arXiv subjects

Daniel Torrent

Publications and source records attributed to Daniel Torrent.

At least 19 recordsLinked to original sources

arXiv2026

Optical free space extreme learning machine for the implementation of emergent complex systems

Cellular automata conform a set of computational models which evolve with a reduced set of simple rules, yet still are able to show extremely complex emergent phenomena such as fractals and universal computation. Despite their apparent simplicity, they have shown great potential in simulating natural systems and solving challenging computational tasks such as classification and image generation. Instead of implementing cellular automata purely at the software level, it is desirable to design novel analog computing platforms that physically evolve following the automata's underlying rules, thereby reducing power requirements and latency. Here, we introduce an optical extreme learning machine for the simulation of a wide range of cellular automata. Our system operates in free space, and uses a spatial light modulator to encode the evolution rules of the system, while coherent wave propagation performs the corresponding computations. Our results demonstrate a simple, fully-programmable, cost and power efficient, and easy to build and align platform for the implementation of a wide range of complex computational systems such as elementary cellular automata, Conway's Game of Life, and two-dimensional Turing machines.

nlin.CG↗

arXiv2019

Majorana-like zero modes in Kekul\'e distorted sonic lattices

Topological phases have recently been realised in bosonic systems. The associated boundary modes between regions of distinct topology have been used to demonstrate robust waveguiding, protected from defects by the topology of the surrounding bulk. A related type of topologically protected state that is not propagating but is bound to a defect has not been demonstrated to date in a bosonic setting. Here we demonstrate numerically and experimentally that an acoustic mode can be topologically bound to a vortex fabricated in a two-dimensional, Kekul\'e-distorted triangular acoustic lattice. Such lattice realises an acoustic analogue of the Jackiw-Rossi mechanism that topologically binds a bound state in a p-wave superconductor vortex. The acoustic bound state is thus a bosonic \edit{\emph{analogue}} of Majorana bound state, \edit{where the two valleys replace particle and hole components}. We numerically show that it is topologically protected against arbitrary symmetry-preserving local perturbations, and remains pinned to the Dirac frequency of the unperturbed lattice regardless of parameter variations. We demonstrate our prediction experimentally by 3D printing the vortex pattern in a plastic matrix and measuring the spectrum of the acoustic response of the device. Despite viscothermal losses, the measured topological resonance remains robust, with its frequency closely matching our simulations.

cond-mat.mes-hall↗

arXiv2019

Direct retrieval method of the effective permittivity and permeability of bulk semi-infinite metamaterials by variable-angle spectroscopic ellipsometry

In this work, we present a simple method for the direct retrieval of the effective permittivity and permeability of a bulk semi-infinite metamaterial from variable-angle spectroscopic ellipsometry measurements. Starting from the well-known Fresnel equations, we derive an analytical expression in which unknown coefficients are fitted to the experimental data using a linear regression model. The effective permittivity and permeability are then determined by solving a simple system and the correct solution is selected based on physical criteria. As an example, the method is applied to the case of a self-assembled metamaterial exhibiting strong isotropic optical magnetism.

physics.optics↗

arXiv2019

Mechanical analogue of a Majorana bound state

The discovery of topologically non-trivial electronic systems has opened a new age in condensed matter research. From topological insulators to topological superconductors and Weyl semimetals, it is now understood that some of the most remarkable and robust phases in electronic systems (e.g., Quantum Hall or Anomalous Quantum Hall) are the result of topological protection. These powerful ideas have recently begun to be explored also in bosonic systems. Topologically protected acoustic, mechanical, and optical edge states have been demonstrated in a number of systems that recreate the requisite topological conditions. Such states that propagate without backscattering could find important applications in communications and energy technologies. In this work we demonstrate the mechanical analogue of a topologically bound state, a different class of non-propagating protected state that cannot be destroyed by local perturbations. These are well known in electronic systems, such as Majorana bound states in topological superconductors, but remain largely unexplored in a bosonic setting. We implement topological binding by creating a Kekul\'e distortion vortex on a two-dimensional mechanical honeycomb superlattice.

cond-mat.mes-hall↗

arXiv2018

Gradient index phononic crystals and metamaterials

Phononic crystals and acoustic metamaterials are periodic structures whose effective properties can be tailored at will to achieve extreme control on wave propagation. Their refractive index is obtained from the homogenization of the infinite periodic system, but it is possible to locally change the properties of a finite crystal in such a way that it results in an effective gradient of the refractive index (GRIN). In such case the propagation of waves can be accurately described by means of ray theory, and different refractive devices can be designed in the framework of wave propagation in inhomogeneous media. In this paper we review the different devices that have been studied for the control of both bulk or guided acoustic waves based on graded phononic crystals.

physics.app-ph↗

arXiv2018

Valley Hall phases in Kagome lattices

We report the finding of the analogous valley Hall effect in phononic systems arising from mirror symmetry breaking, in addition to spatial inversion symmetry breaking. We study topological phases of plates and spring-mass models in Kagome and modified Kagome arrangements. By breaking the inversion symmetry it is well known that a defined valley Chern number arises. We also show that effectively, breaking the mirror symmetry leads to the same topological invariant. Based on the bulk-edge correspondence principle, protected edge states appear at interfaces between two lattices of different valley Chern numbers. By means a plane wave expansion method and the multiple scattering theory for periodic and finite systems respectively, we computed the Berry curvature, the band inversion, mode shapes and edge modes in plate systems. We also find that appropriate multi-point excitations in finite system gives rise to propagating waves along a one-way path only.

cond-mat.mes-hall↗

arXiv2018

Ultrathin acoustic parity-time symmetric metasurface cloak

Invisibility or unhearability cloaks have been made possible by using metamaterials making light or sound flow around obstacle without the trace of reflections or shadows. Metamaterials are known for being flexible building units that can mimic a host of unusual and extreme material responses, which are essential when engineering artificial material properties to realize a coordinate transforming cloak. Bending and stretching the coordinate grid in space requires stringent material parameters, therefore, small inaccuracies and inevitable material losses become sources for unwanted scattering that are decremental to the desired effect. These obstacles further limit the possibility to achieve a robust concealment of sizeable objects from either radar or sonar detection. By using a elaborate arrangement of gain and lossy acoustic media respecting parity-time symmetry, we built an one-way unhearability cloak capable to hide objects seven times larger than acoustic wavelength. Generally speaking, our approach has no limits in terms of working frequency, shape, or size, specifically though, we demonstrate how, in principle, an object of the size of a human can be hidden from audible sound.

physics.app-ph↗

arXiv2018

The Inverse Grating Problem: Efficient Design of Anomalous Flexural Wave Reflectors and Refractors

We present an extensive formulation of the inverse grating problem for exural waves, in which the energy of each diffracted mode is selected and the grating configuration is then obtained by solving a linear system of equations. The grating is designed as a lineal periodic repetition of a unit cell comprising a cluster of resonators attached at points whose physical properties are directly derived by inversion of a given matrix. Although both active and passive attachments can be required in the most general case, it is possible to find configurations with only passive, i.e. damped, solutions. This inverse design approach presents an alternative to the design of metasurfaces for exural waves overcoming the limitations of gradient phase metasurfaces, which require a continuous variation of the surface's impedance. When the grating is designed in such a way that all the energy is channeled to a single diffracted mode, it behaves as an anomalous refractor or re ector. The negative refractor is analyzed in depth, and it is shown that with only three scatterers per unit cell is it possible to build such a device with unitary efficiency.

physics.app-ph↗

arXiv2018

Engineered diffraction gratings for acoustic cloaking

We show that engineered diffraction gratings can considerably simplify the design and performance of acoustic devices. Acoustic reflecting gratings based on rectangular cavities drilled on an acoustically rigid surface are designed in such a way that all the incident energy is channeled towards the diffracted mode traveling in the oposite direction of the incident field (retroreflection effect), and this effect is used to cloak an object placed over a rigid surface. Axisymmetric gratings consisting in rigid surfaces with just one groove per unit cell are used to design acoustic thin acoustic carpet cloaks. Finally, full wave numerical simulations are performed and a conical carpet cloak is experimentally tested, showing an excellent scattering cancellation effect.

physics.app-ph↗

arXiv2018

Acoustic Anomalous Reflectors Based on Diffraction Grating Engineering

We present an efficient method for the design of anomalous reflectors for acoustic waves. The approach is based on the fact that the anomalous reflector is actually a diffraction grating in which the amplitude of all the modes is negligible except the one traveling towards the desired direction. A supercell of drilled holes in an acoustically rigid surface is proposed as the basic unit cell, and analytical expressions for an inverse diffraction problem are derived. It is found that the the number of holes required for the realization of an anomalous reflector is equal to the number of diffracted modes to cancel, and this number depends on the relationship between the incident and reflected angles. Then, the "retrorreflection" effect is obtained by just one hole per unit cell, also with only two holes it is possible to change the reflection angle of a normally incident wave and five holes are enough to design a general retroreflector changing the incident and reflected angles at oblique incidence. Finally, the concept of Snell's law violation is extended not only to the incident and reflected angles, but also to the plane in which it happens, and a device based on a single hole in a square lattice is designed in such a way that the reflection plane is rotated $\pi/4$ with respect to the plane of incidence. Numerical simulations are performed to support the predictions of the analytical expressions, and an excellent agreement is found.

physics.app-ph↗

arXiv2018

Analysis of the Robustness of Conventional and Topologically Protected Edge States in Phononic Crystal Plates

In this work we theoretically study the interface acoustic states of resonators on a thin plate with topologically protected and conventional designs. Topologically protected interface state is first analyzed by employing the conception of breaking inversion symmetry within the unit cell of a honeycomb lattice for cylindrical and spherical resonators; we further demonstrate the robustness of the wave propagation along a zig-zag path containing sharp corners, defect and disorder. The wave propagation ceases to be preserved if we increase the degree of disorder along the zig-zag path. In parallel, the conventional interface state is also designed and compared to the same situations. We found that the conventional interface state suffers back scattering in the zig-zag path while it can show a more confined wave transport in some cases. The presence of a defect along the propagation path scatters the conventional interface wave and in particular can prohibit a full propagation in presence of a localized state at the defect. If the zig-zag path is made disordered, the propagation of the conventional interface mode can be conserved at given frequencies for a low random degree and disappears for higher random degree as the interface bands become flat in dispersion and turn to localized states. Finally, we show that the immunity of the topologically protected design needs the interface to be surrounded by at least two hexagons of the phononic crystals on both sides, especially at the sharp corners in the zig-zag path, while the conventional design only needs one hexagon bulk media with the advantage of compact wave transport. This work puts a step forward for the interface states in micro-/nano-scale characterization and figures out the behaviors for both topologically protected and conventional interface states.

cond-mat.mtrl-sci↗

arXiv2017

Loss Compensation in Time-Dependent Elastic Metamaterials

Materials with properties that are modulated in time are known to display wave phenomena showing energy increasing with time, with the rate mediated by the modulation. Until now there has been no accounting for material dissipation, which clearly counteracts energy growth. This paper provides an exact expression for the amplitude of elastic or acoustic waves propagating in lossy materials with properties that are periodically modulated in time. It is found that these materials can support a special propagation regime in which waves travel at constant amplitude, with temporal modulation compensating for the normal energy dissipation. We derive a general condition under which amplification due to time-dependent properties offsets the material dissipation. This identity relates band-gap properties associated with the temporal modulation and the average of the viscosity coefficient, thereby providing a simple recipe for the design of loss-compensated mechanical metamaterials.

physics.app-ph↗

arXiv2017

Non-Reciprocal Thermal Material by Spatio-Temporal Modulation

The thermal properties of a material with a spatio-temporal modulation in both the thermal conductivity and the mass density are studied. The special configuration studied here consists of a modulation in a wave-like fashion. It is found that these materials behaves, in an effective way, as materials with an internal convection-like term that provides them of non-reciprocal properties, in the sense that the flow of heat has different properties when it propagates in the same direction or in the opposite one to the modulation of the parameters. An effective medium description is presented which accurately describes the modulated material, and numerical simulations supports both the non-reciprocal properties and the effective medium description. It is found that these materials are promising candidates for the design of thermal diodes and other advanced devices for the control of the heat flow at all scales.

cond-mat.mtrl-sci↗

arXiv2016

Hierarchical self-assembly of nanoparticles for optical metamaterials

Hierarchical self-assembly arranges nanostructures at different length scales. It gradually becomes an effective method of fabricating artificial metamaterials from composite nanostructures tailored for a particular response. Hierarchical self-assembly overcomes shortcomings of "top-down" methods by significantly reducing fabrication time and making it possible to form bulk materials. Here we report an application of hierarchical self-assembly of metal nanoparticles for the creation of the first isotropic optical metamaterial with strong artificial magnetism in blue light. We have used colloidal self-assembly to create artificial "nanomolecules" that generate the desired magnetic response and microfluidic self-assembly to produce a bulk metastructure. We demonstrate that the magnetic response of the final material is accurately described by an isotropic magnetic permeability that satisfies the principle of locality. Our approach unlocks the fabrication of large volumes of composite nanomaterials. Moreover, the spatial disorder inherent to this "bottom-up" method holds the key to solving the non-locality problem. The technique can be readily extended to the future generations of low-loss optical metamaterials made of dielectric nano-blocks to bypass the limitations of optical losses associated with plasmonic resonances in noble metals.

cond-mat.mtrl-sci↗

arXiv2015

Resonant and Non-Local Properties of Phononic Metasolids

We derive a general theory of effective properties in metasolids based on phononic crystals with low frequency resonances. We demonstrate that in general these structures need to be described by means of a frequency-dependent and non-local anisotropic mass density, stiffness tensor and a third- rank coupling tensor, which shows that they behave like a non-local Willis medium. The effect of non-locality and coupling tensor manifest themselves for some particular resonances whereas they become negligible for other resonances. Considering the example of a two-dimensional phononic crystal, consisting of triangular arrangements of cylindrical shells in an elastic matrix, we show that its mass density tensor is strongly resonant and anisotropic presenting both positive and negative divergent values, while becoming scalar in the quasi-static limit. Moreover, it is found that the negative value of transverse component of the mass density is induced by a dipolar resonance, while that of the vertical component is induced by a monopolar one. Finally, the dispersion relation obtained by the effective parameters of the crystal is compared with the band structure, showing a good agreement for the low-wave number region, although the non-local effects are important given the existence of some resonant values of the wave number.

cond-mat.mtrl-sci↗

arXiv2015

Transparent gradient index lens for underwater sound based on phase advance

Spatial gradients in refractive index have been used extensively in acoustic metamaterial applications to control wave propagation through phase delay. This study reports the design and experimental realization of an acoustic gradient index lens using a sonic crystal lattice that is impedance matched to water over a broad bandwidth. In contrast to previous designs, the underlying lattice features refractive indices that are lower than the water background, which facilitates propagation control based on a phase advance as opposed to a delay. The index gradient is achieved by varying the filling fraction of hollow, air-filled aluminum tubes that individually exhibit a higher sound speed than water and matched impedance. Acoustic focusing is observed over a broad bandwidth of frequencies in the homogenization limit of the lattice, with intensity magnifications in excess of 7 dB. An anisotropic lattice design facilitates a flat-faceted geometry with low backscattering at 18 dB below the incident sound pressure level. Three dimensional Rayleigh-Sommerfeld integration that accounts for the anisotropic refraction is used to accurately predict the experimentally measured focal patterns.

cond-mat.mtrl-sci↗

arXiv2015

Gradient Index Devices for the Simultaneous Focusing of the S0 and A0 Lamb Modes

Gradient index devices for the simultaneous control of the S 0 and A 0 Lamb modes are presented and numerically analysed. It is found a special relationship between the refractive indexes for these two modes which allows their simultaneous control by means of graded phononic crystals and thickness variations of the plate. A flat gradient index lens and a circular Luneburg lens are designed. Numerical simulations show that the performance of these devices is good for the two modes in a broadband frequency region, and that this approach can be used to design more advanced refractive devices for the total control of guided vibrational modes.

physics.class-ph↗

arXiv2014

Omnidirectional refractive devices for flexural waves based on graded phononic crystals

Different omnidirectional refractive devices for flexural waves in thin plates are proposed and numerically analyzed. Their realization is explained by means phononic crystal plates, where a previously developed homogenization theory is employed for the design of graded index refractive devices. These devices consist of a circular cluster of inclusions with properly designed gradient in their radius. With this approach, the Luneburg and Maxwell lenses and a family of beam splitters for flexural waves are proposed and analyzed. Results show that these devices work properly in a broadband frequency region, being therefore an efficient approach for the design of refractive devices specially interesting for nano-scale applications.

cond-mat.mtrl-sci↗