arXiv subjects

Francisco J. Rodríguez-Fortuño

Publications and source records attributed to Francisco J. Rodríguez-Fortuño.

At least 19 recordsLinked to original sources

arXiv2026

Surface Plasmon Polariton Excitation in Time-modulated Media

Surface plasmon polaritons (SPPs) are central to application areas such as sensing, energy harvesting, and nanoscale optics, and are typically excited via spatial structuring -- an approach lacking dynamic control. We demonstrate that time modulation of a dispersive Drude-like metal allows the excitation and out-coupling of SPPs, thereby modelling realistic transparent conducting oxides; this establishes a pathway for the active control and extraction of plasmons in experimentally viable time-varying systems. Using numerical simulations we show that time modulation facilitates both the launching and radiation of surface plasmons with frequencies governed by the dispersion of the bounding media. Our results also reveal the generation of time-reflected waves at step-like temporal interfaces and the emergence of a magnetostatic mode required for matching boundary conditions at the interface.

physics.optics↗

arXiv2026

Time diffraction of optical helicity

If in a classic double-slit interference experiment the light that passes though one slit is in an orthogonal polarisation state to that of the second slit, no visible interference fringes emerge. Interference fringes are instead present in a different observable that relates to the difference in energy carried by two orthogonal polarisation components, causing spatially varying polarisation patterns: the optical helicity density in the case of linearly polarised fields. Here, we develop a generalised treatment of interference in space and time, and experimentally demonstrate the helicity patterns produced by interference of orthogonally polarised pulses, which would be produced by two temporal slits in time diffraction observations. The developed approach may be important for understanding and applications of complex polarisation patterns and polarisation effects in time varying media.

physics.optics↗

arXiv2026

Dipole light-matter interactions in the bispinor formalism

The conventional formulation of power absorption, optical forces, and torques on dipolar particles involve lenghty and cumbersome expressions that obscure their shared physical origin. We apply a bispinor formalism that unifies these disparate phenomena in a very general case including chiral and nonreciprocal particles. This reveals that force, torque, absorbed power, and absorbed helicity rate can all be concisely expressed in terms of broken symmetries, and leads to the fundamental inequalities that dipolar particles' cross-sections must satisfy. This framework uncovers profound connections normally hidden behind complex algebra -- for instance, pressure forces depend exclusively on the difference in linear momenta of different light components and the corresponding breaking of symmetry by a particle, and optical recoil forces depend exclusively on helicity cross sections -- providing clarity, conciseness, and a powerful predictive tool for arbitrary dipole interactions.

physics.optics↗

arXiv2026

Radiation forces and torques in optics and acoustics

The mechanical action of various kinds of waves has been recognized for several centuries. The first tide of scientific interest in wave-induced forces and torques emerged at the turn of the 20th century, with the development of wave theories and the concepts of wave momentum and angular momentum. A second surge occurred in the past several decades, driven by technological breakthroughs: the invention of lasers and the controlled generation of structured wave fields. This resulted in major discoveries, including optical trapping and manipulation of small particles, from atomic to micro sizes, as well as acoustic manipulation of larger particles, including biological cells and samples. Nowadays, radiation forces and torques underpin numerous applications: optical and acoustic tweezers, acoustofluidic sorting of biological cells, optomechanical systems operating in both classical and quantum regimes, solar sails, quantum simulators, volumetric displays, etc. In this review, we present a unifying perspective on optical and acoustic forces and torques acting on various particles, addressing both their theoretical foundations and key applications. Our approach relies on the universal connection between the local energy, momentum, and spin densities of wave fields and the principal forces and torques exerted on small particles. Moreover, we describe important cases of nontrivial (e.g., lateral and pulling) forces and complex (e.g., chiral and anisotropic) particles. We also highlight significant experimental achievements involving optical and acoustic manipulation in structured wave fields. Our aim is to illuminate the common fundamental origins and close interconnections between the mechanical actions of optical and acoustic fields, thereby fostering a deeper understanding and advancing the development of optomechanical and acoustomechanical applications.

physics.optics↗

arXiv2026

A robust and efficient method to calculate electromagnetic modes on a cylindrical step-index nanofibre

The accurate calculation of guided electromagnetic modes in optical nanofibres is critical for applications in nanophotonics, from quantum interfaces to vectorial light sensing. Standard textbook methods rely on solving a $4\times4$ matrix eigenvalue problem to find the modal fields. While widely used, this approach has a subtle but significant flaw: the final determination of the field amplitudes requires finding the numerical null space of a theoretically singular matrix, an ill-conditioned problem that introduces large relative errors in the small but physically crucial longitudinal field components. In this work, we introduce a fundamentally more robust and efficient semi-analytical method. By starting from the foundational symmetries of the cylindrical waveguide and employing a judicious normalisation of the field amplitudes, we demonstrate that the problem can be analytically reduced to a much simpler $2\times2$ system. This reformulation yields two decisive advantages: the dispersion relation is obtained numerically from a simple and well-behaved transcendental equation, and more importantly, the modal field amplitudes are subsequently determined \emph{analytically}. Our approach completely bypasses the numerical null space calculation, thereby ensuring the accuracy of the full vectorial field structure. This method provides a powerful and reliable tool for the design and analysis of nanofibre-based devices, particularly for applications in chiral quantum optics and nanophotonics where precise knowledge of field polarisation and specifically of the longitudinal components is paramount.

physics.optics↗

arXiv2025

On the origin and properties of dipolar recoil force and torque

The recoil optical force and torque acting on an electromagnetic dipole are typically derived by computing the imbalance in radiated linear and angular electromagnetic momentum coming from the source, using Maxwell stress tensor integration. This quantifies the recoil's outcome without revealing its physical origin or the underlying forces that produce it. The recoil force and torque exist even in the absence of external illumination, such that an isolated dipole emitter can experience them. In contrast to the other terms, the recoil terms are odd under time reversal. To clarify their nature and properties, we re-derive the recoil force from first principles using the total Lorentz force on a system of charges that form a simultaneous electric and magnetic dipole. The results agree with the standard momentum-based derivation and reveal their ultimate origin from retardation effects -- arising from the finite speed of light -- in the mutual interactions between charges. This result provides fundamental insight into the recoil mechanism, offering a clearer conceptual foundation for future theoretical and experimental studies of light-induced forces.

physics.optics↗

arXiv2025

Plasmonic Brownian Ratchets for Directed Transport of Analytes

Controlled long-range transport of micro- and nano-scale objects is a key requirement in lab-on-a-chip and microfluidic applications, enabling the efficient capture, concentration, manipulation, and detection of analytes. Traditional methods such as microfluidic pumps and optical trapping face challenges including high power consumption and limited range of action. This study introduces a plasmonic Brownian ratchet designed for the directed transport of dielectric nanometer-sized particles at low optical powers. Through numerical simulations, the ratchet geometry was optimized to enhance electric fields, optical forces, and trapping potentials. Experimentally, the plasmonic ratchet demonstrated the ability to rectify random thermal motion of 40 nm polysterene spheres over extended distances in a specific direction, achieving velocities up to 2.4 $μ$m/s at excitation powers as low as 0.785 kW/cm^2. This plasmonic ratchet offers a robust and efficient solution for the targeted delivery and concentration of nanoscale analytes on chips, with significant implications for advancing applications in the life sciences.

cond-mat.mes-hall↗

arXiv2025

Bulk-to-surface coupling of spatiotemporal vortices

We consider the coupling of bulk space-time-structured waves, such as spatiotemporal vortex pulses (STVPs), to surface waves, such as surface plasmon-polaritons (SPPs). For resonant coupling that preserves the frequency and tangent wavevector components, it is challenging to transfer the space-time wave structure because of the difference between the bulk and surface wave dispersions. We describe three mechanisms allowing for the bulk-to-surface conversion of STVPs: (i) a suitable tilt of the pulse spectrum in the $(ω,{\bf k})$ space; (ii) confinement of the incident pulse in the direction orthogonal to the vortex; and (iii) losses in the surface waves. We supply general considerations with numerical simulations of the STVP-to-SPP coupling at a dielectric-prism/metal-film interface.

physics.optics↗

arXiv2025

Fresnel reflection coefficients in the Fourier domain for a planar surface in uniform motion parallel to its interface

The optical reflection coefficient of a dielectric medium moving uniformly in the plane spanned by its surface is rigorously calculated using classical electrodynamics and special relativity, and expressed in the Fourier domain, as a function of the incident frequency and wavevector, valid in both the far- and near-field regimes. It is found that cross-polarisation appears as a consequence of the motion, except when it is directed along the plane of incidence. As an example, using a Drude model for the permittivity of the surface at rest, the dispersion relation of its surface modes is calculated. A tilting of the dispersion relation is observed, leading to movement-induced surface plasmon unidirectionality and non-reciprocity.

physics.optics↗

arXiv2025

Non-linear bistability in pulsed optical traps

Optical trapping, also known as optical tweezing or optical levitation, is a technique that uses highly focused laser beams to manipulate micro- and nanoscopic particles. In optical traps driven by high-energy pulses, material non-linearity can result in unusual opto-mechanical effects, such as displaced equilibrium points. However, existing theoretical models of non-linear optical force on small particles consider smooth material dependence on the incident field strength alone, and not the feedback between the particle permittivity and internal field strength, which is, in turn, a function of the permittivity. The hysteresis effects of optical bistability in pulsed optical traps therefore elude existing optical force models. Here, we investigate a bistable optical trap, set up by counter-propagating ultrashort pulses, in which the optical force exerted on a particle depends not only on the field at the particle's current location, but on the particle's historic trajectory in the trap. The developed formalism will be important for designing optical traps and nanoparticle manipulation in pulsed field for various applications, including potentially time crystal demonstrations.

physics.optics↗

arXiv2024

Electromagnetic symmetry dislocations

Singular optics aims to understand and manipulate light's topological defects, pioneered by the discovery that phase vortex lines, strands of destructive interference, naturally occur in scalar wave fields. Monochromatic electromagnetic fields, however, are described by complex three-dimensional vectors that make individual scalar phase vortices in their vector components, which depend on the choice of co-ordinate basis, less meaningful. Instead, polarisation singularities can capture the vector texture of complicated, even non-paraxial light, with separate spatial descriptions for the electric $\mathbf{E}$ and magnetic $\mathbf{H}$ fields. But polarisation textures, too, are basis-dependent, because the laws of electromagnetism can be expressed not only by separate $\mathbf{E}$ and $\mathbf{H}$ fields, but by linear combinations of the two. We instead propose fundamental, basis-independent topological features generic in monochromatic electromagnetic fields: one- and two-dimensional structures that relate to time-averaged symmetries, including parity, duality and time-reversal, held locally by the combined electric and magnetic field polarisation geometry.

physics.optics↗

arXiv2024

Mixing Skyrmions and Merons in Topological Quasicrystals of Evanescent Optical Field

Photonic skyrmion and meron lattices are structured light fields with topologically protected textures, analogous to magnetic skyrmions and merons. Here, we report the theoretical existence of mixed skyrmion and meron quasicrystals in an evanescent optical field. Topological quasiperiodic tilings of even and odd point group symmetries are demonstrated in both the electric field and spin angular momentum. These quasicrystals contain both skyrmions and merons of Néel-type topology. Interestingly, the quasiperiodic tilings are in agreement with the observations of quasiperiodic arrangements of carbon nanoparticles in water driven by ultrasound, and pave the way towards engineering hybrid topological states of light which may have potential applications in optical manipulation, metrology and information processing.

physics.optics↗

arXiv2024

The electromagnetic symmetry sphere: a framework for energy, momentum, spin and other electromagnetic quantities

Electromagnetic quantities such as energy density, momentum, spin, and helicity bring meaning and intuition to electromagnetism and possess intricate interrelations, particularly prominent in complex non-paraxial near-fields. These quantities are conventionally expressed using electric and magnetic field vectors, yet the electric-magnetic basis is one among other often overlooked alternatives, including parallel-antiparallel and right-left-handed helicity bases, related to the parity and duality symmetries of electromagnetism. Projecting time-harmonic electromagnetic fields into a variety of bases allows re-interpreting established quantities and reveals underlying mathematical structures: a Bloch sphere which describes asymmetries in electromagnetic energy, a systematic path to unify and uncover relations between electromagnetic quantities, and the unlocking of symmetry-driven equations in light-matter interaction.

physics.optics↗

arXiv2024

Longitudinal chiral forces in photonic integrated waveguides to separate particles with realistically small chirality

Chiral optical forces exhibit opposite signs for the two enantiomeric versions of a chiral molecule or particle. If large enough, these forces might be able to separate enantiomers all optically, which would find numerous applications in different fields, from pharmacology to chemistry. Longitudinal chiral forces are especially promising for tackling the challenging scenario of separating particles of realistically small chiralities. In this work, we study the longitudinal chiral forces arising in dielectric integrated waveguides when the quasi-TE and quasi-TM modes are combined as well as their application to separate absorbing and non-absorbing chiral particles. We show that chiral gradient forces dominate in the scenario of beating of non-denegerate TE and TM modes when considering non-absorbing particles. For absorbing particles, the superposition of degenerate TE and TM modes can lead to chiral forces that are kept along the whole waveguide length. We accompany the calculations of the forces with particle tracking simulations for specific radii and chirality parameters. We show that longitudinal forces can separate non-absorbing chiral nanoparticles in water even for relatively low values of the particle chirality and absorbing particles with arbitrarily low values of chirality can be effectively separated after enough interaction time.

physics.optics↗

arXiv2024

Propagation of focused scalar and vector vortex beams in anisotropic media: A semi-analytical approach

In the field of structured light, the study of optical vortices and their vectorial extension--vectorial vortex beams--has garnered substantial interest due to their unique phase and polarisation properties, which make them appealing for many potential applications. Combining the advantages of vortex beams and anisotropic materials, new possibilities for electromagnetic field tailoring can be achieved in nonlinear optics, quantum and topological photonics. These applications call for a comprehensive modelling framework that accounts for properties of both anisotropic materials and vector vortex beams. In this paper, we describe a semi-analytical model that extends the vectorial diffraction theory to focused vortex beams propagating through a uniaxial slab, considering the cases of scalar and vectorial vortexes in the common framework of a Laguerre-Gaussian modes. The model aims to provide a comprehensive description of the methodology, enabling the implementation of complex beams transmission through, reflection from and propagation in uniaxial anisotropic materials for specific applications. We apply the developed approach to propagation of high-order vortex beams in uniaxial materials with various dispersion characteristics> elliptic, hyperbolic and epsilon-near-zero regimes. We show how variations of the medium anisotropy modify the beam structure due to the vectorial nature of their interaction, which results from the different permittivities of the medium for transverse and longitudinal field components. The applicability of the approach can be extended to artificially structured media if they can described by effective medium parameters. The developed formalism will be useful for modelling of interaction of complex beams with uniaxial materials, allowing a common framework for a large variety of situations, which can also be extended beyond the electromagnetic waves.

physics.optics↗

arXiv2024

Non-Diffracting Polarisation Features around Far-Field Zeros of Electromagnetic Radiation

Light from any physical source diffracts over space, as spherical wavefronts grow and energy density is spread out. Diffractive effects pose fundamental limits to light-based technologies, including communications, spectroscopy, and metrology. Polarisation becomes paraxial in the far field limit and, by ignoring longitudinal field components, the rich physics of non-paraxial fields which exist in near-fields or a beam's tight focus are lost. The longitudinal field cannot, however, be ignored when transverse field components vanish (in a transverse field zero) and carry a small non-paraxial region to infinity. We show that a transverse field zero is always accompanied by non-diffracting polarisation structures, whose geometries are independent of the distance to the source, including an enclosing intensity ratio tube, and parallel, non-diverging polarisation singularities. We illustrate these features in multipole radiation and in double slit interference, two examples which have time-fixed transverse field zeros. Non-diffracting structures with changing position are coupled to time-varying zeros, which are present in all far field radiation.

physics.optics↗

arXiv2023

Chiral forces in longitudinally invariant dielectric photonic waveguides

Optical forces can be chiral when they exhibit opposite signs for the two enantiomeric versions of a chiral molecule or particle. Such forces could be eventually used to separate enantiomers, which could find application in numerous disciplines. Here, we analyze numerically the optical chiral forces arising in the basic element of photonic integrated circuitry: a dielectric waveguide with rectangular cross-section. Such waveguides are inherently lossless thus generating chiral forces that are invariant in the longitudinal direction and therefore enable enantiomeric separation over long (cm-scale) distances. Assuming Brownian motion in a liquid environment, we calculate first the force strength and time span needed to perform the separation of chiral nanoparticles as a function of the radii. Then we analyze the chiral forces produced by the fundamental quasi-TE guided mode in a silicon nitride waveguide and show that it can lead to enantiomeric separation via the transverse spin at short wavelengths (405 nm). At longer wavelengths (1310 nm), the proper combination of degenerate quasi-TE and quasi-TM modes would result in a quasi-circularly polarized mode with intrinsic chirality (helicity), leading to chiral gradient forces that also enable the enantiomeric separation of smaller nanoparticles. We report particle tracking simulations where the optical force field produced by a quasi-TE and a quasi-circular mode proved to separate enantiomers under a time span of two seconds. Our results suggest the viability of enantiomeric separation using simple photonic integrated circuits, though different wavelength windows should be selected according to the nanoparticle size.

physics.optics↗

arXiv2023

A decomposition of light's spin angular momentum density

Light carries intrinsic spin angular momentum (SAM) when the electric or magnetic field vector rotates over time. A familiar vector equation calculates the direction of light's SAM density using the right hand rule with reference to the electric and magnetic polarisation ellipses. Using Maxwell's equations, this vector equation can be decomposed into a sum of two distinct terms, akin to the well-known Poynting vector decomposition into orbital and spin currents. We present the first general study of this spin decomposition, showing that the two terms, which we call canonical and Poynting spin, are chiral analogies to the canonical and spin momenta of light in its interaction with matter. Both canonical and Poynting spin incorporate spatial variation of the electric and magnetic fields and are influenced by optical orbital angular momentum (OAM). The decomposition allows us to show that the OAM of a linearly polarised vortex beam can impart a first-order preferential force to chiral matter in the absence of spin.

physics.optics↗