonon-electron systems, allow us to leverage the transfer of crystal momentum; we mode-convert Rayleigh surface waves into bulk waves that form tunable beams. Experiments, theory and simulation verify that these tailored Umklapp mechanisms play a key role in coupling surface Rayleigh waves to reversed bulk shear and compressional waves independently, thereby creating passive self-phased arrays allowing for tunable redirection and wave focusing within the bulk medium.

Journal article

Archer AJ, Wolgamot HA, Orszaghova J, Bennetts LG, Peter MA, Craster RVet al., 2020, Experimental realization of broadband control of water-wave-energy amplification in chirped arrays, Physical Review Fluids, Vol: 5, Pages: 62801(R) – 1-62801(R) – 8, ISSN: 2469-990X

Water waves in natural environments are typically broadband, nonlinear anddynamic phenomena. Taking concepts developed for slow light in optics, weaddress the challenge of designing arrays to control the spatial distributionof wave energy, and amplify target frequencies at specified locations.Experiments on incident waves interacting with a chirped array of eightvertical cylinders demonstrate significant amplifications as predictednumerically, and provide motivation for application to energy harvesting.

Journal article

Chaplain GJ, Pajer D, De Ponti JM, Craster RVet al., 2020, Delineating rainbow reflection and trapping with applications for energy harvesting, New Journal of Physics, Vol: 22, Pages: 1-12, ISSN: 1367-2630

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 focusing of energy.

Journal article

Chaplain GJ, Craster R, 2020, Ultrathin entirely flat Umklapp lenses, Physical Review B: Condensed Matter and Materials Physics, Vol: 101, Pages: 155430 – 1-155430 – 9, ISSN: 1098-0121

We design ultra-thin, entirely flat, dielectric lenses using crystal momentum transfer, so-called Umklapp processes, achieving the required wave control for a new mechanism of flat lensing; physically, these lenses take advantage of abrupt changes in the periodicity of a structured line array so there is an overlap between the first Brillouin zone of one medium with the second Brillouin zone of the other. At the interface between regions of different periodicity, surface, array guided waves hybridize into reversed propagating beams directed into the material exterior to the array. This control, and redirection, of waves then enables the device to emulate a Pendry-Veselago lens that is one unit cell in width, with no need for an explicit negative refractive index. Simulations using an array embedded in an idealized slab of silicon nitride (Si3N4) in air, operating at visible wavelengths between 420–500THz demonstrate the effect.

Journal article

Makwana M, Laforge N, Craster R, Dupont G, Guenneau S, Laude V, Kadic Met al., 2020, Experimental observations of topologically guided water waves within non-hexagonal structures, Applied Physics Letters, Vol: 116, Pages: 131603-1-131603-5, ISSN: 0003-6951

We investigate symmetry-protected topological water waves within a strategically engineered square lattice system. Thus far, symmetry-protected topological modes in hexagonal systems have primarily been studied in electromagnetism and acoustics, i.e. dispersionless media. Herein, we show experimentally how crucial geometrical properties of square structures allow for topological transport that is ordinarily forbidden within conventional hexagonal structures. We perform numerical simulations that take into account the inherent dispersion within water waves and devise a topological insulator that supports symmetry-protected transport along the domain walls. Our measurements, viewed with a high-speed camera under stroboscopic illumination, unambiguously demonstrate the valley-locked transport of water waves within a non-hexagonal structure. Due to the tunability of the energy's directionality by geometry, our results could be used for developing highly-efficient energy harvesters, filters and beam-splitters within dispersive media.

Journal article

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