arXiv subjects

Jorge Olmos-Trigo

Publications and source records attributed to Jorge Olmos-Trigo.

At least 19 recordsLinked to original sources

arXiv2026

A Universal Magnetoelectric Limit for Chiral and Tellegen Bi-Isotropic Scatterers

We reveal the existence of a universal upper bound on the magnetoelectric coupling of any bi-isotropic nanoparticle. The bound arises solely from energy conservation, making it independent of the specific material properties of the nanoparticle and illumination conditions. Moreover, the bound does not rely on reciprocity, being identical for reciprocal (chiral) and non-reciprocal (Tellegen) nanoparticles. We further show that the chiral Mie coefficient of spherical particles of arbitrary optical size obeys the same bound across all multipolar scattering channels. Our results introduce a universal metric on the magnetoelectric coupling of bi-isotropic objects, setting identical limits on chiral and Tellegen light-matter interactions at the single particle level.

physics.optics↗

arXiv2026

Detection of Hybrid Optical Anapoles in Dielectric Microspheres

Nonradiating optical anapoles are special configurations of charge_current distributions that do not radiate. It was theoretically predicted that, for microspheres, electric and magnetic dipolar coefficients can simultaneously vanish by engineering the incident light, leading to the excitation of nonradiating hybrid optical anapoles. In this work, the experimental detection of hybrid optical anapoles in dielectric microspheres (TiO2) is reported using dual detection optical spectroscopy, developed to enable sequential measurement of forward and backward scattering under tightly_focused Gaussian beam (TFGB) illumination. The results show that the excitation of TiO2 microspheres (diameter, d approx. 1 um) under TFGB illumination leads to the appearance of scattering minima in both the forward and backward directions within specific wavelength ranges. These scattering minima are found to be due to vanishing electric and magnetic dipolar coefficients associated with hybrid optical anapoles. The ability to confine electromagnetic fields associated with hybrid optical anapoles can give rise to several novel optical phenomena and applications.

physics.optics↗

arXiv2026

Characterizing the Backscattered Spectrum of Mie Spheres

This study describes both experimentally and theoretically an important hitherto undiscovered feature of the scattering of micron_sized spherical objects when illuminated with highly focused circularly polarized light. This is a regime of high experimental relevance which has not been described in full detail. The experiments are complemented with the analytical formulas explaining the field scattered directed toward the backward hemispace. In particular, it is proven that this field shows a very regular oscillatory dependency with the optical size. This phenomenon is typically hidden in the total scattered field, as the field is scattered much less toward the backward hemisphere than toward the forward one. These regular oscillations are measured experimentally. It is proven that, by analyzing them, it is possible to determine the index of refraction of isolated micron_sized particles, opening new paths for applications in sensing and metrology.

physics.optics↗

arXiv2026

Optical forces, helicity, angular momentum and how they are all intertwined

The theoretical description of optical forces and torques on micron_sized particles is a crucial area of research and has formed the foundation for advancements in optical trapping and manipulation technologies. In this study, we derive analytical expressions for optical forces and torques on micron_sized spherical particles illuminated by focused Laguerre_Gaussian (LG) beams, employing the well_defined helicity multipolar decomposition of electromagnetic fields and Mie theory. We developed a multifunctional program, Multipolar Optical Forces Toolbox, based on this theoretical framework. The program, available on GitHub, was used to generate optical trapping stability maps. These maps predict trap stability across a wide range of system parameters and serve as a practical tool for designing advanced optical trapping experiments. Our analysis reveals the important role of helicity p and orbital angular momentum l on the dynamics of particles trapped off_axis in LG beams and demonstrates the unique nature of the tangential torque. Our findings also highlight notable differences in longitudinal optical forces resulting from pure helicity modifications in Gaussian beams. Furthermore, we showcase the ability of LG beams to isolate Mie resonances, offering a novel approach to locate the spectral positions of the resonances of high multipolar modes. These insights deepen the understanding of helicity in LG optical traps and pave the way for the development of more advanced optical manipulation techniques.

physics.optics↗

arXiv2026

On-Axis Optical Trapping with Vortex Beams: The Role of the Multipolar Decomposition

Optical trapping is a well_established, decades old technology with applications in several fields of research. The most common scenario deals with particles that tend to be centered on the brightest part of the optical trap. Consequently, the optical forces keep the particle away from the dark zones of the beam. However, this is not the case when a focused doughnut_shaped beam generates on_axis trapping. In this system, the particle is centered on the intensity minima of the laser beam and the bright annular part lies on the periphery of the particle. Researchers have shown great interest in this phenomenon due to its advantage of reducing light interaction with trapped particles and the intriguing increase in the trapping strength. This work presents experimental and theoretical results that extend the analysis of on_axis trapping with light vortex beams. Specifically, in our experiments, we trap micron_sized spherical silica (SiO2) particles in water and we measure, through the power spectrum density method, the trap stiffness constant \k{appa} generated by vortex beams with different topological charge orders. The optical forces are calculated from the exact solutions of the electromagnetic fields provided by the generalized Lorentz_Mie theory. We show a remarkable agreement between the theoretical prediction and the experimental measurements of \k{appa}. Moreover, our numerical model gives us information about the electromagnetic fields inside the particle, offering valuable insights into the influence of the electromagnetic fields present in the vortex beam trapping scenario.

physics.optics↗

arXiv2026

An Exact Energy Conservation Law for Magneto-Optical Nanoparticles

Energy conservation imposes fundamental bounds on the polarizabilities of nanoparticles (NPs). While such bounds are well established for isotropic and bianisotropic NPs, they remain unexplored for magneto-optical NPs. Here, we derive the exact energy-conservation law governing the electric and magnetic dipolar response of axially symmetric magneto-optical NPs under general illumination conditions and arbitrary external magnetic fields. Two central results follow from energy conservation: (i) purely magneto-optical scattering, where the non-magnetic polarizability vanishes, is fundamentally forbidden, and (ii) strong magneto-optical scattering regimes, in which the magneto-optical polarizability dominates, are intriguingly allowed.

physics.optics↗

arXiv2024

Solving Maxwell's Equations Using Polarimetry Alone

Maxwell's equations are solved when the amplitude and phase of the electromagnetic field are determined at all points in space. Generally, the Stokes parameters can only capture the amplitude and polarization state of the electromagnetic field in the radiation (far) zone. Therefore, the measurement of the Stokes parameters is, in general, insufficient to solve Maxwell's equations. In this Letter, we solve Maxwell's equations for a set of objects widely used in Nanophotonics using the Stokes parameters alone. Our method for solving Maxwell's equations endows the Stokes parameters an even more fundamental role in the electromagnetic scattering theory.

physics.optics↗

arXiv2024

The Stokes Vector Measurement

The multipolar expansion of the electromagnetic field plays a key role in the study of light-matter interactions. All the information about the radiation and coupling between the incident wavefield and the object is embodied in the electric and magnetic scattering coefficients $\{a_{\ell m}, b_{\ell m} \}$ of the expansion. However, the experimental determination of $\{a_{\ell m}, b_{\ell m} \}$ requires measuring the components of the scattered field in all directions, something that is exceptionally challenging. Here, we demonstrate that a single measurement of the Stokes vector unlocks access to the quadrivector $ \mathbf{D}_{\ell m} = \left[|a_{\ell m}|^2, |b_{\ell m}|^2, \Re \{ a_{\ell m} b^*_{\ell m} \}, \Im \{ a_{\ell m} b^*_{\ell m} \} \right]$. Thus, our Stokes polarimetry method allows us to capture $|a_{\ell m}|^2$ and $|b_{\ell m}|^2$ separately, a distinction that can not be achieved by measuring the total energy of the scattered field via an integrating sphere. Importantly, we demonstrate the robustness of our Stokes polarimetry method, showing its fidelity with just two measurements of the Stokes vector at different scattering angles. Our findings, supported by analytical theory and exact numerical simulations, can find applications in Nanophotonics and greatly facilitate routine light-scattering measurements in optical laboratories.

physics.optics↗

arXiv2023

Characterizing cylindrical particles upon local measurements of two Stokes parameters

Researchers routinely characterize optical samples by computing the scattering cross-section. However, the experimental determination of this magnitude requires the measurement and integration of the components of the scattered field in all directions. Here, we propose a method to determine the scattering cross-section and global polarization state of radiation through measurements of two Stokes parameters at an angle of choice in far-field. The method applies to cylindrically symmetric samples whose optical response is well-described by a single multipolar order j. Moreover, the formalism is applicable for a wide range of different illuminations, and it only requires the use of a single camera and conventional wave plates. Our findings significantly reduce the complexity of routine characterization measurements for cylindrical samples in optical laboratories.

physics.optics↗

arXiv2023

Spheres of maximum electromagnetic chirality

The search for objects that yield maximum electromagnetic chirality in their emitted wavefield has garnered significant attention in recent years. However, achieving such maximum chirality is challenging, as it typically requires complex chiral metamaterials. Here we demonstrate that chiral spheres can yield maximum chirality in their emitted wavefield. Specifically, we analytically find the spectral trajectories at which chiral spheres become optically transparent to a given helicity of the incident field, while for its opposite helicity, they behave as dual objects, i.e., on scattering, they preserve helicity. Since chiral spheres behave as dual objects at the first Kerker condition of zero backscattering, we significantly simplify this condition in terms of a Riccati-Bessel function. Importantly, all our results are exact and applicable regardless of the multipolar order, refractive index contrast, optical size, and intrinsic chirality of the chiral sphere. Thus, our exact findings can serve as building blocks for designing novel metasurfaces or metamaterials with maximum electromagnetic chirality properties.

physics.optics↗

arXiv2023

Resonant helicity mixing of electromagnetic waves propagating through matter

Dual scatterers preserve the helicity of an incident field, whereas antidual scatterers flip it completely. In this setting of linear electromagnetic scattering theory, we provide a completely general proof on the non-existence of passive antidual scatterers. However, we show that scatterers fulfilling the refractive index matching condition flip the helicity of the fields very efficiently without being in contradiction with the law of energy conservation. Moreover, we find that this condition is paired with the impedance matching condition in several contexts of electromagnetism and, in particular, within Fresnel's and Mie's scattering problems. Finally, we show that index-matched media induce a resonant helicity mixing on the propagating electromagnetic waves. We reach to this conclusion by identifying that the refractive index matching condition leads to the phenomenon of avoided level-crossing. Our contribution not only closes a historical discussion within the Nanophotonics community, but also opens up new possibilities in the fields of Metamaterials and Particle Physics.

physics.optics↗

arXiv2023

Capturing near-field circular dichroism enhancements from far-field measurements

Molecular Circular dichroism (CD) spectroscopy faces significant limitations due to the inherent weakness of chiroptical light-matter interactions. In this view, resonant optical antennas constitute a promising solution to this problem since they can be tuned to increase the CD enhancement factor, fCD, a magnitude describing the electromagnetic near-field enhancement of scatterers associated with a given helicity. Here, we derive an exact multipolar expansion of fCD, which is valid to deduce the integrated near-field CD enhancements of chiral molecules in the presence of scatterers of any size and shape under general illumination conditions. Based on our analytical findings, we show that the near-field fCD factor can be related to magnitudes that can be computed in the far-field, i.e., the scattering cross-section and the helicity expectation value. Moreover, we show that in the case of lossless cylindrically symmetric samples, the near-field fCD factor can be inferred experimentally only from two far-field measurements at specific scattering angles. Our contribution paves the way for the experimental characterization of devices capable of enhancing molecular CD spectroscopy.

physics.optics↗

arXiv2022

Optical mirages from spinless beams

Spin-orbit interactions of light are ubiquitous in multiple branches of nanophotonics, including optical wave localization. In that framework, it is widely accepted that circularly polarized beams lead to spin-dependent apparent shifts of dipolar targets commonly referred to as optical mirages. In contrast, these optical mirages vanish when the illumination comes from a spinless beam such as a linearly polarized wave. Here we show that optical localization errors emerge for particles sustaining electric and magnetic dipolar response under the illumination of spinless beams. As an example, we calculate the optical mirage for the scattering by a high refractive index nanosphere under the illumination of a linearly polarized plane wave carrying null spin, orbital, and total angular momentum. Our results point to an overlooked interference between the electric and magnetic dipoles rather than the spin-orbit interactions of light as the origin for the tilted position of the nanosphere.

physics.optics↗

arXiv2022

On the helicity conservation for Mie optical cavities

The study of helicity in the context of light-matter interactions is an increasing area of research. However, some fundamental aspects of the helicity content of light fields inside scatterers have been overlooked. In this work, we demonstrate that the helicity of light fields inside lossless spherical cavities cannot be either conserved or sign-flipped as a result of a scattering event. The underlying reason is that the internal electric and magnetic Mie coefficients cannot simultaneously oscillate with equal amplitude and equal/opposite phase. Our analytical demonstration is fulfilled regardless of the refractive index, sphere size, and multipolar order. In addition, we show that the helicity of light fields inside lossy spheres can be conserved. This fact is in striking contrast to the behavior of the scattered field, whose helicity content cannot be conserved precisely when the sphere has losses. Finally, we show that the helicity content of internal fields can be flipped for materials with gain.

physics.optics↗

arXiv2022

Tailoring accidental double bound states in the continuum in all-dielectric metasurfaces

Bound states in the continuum (BICs) have been thoroughly investigated due to their formally divergent Q-factor, especially those emerging in all-dielectric, nanostructured metasurfaces from symmetry protection at the $Γ$ point (in-plane wavevector $k_{||}=0$). Less attention has been paid to accidental BICs that may appear at any other $k_{||}\not =0$ in the band structure of supported modes, being in turn difficult to predict. Here we make use of a coupled electric/magnetic dipole model to determine analytical conditions for the emergence of accidental BICs, valid for any planar array of meta-atoms that can be described by dipolar resonances, which is the case of many nanostructures in the optical domain. This is explored for all-dielectric nanospheres through explicit analytical conditions that allow us in turn to predict accidental BIC positions in the parameter space $(ω,\bf{k_{||}}$). Finally, such conditions are exploited to determine not only single, but also double (for both linear polarizations) accidental BICs occurring at the same position in the dispersion relation $ω-\bf{k_{||}}$ for realistic semiconductor nanodisk meta-atoms. This might pave the way to a variety of BIC-enhanced light-matter interaction phenomena at the nanoscale such as lasing or non-linear conversion, that benefit from emerging at wavevectors away from the $Γ$ point (off-normal incidence) overlapping for both linear polarizations.

physics.optics↗

arXiv2022

Correlations between helicity and optical losses within general electromagnetic scattering theory

Helicity preserving nanostructures and metasurfaces have been recently proposed as suitable candidates to enhance spectroscopic features of chiral matter. With this in mind, we highlight that losses in the constituent nonmagnetic materials dramatically affect the possibility of constructing structures which conserve helicity. We first present a general procedure that permits the evaluation of the normalized helicity expectation value, $\langle\hatΛ\rangle$, i.e. the observable that permits the identification of helicity preserving scatterers. We then apply this procedure to the case of a chiral sphere, which in an orientation averaged picture can capture the optical response of chiral inorganic nanostructures, obtaining a widely applicable analytical expression of $\langle\hatΛ\rangle$ for this type of objects. Finally, we numerically show that optical losses impose an upper bound to the helicity expectation value on nonmagnetic core-shells and chiral spheres.

physics.optics↗

arXiv2021

Challenges on Optical Printing of Colloidal Nanoparticles

While colloidal chemistry provides ways to obtain a great variety of nanoparticles, with different shapes, sizes, material composition, and surface functions, their controlled deposition and combination on arbitrary positions of substrates remains a considerable challenge. Over the last ten years, optical printing arose as a versatile method to achieve this purpose for different kinds of nanoparticles. In this article we review the state of the art of optical printing of single nanoparticles, and discuss its strengths, limitations, and future perspectives, by focusing on four main challenges: printing accuracy, resolution, selectivity, and nanoparticles photostability.

cond-mat.mes-hall↗

arXiv2020

Multiple Kerker anapoles in dielectric microspheres

High refractive index dielectric spheres present remarkable light-scattering properties in the spectral range dominated by dipolar modes. However, most of these properties are absent for larger spheres under plane wave illumination. Here, we propose to unravel dipolar regimes regardless of the sphere size and refractive index by illuminating with a pure dipolar field. This type of illumination ensures that the scattering response of the sphere is purely dipolar. In this scenario, we show that Kerker conditions are not only related to duality symmetry and a strong backward-to-forward asymmetric light-scattering, but also to the appearance of non-radiating sources: the so-called hybrid anapoles. Finally, we show that all the abovementioned scattering features under dipolar illumination are reproducible with an experimentally accessible tightly-focused Gaussian beam.

physics.optics↗