Publications

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

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

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

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

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

Reshef O, Aharonovich I, Armani AM, Gigan S, Grange R, Kats MA, Sapienza Ret al., 2020,

How to organize an online conference

, Nature Reviews Materials, Vol: 5, Pages: 253-256, ISSN: 2058-8437

The first online-only meeting in photonics, held on 13 January 2020, was a resounding success, with 1100 researchers participating remotely to discuss the latest advances in photonics. Here, the organizers share their tips and advice on how to organize an online conference.

Journal article

Proctor M, Huidobro PA, Maier SA, Craster RV, Makwana MPet al., 2020,

Manipulating topological valley modes in plasmonic metasurfaces

, Nanophotonics, Vol: 9, Pages: 657-665, ISSN: 2192-8606

The coupled light-matter modes supported by plasmonic metasurfaces can be combined with topological principles to yield subwavelength topological valley states of light. We give a systematic presentation of the topological valley states available for lattices of metallic nanoparticles: All possible lattices with hexagonal symmetry are considered, as well as valley states emerging on a square lattice. Several unique effects which have yet to be explored in plasmonics are identified, such as robust guiding, filtering and splitting of modes, as well as dual-band effects. We demonstrate these by means of scattering computations based on the coupled dipole method that encompass the full electromagnetic interactions between nanoparticles.

Journal article

Septiadi D, Barna V, Saxena D, Sapienza R, Genovese D, De Cola Let al., 2020,

Biolasing from individual cells in a low-Q resonator enables spectral fingerprinting

, Advanced Optical Materials, Vol: 8, Pages: 1-8, ISSN: 2195-1071

Lasing from cells has recently been subject of thorough investigation because of the potential for sensitive and fast biosensing. Yet, lasing from individual cells has been studied in high‐quality resonators, resulting in limited dependence of the lasing properties on the cellular microenvironment. Here, lasing is triggered by cells floating in a low quality factor resonator composed of a disposable poly(methyl methacrylate) (PMMA) cell counting‐slide, hence in absence of conventional high‐reflectivity optical cavities. The exceptional spectral narrowing and the steep slope increase in the input–output energy diagram prove occurrence of laser action in presence of cells. The observed biolasing is an intrinsically dynamic signal, with large fluctuations in intensity and spectrum determined by the optical properties of the individual cell passing through the pump beam. Numerical simulations of the scattering efficiency rule out the possibility of optical feedback from either WGM (whispering gallery mode) or multiple scattering within the cell, and point to the enhanced directional scattering field as the crucial contribution of cells to the laser action. Finally, principal component analysis of lasing spectra measured from freely diffusing cells yields spectral fingerprints of cell populations, which allows discriminating cancer from healthy Rattus glial cells with high degree of confidence.

Journal article

Craster R, Maria de Ponti J, Colombi A, ardito R, braghin F, corigliano Aet al., 2020,

Graded elastic metasurface for enhanced energy harvesting

, New Journal of Physics, Vol: 22, Pages: 1-11, ISSN: 1367-2630

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

Journal article

Toan VN, Nhat VP, Hanh HM, Dung CD, Hai HL, Sapienza R, Van-Duong Tet al., 2019,

Protein-based microsphere biolasers fabricated by dehydration

, SOFT MATTER, Vol: 15, Pages: 9721-9726, ISSN: 1744-683X

Journal article

Sortina L, Zotev PG, Mignuzzi S, Cambiasso J, SChmidt D, Genco A, Abmann M, Bayer M, Maier SA, Sapienza R, Tartakovskii AIet al., 2019,

Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas

, Nature Communications, Vol: 50, Pages: 1-8, ISSN: 2041-1723

Unique structural and optical properties of atomically thin transition metal dichalcogenides (TMDs) enable in principle their efficient coupling to photonic cavities having the optical mode volume below the diffraction limit. So far, this has only been demonstrated by coupling TMDs with plasmonic modes in metallic nano-structures, which exhibit strong energy dissipation limiting their potential applications in devices. Here, we present an alternative approach for realisation of ultra-compact cavities interacting with two-dimensional semiconductors: we use mono- and bilayer TMD WSe2 coupled to low-loss high-refractive-index gallium phosphide (GaP) nano-antennas. We observe a photoluminescence (PL) enhancement exceeding 104 compared with WSe2 placed on the planar GaP, and trace its origin to a combination of enhancement of the spontaneous light emission rate, favourable modification of the PL directionality and enhanced optical excitation efficiency, all occurring as a result of WSe2 coupling with strongly confined photonic modes of the nano-antennas. Further effect of the coupling is observed in the polarisation dependence of WSe2 PL, and in the Raman scattering signal enhancement exceeding 103. Our findings reveal high-index dielectric nano-structures as a promising platform for engineering light-matter coupling in two-dimensional semiconductors.

Journal article

Ungureanu B, Guenneau S, Achaoui Y, Diatta A, Farhat M, Hutridurga H, Craster RV, Enoch S, Brule Set al., 2019,

The influence of building interactions on seismic and elastic body waves

, EPJ Applied Metamaterials, Vol: 6, Pages: 1-12, ISSN: 2272-2394

We outline some recent research advances on the control of elastic waves in thin and thick plates, that have occurred since the large scale experiment [S. Brûlé, Phys. Rev. Lett. 112, 133901 (2014)] that demonstrated significant interaction of surface seismic waves with holes structuring sedimentary soils at the meter scale. We further investigate the seismic wave trajectories of compressional body waves in soils structured with buildings. A significant substitution of soils by inclusions, acting as foundations, raises the question of the effective dynamic properties of these structured soils. Buildings, in the case of perfect elastic conditions for both soil and buildings, are shown to interact and strongly influence elastic body waves; such site-city seismic interactions were pointed out in [Guéguen et al., Bull. Seismol. Soc. Am. 92, 794–811 (2002)], and we investigate a variety of scenarios to illustrate the variety of behaviours possible.

Journal article

Makwana M, Craster R, Guenneau S, 2019,

Topological beam-splitting in photonic crystals

, Optics Express, Vol: 27, Pages: 16088-16102, ISSN: 1094-4087

We create a passive wave splitter, created purely by geometry, to engineer three-way beam splitting in electromagnetism in transverse electric and magnetic polarisation. We do so by considering arrangements of Indium Phosphide dielectric pillars in air, in particular we place several inclusions within a cell that is then extended periodically upon a square lattice. Hexagonal lattice structures are more commonly used in topological valleytronics but, as we discuss, three-way splitting is only possible using a square, or rectangular, lattice. To achieve splitting and transport around a sharp bend we use accidental, and not symmetry-induced, Dirac cones. Within each cell pillars are either arranged around a triangle or square; we demonstrate the mechanism of splitting and why it does not occur for one of the cases. The theory is developed and full scattering simulations demonstrate the effectiveness of the proposed designs.

Journal article

Palmer S, Xiao X, Pazos-Perez N, Guerrini L, Correa-Duarte M, Maier S, Craster R, Alvarez-Puebla R, Giannini Vet al., 2019,

Extraordinarily transparent compact metallic metamaterials

, Nature Communications, Vol: 10, ISSN: 2041-1723

The design of achromatic optical components requires materials with high transparency and low dispersion. We show that although metals are highly opaque, densely packed arrays of metallic nanoparticles can be more transparent to infrared radiation than dielectrics such as germanium, even when the arrays are over 75% metal by volume. Such arrays form effective dielectrics that are virtually dispersion-free over ultra-broadband ranges of wavelengths from microns up to millimeters or more. Furthermore, the local refractive indices may be tuned by altering the size, shape, and spacing of the nanoparticles, allowing the design of gradient-index lenses that guide and focus light on the microscale. The electric field is also strongly concentrated in the gaps between the metallic nanoparticles, and the simultaneous focusing and squeezing of the electric field produces strong ‘doubly-enhanced’ hotspots which could boost measurements made using infrared spectroscopy and other non-linear processes over a broad range of frequencies.

Conference paper

Mignuzzi S, Cambiasso J, Vezzoli S, Horsley SAR, Barnes WL, Maier SA, Sapienza Ret al., 2019,

Dielectric nanocavities with enhanced local density of states

We present inverse-designed lossless dielectric nanocavities with enhanced local density of optical states. Photon counting statistics from fluorescent molecules allows determining strong field confinement and single-molecule detection at micromolar concentration in liquid.

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Journal article

Dubois M, Perchoux J, Vanel AL, Tronche C, Achaoui Y, Dupont G, Bertling K, Rakic AD, Antonakakis T, Enoch S, Abdeddaim R, Craster RV, Guenneau Set al., 2019,

Acoustic flat lensing using an indefinite medium

, Physical Review B: Condensed Matter and Materials Physics, Vol: 99, ISSN: 1098-0121

Acoustic flat lensing is achieved here by tuning a phononic array to have indefinite medium behavior in a narrow frequency spectral region along the acoustic branch in the irreducible Brillouin zone (IBZ). This is confirmed by the occurrence of a flat band along an unusual path in the IBZ and by interpreting the intersection point of isofrequency contours on the corresponding isofrequency surface; coherent directive collimated beams are formed whose reflection from the array surfaces create lensing. Theoretical predictions using a mass-spring lattice approximation of the phononic crystal (PC) are corroborated by time-domain experiments, airborne acoustic waves generated by a source with a frequency centered about 10.6 kHz, placed at three different distances from one side of a finite PC slab, constructed from polymeric spheres, yielding distinctive focal spots on the other side. These experiments evaluate the pressure field using optical feedback interferometry and demonstrate precise control of the three-dimensional wave trajectory through a sonic crystal.

Journal article

Vanel AL, Craster RV, Schnitzer O, 2019,

Asymptotic modelling of phononic box crystals

, SIAM Journal on Applied Mathematics, Vol: 79, Pages: 506-524, ISSN: 0036-1399

We introduce phononic box crystals, namely arrays of adjoined perforated boxes, as a three-dimensional prototype for an unusual class of subwavelength metamaterials based on directly coupling resonating elements. In this case, when the holes coupling the boxes are small, we create networks of Helmholtz resonators with nearest-neighbour interactions. We use matched asymptotic expansions, in the small hole limit, to derive simple, yet asymptotically accurate, discrete wave equations governing the pressure field. These network equations readily furnish analytical dispersion relations for box arrays, slabs and crystals, that agree favourably with finite-element simulations of the physical problem. Our results reveal that the entire acoustic branch is uniformly squeezed into a subwavelength regime; consequently, phononic box crystals exhibit nonlinear-dispersion effects (such as dynamic anisotropy) in a relatively wide band, as well as a high effective refractive index in the long-wavelength limit. We also study the sound field produced by sources placed within one of the boxes by comparing and contrasting monopole- with dipole-type forcing; for the former the pressure field is asymptotically enhanced whilst for the latter there is no asymptotic enhancement and the translation from the microscale to the discrete description entails evaluating singular limits, using a regularized and efficient scheme, of the Neumann's Green's function for a cube. We conclude with an example of using our asymptotic framework to calculate localized modes trapped within a defected box array.

Journal article

Chaplain GJ, Makwana MP, Craster R, 2019,

Rayleigh-Bloch, topological edge and interface waves for structured elastic plates

, Wave Motion, Vol: 86, Pages: 162-174, ISSN: 0165-2125

Galvanised by the emergent fields of metamaterials and topological wave physics, there is currently much interest in controlling wave propagation along structured arrays, and interfacial waves between geometrically different crystal arrangements. We model array and interface waves for structured thin elastic plates, so-called platonic crystals, that share many analogies with their electromagnetic and acoustic counterparts, photonic and phononic crystals, and much of what we present carries across to those systems. These crystals support several forms of edge or array-guided modes, that decay perpendicular to their direction of propagation. To rapidly, and accurately, characterise these modes and their decay we develop a spectral Galerkin method, using a Fourier–Hermite basis, to provide highly accurate dispersion diagrams and mode-shapes, that are confirmed with full scattering simulations. We illustrate this approach using Rayleigh Bloch modes, and generalise high frequency homogenisation, along a line array, to extract the envelope wavelength along the array. Rayleigh–Bloch modes along graded arrays of rings of point masses are investigated and novel forms of the rainbow trapping effect and wave hybridisation are demonstrated. Finally, the method is used to investigate the dispersion curves and mode-shapes of interfacial waves created by geometrical differences in adjoining media.

Journal article

Movchan AB, McPhedran RC, Carta G, Craster RVet al., 2019,

Platonic localisation: one ring to bind them

, Archive of Applied Mechanics, Vol: 89, Pages: 521-533, ISSN: 0939-1533

In this paper, we present an asymptotic model describing localised flexural vibrations along a structured ring containing point masses or spring–mass resonators in an elastic plate. The values for the required masses and stiffnesses of resonators are derived in a closed analytical form. It is shown that spring–mass resonators can be tuned to produce a “negative inertia” input, which is used to enhance localisation of waveforms around the structured ring. Theoretical findings are accompanied by numerical simulations, which show exponentially localised and leaky modes for different frequency regimes.

Journal article

Mignuzzi S, Vezzoli S, Horsley SAR, Barnes WL, Maier SA, Sapienza Ret al., 2019,

Nanoscale design of the local density of optical states

, Nano Letters, Vol: 19, Pages: 1613-1617, ISSN: 1530-6984

We propose a design concept for tailoring the local density of optical states (LDOS) in dielectric nanostructures, based on the phase distribution of the scattered optical fields induced by point-like emitters. First we demonstrate that the LDOS can be expressed in terms of a coherent summation of constructive and destructive contributions. By using an iterative approach, dielectric nanostructures can be designed to effectively remove the destructive terms. In this way, dielectric Mie resonators, featuring low LDOS for electric dipoles, can be reshaped to enable enhancements of 3 orders of magnitude. To demonstrate the generality of the method, we also design nanocavities that enhance the radiated power of a circular dipole, a quadrupole, and an arbitrary collection of coherent dipoles. Our concept provides a powerful tool for high-performance dielectric resonators and affords fundamental insights into lightmatter coupling at the nanoscale.

Journal article

Jacucci G, Onelli OD, De Luca A, Bertolotti J, Sapienza R, Vignolini Set al., 2019,

Coherent backscattering of light by an anisotropic biological network.

, Interface Focus, Vol: 9, Pages: 20180050-20180050, ISSN: 2042-8898

The scattering strength of a random medium relies on the geometry and spatial distribution of its components as well as on their refractive index. Anisotropy can, therefore, play a major role in the optimization of the scattering efficiency in both biological and synthetic materials. In this study, we show that, by exploiting the coherent backscattering phenomenon, it is possible to characterize the optical anisotropy in Cyphochilus beetle scales without the need to change their orientation or their thickness. For this reason, such a static and easily accessible experimental approach is particularly suitable for the study of biological specimens. Moreover, estimation of the anisotropy in Cyphochilus beetle scales might provide inspiration for improving the scattering strength of artificial white materials.

Journal article

Gaio M, Saxena D, Bertolotti J, Pisignano D, Camposeo A, Sapienza Ret al., 2019,

A nanophotonic laser on a graph

, Nature Communications, ISSN: 2041-1723

Nanophotonic architectures for classical and quantum optical technology canboost light-matter interaction via sculpturing the optical modes, formingcavities and designing long-range propagation channels. Conventional photonicschemes minimise multiple scattering to realise a miniaturised version ofmacroscopic beam-splitters, interferometers and optical cavities for lightpropagation and lasing. Here instead, we introduce a nanophotonic network builtfrom multiple paths and interference, to control and enhance light-matterinteraction via light localisation beyond single scattering. The network isbuilt from a mesh of subwavelength waveguides, and can sustain localised modesand mirror-less light trapping stemming from interference over hundreds ofnodes. When optical gain is added, these modes can easily lase, reaching$\sim$100 pm linewidths. We introduce a graph solution to the Maxwell'sequation which describes light on the network, and predicts lasing action. Inthis framework, the network optical modes can be designed via the networkconnectivity and topology, and lasing can be tailored and enhanced by thenetwork shape. Nanophotonic networks pave the way for new laser devicearchitectures, which can be used for sensitive biosensing and on-chip opticalinformation processing.

Journal article

Berte R, Della Picca F, Poblet M, Li Y, Cortes E, Craster RV, Maier SA, Bragas AVet al., 2018,

Acoustic far-field hypersonic surface wave detection with single plasmonic nanoantennas

, Physical Review Letters, Vol: 121, ISSN: 0031-9007

The optical properties of small metallic particles allow us to bridge the gap between the myriad of subdiffraction local phenomena and macroscopic optical elements. The optomechanical coupling between mechanical vibrations of Au nanoparticles and their optical response due to collective electronic oscillations leads to the emission and the detection of surface acoustic waves (SAWs) by single metallic nanoantennas. We take two Au nanoparticles, one acting as a source and the other as a receptor of SAWs and, even though these antennas are separated by distances orders of magnitude larger than the characteristic subnanometric displacements of vibrations, we probe the frequency content, wave speed, and amplitude decay of SAWs originating from the damping of coherent mechanical modes of the source. Two-color pump-probe experiments and numerical methods reveal the characteristic Rayleigh wave behavior of emitted SAWs, and show that the SAW-induced optical modulation of the receptor antenna allows us to accurately probe the frequency of the source, even when the eigenmodes of source and receptor are detuned.

Journal article

Makwana MP, Craster R, 2018,

Designing multidirectional energy splitters and topological valley supernetworks

, PHYSICAL REVIEW B, Vol: 98, ISSN: 2469-9950

Journal article

Makwana MP, Craster R, 2018,

Geometrically navigating topological plate modes around gentle and sharp bends

, PHYSICAL REVIEW B, Vol: 98, ISSN: 2469-9950

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Tailored elastic surface to body wave Umklapp conversion

A nanophotonic laser on a graph