arXiv subjects

Dani Torrent

Publications and source records attributed to Dani Torrent.

12 recordsLinked to original sources

arXiv2026

Near-Field Mechanical Fingerprints for THz Sensing of 'Hidden' Nanoparticles in Complex Media

Terahertz (THz) spectroscopy holds transformative potential for non-invasive sensing, yet characterizing individual nanoparticles in complex biological environments remains challenging due to the far-field diffraction limit. While near-field dipolar theory is well established, its application to characterizing/identifying nanoparticles immersed in complex media at THz frequencies is largely unexplored. This work utilizes numerical simulations of magneto-optical (MO) heterodimers -- comprising n-doped Indium Antimonide (n-InSb) and isotropic or birefringent particles (e.g., SiO2, GaSe) -- under counter-propagating, circularly polarized THz illumination. We demonstrate that while far-field observables like absorption cross-sections are often dominated by the MO-active particle, mechanical variables-specifically induced binding forces and spin/orbital torques-exhibit superior sensitivity for detecting "hidden" neighboring components. Because these mechanical signatures depend directly on near-field interactions, they provide higher information density regarding interparticle coupling. Key findings reveal material-specific spectral "hotspots" and "zeros" that serve as robust calibration markers even within dispersive biological surrogates. We show that the spin torque on non-MO particles is significantly modified by MO-neighbor proximity, a phenomenon controllable via static magnetic fields. Furthermore, these variables exhibit high angular sensitivity in perpendicular configurations. Our results provide a roadmap for using optomechanical signatures as high-resolution detectors for in-vivo diagnostics, signal transduction, and low-energy nanocircuit control.

physics.app-ph↗

arXiv2024

Modeling Realistic Dynamics of Nanoparticle Dimers for Magneto-Optical Matter

Traditional approaches to optical matter often involve complex illumination fields with costly and unstable setups, requiring strong gradient forces, high-intensity laser spots that could harm samples, and substrate support. For binding, attractive inter-particle forces may not be sufficient to assemble systems due to unbalanced components such as centrifugal forces or collisions. In previous work, magneto-optical nanoparticles illuminated with two counter-propagating circularly polarized waves were optically bound under quasi-static conditions. However, the dynamics of such nano systems were not thoroughly considered. Here, a general framework to study magneto-optical (MO) matter is introduced, controllable by static magnetic fields. Dynamic binding between two n-doped InSb nanoparticles, which exhibit surface plasmons at THz frequencies, is recreated. Additionally, the reported examples may represent a novel approach of low-energy illumination sources without gradient components. This numerical framework, grounded in Langevin dynamics and realistic collision phenomena, serves as a robust and universal tool for exploring various optomechanical designs. The impact of thermal noise, collision types, initial conditions, and resonance excitation on a dimer system is examined. Dimers tuned initially at magneto-optical resonance can bind in stable, average positions even when particle-particle collisions are present. The current methodology provides essential knowledge for studying optical binding and the dynamics of any small-scale cluster of interacting sub-units.

physics.optics↗

arXiv2023

Observation of Two-Dimensional Acoustic Bound States in the Continuum

The design of devices based on acoustic or optical fields requires the fabrication of cavities and structures capable of efficiently trapping these waves. A special type of cavity can be designed to support resonances with a theoretically infinite quality factor, named bound states in the continuum or BICs. The experimental measurement of such modes is still a challenging problem, as they are, by definition, not accessible from external perturbations. Therefore, current reported works rely on indirect measurements that are based on the traces left by these modes on external properties of one-dimensional systems. Here we report on the theoretical design and experimental realization of a two-dimensional, fully open acoustic resonator supporting BICs. This BIC, whose symmetry is chosen during design by properly tailoring the geometrical properties of the system, is completely accessible and allows for the direct measurement of the whole pressure field and properties. We experimentally demonstrate its existence with high quality factor and field enhancement properties.

physics.class-ph↗

arXiv2022

Bound states in the continuum in circular clusters of scatterers

In this work, we study the localization of flexural waves in highly symmetric clusters of scatterers. It is shown that when the scatterers are placed regularly in the perimeter of a circumference the quality factor of the resonances strongly increases with the number of scatterers in the cluster. It is also found that in the continuous limit, that is to say, when the number of scatterers tends to infinite, the quality factor is infinite so that the modes belong to the class of the so called bound states in the continuum or BICs, and an analytical expression for the resonant frequency is found. These modes have different multipolar symmetries, and we show that for high multipolar orders the modes tend to localize at the border of the circumference, forming therefore a whishpering gallery mode with an extraordinarily high quality factor. Numerical experiments are performed to check the robustness of these modes under different types of disorder and also to study their excitation from the far field. Although we have focused our study to flexural waves, the methodology presented in this work can be applied to other classical waves, like electromagnetic or acoustic waves, being therefore a promissing approach for the design of high quality resonators based on finite clusters of scatterers.

physics.class-ph↗

arXiv2022

High-Quality Resonances in Quasi-Periodic Clusters of Scatterers for Flexural Waves

Multiple scattering theory is applied to the study of clusters of point-like scatterers attached to a thin elastic plate and arranged in quasi-periodic distributions. Two type of structures are specifically considered: the twisted bilayer and the quasi-periodic line. The former consists in a couple of two-dimensional lattices rotated a relative angle, so that the cluster forms a moir\'e pattern. The latter can be seen as a periodic one-dimensional lattice where an incommensurate modulation is superimposed. Multiple scattering theory allows for the fast an efficient calculation of the resonant modes of these structures as well as for their quality factor, which is thoroughly analyzed in this work. The results show that quasi-periodic structures present a large density of states with high quality factors, being therefore a promising way for the design of high quality wave-localization devices.

physics.class-ph↗

arXiv2022

Far-field Perfect Imaging with Time Modulated Gratings

We study the capabilities of time-modulated diffraction gratings as imaging devices. It is shown that a time-dependent but transversally homogeneous slab can be used to make a perfect image of an object in the far-field, since all the evanescent modes couple to propagative time-diffracted orders. It is found that, if the image to be obtained is axially symmetric, it can be recovered by measuring the time-signal at a single point, without the need of performing a spatial scan, so that time gratings can act as well as single-pixel imaging devices. In the case of having an object without axial symmetry, the time-grating can be combined with a spatial grating, and then the full image can be recovered again with a measurement at a single point. We apply the theory of compressive sensing to optimize the recovery method and numerical examples are provided. We show therefore that time-modulated gratings can be used to perfectly recover the image of an object in the far field and after measuring at a single point in the space, being therefore a promising approach to superesoloution and ultra-fast imaging.

physics.optics↗

arXiv2021

Zak's Phase in Non-Symmetric One-Dimensional Crystals

In this work, we derive some analytical properties of Berry's phase in one-dimensional quantum and classical crystals, also named Zak's phase, when computed with a Fourier basis. We show that Zak's phase can be divided in two terms: a global phase required to make the Bloch wave periodic in the Brillouin zone and an internal phase which measures the relative delay of the different Fourier terms within the Brillouin zone. While the former phase is dependent on the origin of coordinates of the unit cell, the latter is independent of it, so that it can be interpreted as an internal property of the band itself. We show that this internal phase is always zero for a symmetric crystal while it can take any value when this symmetry is broken, showing therefore that it can be interpreted as a measure of the assymetry of the band. Since for a symmetric crystal Zak's phase is entirely determined by the global part, we show that this can be easily calculated by means of the parity of the Fourier terms at the center and edge of the Brillouin zone, being therefore unnecessary the integration of the modes through the unit cell and the entire Brillouin zone. We provide numerical examples analyzing the internal part for both electronic and classical waves (acoustic or photonic). We analyze the weakest electronic potential capable of presenting asymmetry, as well as the double-Dirac delta potential, and in both examples it is found that the internal phase varies continuously as a function of a symmetry-control parameter, but it is zero when the crystal is symmetric. For classical waves, the layered material is analyzed. Although Zak's phase has been mainly studied in connection with the existence of edge states in finite crystals, we consider that the study of the internal phase can be more relevant to understand bulk properties of quantum and classical crystals.

cond-mat.mtrl-sci↗

arXiv2021

Edge modes for flexural waves in quasi-periodic linear arrays of scatterers

We present a multiple scattering analysis of robust interface states for flexural waves in thin elastic plates. We show that finite clusters of linear arrays of scatterers built on a quasi-periodic arrangement support bounded modes in the two-dimensional space of the plate. The spectrum of these modes plotted against the modulation defining the quasi-periodicity has the shape of a Hofstadter butterfly, which previous works suggest that might support topologically protected modes. Some interface states appear inside the gaps of the butterfly, which are enhanced when one linear cluster is merged with its mirror reflected version. The robustness of these modes is verified by numerical experiments in which different degrees of disorder are introduced in the scatterers, showing that neither the frequency nor the shape of the modes is altered. Since the modes are at the interface between two one-dimensional arrays of scatterers deposited on a two-dimensional space, these modes are not fully surrounded by bulk gaped materials, so that they are more suitable for their excitation by propagating waves. The generality of these results goes beyond flexural waves, since similar results are expected for acoustic or electromagnetic waves.

physics.app-ph↗

arXiv2020

Metaclusters for the Full Control of Mechanical Waves

We present a new method for the control of waves based on inverse multiple scattering theory. Conceived as a generalization of the concept of metagrating, we call metaclusters to a finite set of scatterers whose position and properties are obtained by inverse design once we have defined their response to some external incident field. The particular focus is on designing passive metaclusters that do not require an external source of energy. The method is applied to the propagation of flexural waves in thin plates, and to the design of far field patterns, although its generalization to acoustic or electromagnetic waves is straightforward. Numerical examples are presented to the design of uni and bidirectional "anomalous scatterers", which will bend the scattering energy along a specific direction, "odd pole" scatterers, whose radiation pattern presents an odd number of poles and to the generation of vortical patterns. Finally, some considerations about the optimal design of these metaclusters are discussed.

physics.app-ph↗

arXiv2020

Strong Spatial Dispersion in Time-Modulated Dielectric Media

We present an effective medium description of time-modulated dielectric media. By taking the averaged fields over one modulation period, the relationship between them is derived, defining therefore the different constitutive parameters. In the most general situation, it is found that the effective material is described by means of a spatially and temporally dispersive transverse dielectric function and a constant longitudinal dielectric function. It has been also found that the frequency dependence in the former is weak, in comparison with its wavenumber-dependence (spatial dispersion). Different physical consequences of this spatial dispersion are discussed, with special emphasis in the weak dispersion approximation, limit in which it is found that the effective material behaves as a resonant and isotropic magnetodielectric medium with no additional longitudinal mode, as it is commonly found in spatially dispersive materials. Time-dependent media opens therefore an alternative way of designing dynamically tunable metamaterials.

physics.optics↗

arXiv2020

Dipolar localization of waves in twisted phononic crystal plates

The localization of waves in two-dimensional clusters of scatterers arranged in relatively twisted lattices is studied by multiple scattering theory. It is found that, for a given frequency, it is always possible to find localized modes for a discrete set of rotation angles, analogous to the so-called "magic angles" recently found in two-dimensional materials like graphene. Similarly, for small rotations of the lattices, a large number of resonant frequencies is found, whose position strongly depends on the rotation angle. Moreover, for angles close to those that make the two lattices commensurable a single mode appears that can be easily tuned by the rotation angle. Unlike other twisted materials, where the properties of the bilayers are mainly explained in terms of the dispersion relation of the individual lattices, the special angles in these clusters happen because of the formation of dipolar scatterers due to the relative rotation between the two lattices, enhancing therefore their interaction. While the presented results are valid for any type of wave, the specific case of flexural waves in thin elastic plates is numerically studied, and the different modes found are comprehensively explained in terms of the interaction between pairs of scatterers. The analysis presented here shows that these structures are promising candidates for the inverse design of tunable wave-trapping devices for classical waves.

cond-mat.mtrl-sci↗

arXiv2020

Generalized Elastodynamic Model for Nanophotonics

A self-consistent theory for the classical description of the interaction of light and matter at the nano-scale is presented, which takes into account spatial dispersion. Up to now, the Maxwell equations in nanostructured materials with spatial dispersion have been solved by the introduction of the so-called Additional Boundary Conditions. In this paper, we derive an approach where non-local effects are studied in a precise and uniquely defined way, thus allowing the treatment of all solid-solid interfaces (metals, semiconductors or insulators), as well as solid-vacuum interfaces in the same framework. The theory is based on the derivation of a potential energy for an ensemble of electrons in a given potential, where the deformation of the ensemble is treated as in a solid, including both shear and compressional deformations, instead of a fluid described only by a bulk compressibility like in the hydrodynamical approach. The derived classical equation of motion for the ensemble describes the deformation vector and the corresponding polarization vector as an elastodynamic field, including viscous forces, from which a generalized non-local constitutive equation for the dielectric constant is derived. Boundary conditions are identical to that of elastodynamics and they emerge in a natural way, without any physical hypothesis outside the current description, as it is commonly required in other non-local approaches. This description does not require the discontinuity of any component of the electric, magnetic or polarization fields and, consequently, no bounded currents or charges are present at the interface, which is a more suitable description from the microscopic point of view. It is shown that the method converges to the local boundary conditions in the low spatial dispersion limit for insulators and conductors, quantified by means of a parameter defined as the "characteristic length".

physics.optics↗