arXiv subjects

Miguel A. Porras

Publications and source records attributed to Miguel A. Porras.

At least 19 recordsLinked to original sources

arXiv2026

Observation of helical pulses

Ultrafast spatiotemporal vortex pulses constitute a category within spatiotemporal topological waves. Nevertheless, the experimental realization of helical pulses single or few cycle short vortex pulses characterized by space time nonseparability remains elusive to date. Here, we introduce two complementary methods for experimentally generating such space time nonseparable helical pulses (SNHPs) in the optical and microwave spectral regimes. We achieve few cycle quasi linearly polarized SNHPs by decomposing the optical toroidal pulses into their polarization components. We also generated single cycle nontransverse SNHPs directly from a microwave ultrawideband spiral emitter. These approaches enable the experimental realization of SNHPs and provide a platform for further investigation into their properties and applications, such as nontrivial light-matter interactions and optical communications.

physics.optics↗

arXiv2026

Non-diffracting meronic spin defects of light

Optical vortices are singularity lines where the light field intensity vanishes and its phase is undefined. These threads of darkness are adorned by Gauss's law as lines of pure longitudinal polarization where the polarization plane tilts and winds around. We unveil the resulting spin field as a unique structure which unifies both topological textures and defects, as it includes a point defect of undefined spin surrounded by a meronic texture which spans half the spin unit sphere. Moreover, this intricate topological structure of transverse spin does not spread in propagation, is localized arbitrarily below the wavelength of light and presents highly anisotropic features. Here we describe these hidden topologies of transverse spin embedded in simple scalar vortex beams, highlighting the diversity of topological structures that arise in two different spaces -- the spin unit sphere and the transverse-axial Poincaré sphere -- and discuss the underlying aspects behind their subwavelength localization.

physics.optics↗

arXiv2026

Diffraction-free natural optical skyrmions and their subwavelength confinement around vortices

Diffraction causes waves to spread out as they propagate freely. The tighter the lateral confinement, the faster the spreading. Past research on how to suppress diffraction has been based on wave engineering and has led so far to idealized waves that, in real settings, eventually diffract. Here, we find a propagating light wave structure naturally present in optical vortices, a natural skyrmion, that is exempt from diffraction. Moreover, diffraction-free propagation occurs with lateral confinement at any scale below the wavelength of light. In our experiments, we observe non-diffraction over a propagation distance above three orders of magnitude greater than expected from the skyrmion subwavelength size. We thus provide a factual, real-world form of ideal non-diffracting propagation. This form substantially differs from previous forms of light propagation, including propagating optical skyrmions known to date, and could open up new perspectives in its various applications.

physics.optics↗

arXiv2026

Spontaneous optical skyrmion generation by frequency doubling in underdense plasmas

Optical skyrmions have been widely explored in recent years. Among them, Stokes skyrmions require sophisticated wave engineering or photonic devices for their generation. We show that Stokes skyrmions can emerge spontaneously in second-order harmonic generation in underdense plasmas driven by optical vortices. The nonlinear response produces a structured frequency-doubled field whose polarization texture maps the Poincaré sphere. When plasma inhomogeneities are taken into account, the electron density gradient deforms the skyrmionic texture, enabling topological diagnosis of plasma density.

physics.optics↗

arXiv2026

Roadmap on Attosecond Science

Twenty-five years have passed since the first experimental demonstration of attosecond pulses, marking the advent of our ability to resolve and control electron motion in real time. What began as a technological breakthrough - generating the shortest flashes ever produced - has evolved into a powerful approach for probing and steering electronic dynamics in atoms, molecules, and solids. This roadmap, authored by leading experts in the field, surveys the recent rapid progress in the generation and characterization of attosecond pulses, emerging attosecond measurement and control techniques, and their expanding range of applications. It reviews current and future developments in attosecond light sources, including novel laser technologies, waveform synthesizers, new schemes for high-order harmonic generation, attosecond pulse generation at free-electron lasers, and structured light. Advances in attosecond measurement methodologies are also discussed, encompassing all-attosecond pump-probe spectroscopy, attosecond four-wave mixing, attosecond microscopy, spectroscopy with light transients, and attosecond interferometry. Furthermore, the roadmap addresses applications of attosecond spectroscopy to reveal electron dynamics in molecules and condensed matter systems from both theoretical and experimental perspectives, and highlights emerging directions at the interface with quantum optics and quantum entanglement. Overall, this work aims to serve as a comprehensive resource for navigating the evolving landscape of attosecond science.

physics.optics↗

arXiv2026

Scaling law from orbital angular momentum conservation in harmonic and high-order harmonic generation driven by spatiotemporal light fields

Nonlinear photon upconversion processes driven by diverse forms of structured light are receiving increasing attention. In harmonic and high-order harmonic generation (HG and HHG) with Laguerre-Gauss (LG) beams, linear scaling the driver topological charge (TC) with the harmonic order is equivalent to driver orbital angular momentum (OAM) per photon scaling, and constitutes a proof of OAM conservation. However, with generic driving fields, such as non-LG vortices or spatiotemporal optical vortices, TC and OAM per photon may scale or not in a process in which the OAM is conserved. We find the physical magnitude that scales with generality when the OAM, either longitudinal or transverse, or its intrinsic part, is conserved. This new rule allows for the wealth of phenomena observed in HHG that are unintelligible from the rigid LG rule.

physics.optics↗

arXiv2026

Gouy phase-assisted Zeno effect for protecting light structure in random media

Identifying physical mechanisms that protect the information carried by various forms of structured light is one of the cornerstones of today's classical and quantum communications. Here we show that the purity of orbital angular momentum (OAM) modes can be protected against degradation in random media by leveraging two fundamental features of their own Schrödinger Hamiltonian dynamics, namely, Zeno effect -- frequent observations slow down the evolution -- , and Gouy phase -- the back-action of the observation. Repeated, OAM-dependent Gouy phase kicks imparted along the disturbing path by simple imaging systems trigger the optical Zeno effect that protects the input OAM mode against mode cross-talk that would broaden the OAM spectrum. Given the universality of the mechanism, the Gouy phase-assisted Zeno effect would protect propagation modes other than those of OAM, and the diverse forms of structured light built with them.

physics.optics↗

arXiv2025

Isolated attosecond spatio-temporal optical vortices: Interplay between the topological charge and orbital angular momentum scaling in high harmonic generation

The propagation properties and the nature of the transverse orbital angular momentum (t-OAM) of spatiotemporal optical vortices (STOVs) open new scenarios in high-harmonic generation (HHG), where the richness of the topological charge and OAM up-conversion are exposed. Through advanced numerical simulations, we demonstrate that HHG driven by spatio-spectral optical vortices produces far-field, extreme-ultraviolet STOV harmonics with non-scaling topological charge, i.e., with the same topological charge. This allows for the generation of attosecond STOVs, in contrast to previous works of HHG driven by STOVs, where the topological charge scales with the harmonic order. Our findings evidence that the scaling of the topological charge in HHG driven by spatio-temoral topological fields is not generally connected to that of the up-converted OAM. The up-converted intrinsic OAM does scale with generality with harmonic order in HHG, albeit this scaling does not necessarily imply its conservation.

physics.optics↗

arXiv2025

Enhanced spatiotemporal optical vortices and vortex chains from Hermite-Gauss modes with a tilted pulse front

Hermite-Gaussian (HG) beams are standard modes delivered by continuous or pulsed lasers systems, and pulse-front tilt is one of the most common, detrimental or beneficial, spatiotemporal couplings affecting ultrashort pulses. Combining them, we show that focusing a pulsed HG beam with a tilt generates an elliptical spatiotemporal optical vortex (STOV), or a chain of them. The elliptical STOVs differ from standard STOVs in an additional spatial chirp that is manifested as a wave front temporal rotation, and results in an enhanced transverse orbital angular momentum. The longitudinal field is significantly larger than that of normal STOVs, and may also take the form of an elliptical STOV. Our concept greatly simplifies previous arrangements for the generation of STOVs, whose additional features make them attractive for improving their applications in electron trapping and acceleration, or as driving fields for the generation of higher-order harmonics and other interactions with matter.

physics.optics↗

arXiv2025

Optical Skyrmions of Vortex Darkness

We disclose the existence of a type of optical skyrmion, Gauss-Stokes (GS) skyrmions, that is naturally present in an optical vortex around its phase singularity. Contrary to previous research with optical skyrmions, we neither shape vector beams nor superpose different spatial modes and polarizations. In GS skyrmions, the phase singularity in the transversal field of a single monochromatic beam of uniform polarization (a scalar beam) is concealed by the axial field dictated by Gauss's divergence law, giving rise to a polarization singularity of undefined polarization plane. This singularity is enclosed by a rich skyrmionic polarization texture fulfilling a topological map and covering all the states of transverse-axial polarization. In our experiment, we facilitate the observation of a GS skyrmion with the predicted features using focused fields with enhanced axial component.

physics.optics↗

arXiv2024

Extreme-ultraviolet spatiotemporal vortices via high harmonic generation

Spatiotemporal optical vortices (STOV) are space-time structured light pulses with a unique topology that couples spatial and temporal domains and carry transverse orbital angular momentum (OAM). Up to now, their generation has been limited to the visible and infrared regions of the spectrum. During the last decade, it was shown that through the process of high-order harmonic generation (HHG) it is possible to up-convert spatial optical vortices that carry longitudinal OAM from the near-infrared into the extreme-ultraviolet (EUV), thereby producing vortices with distinct femtosecond and attosecond structure. In this work we demonstrate theoretically and experimentally the generation of EUV spatiotemporal and spatiospectral vortices using near infrared STOV driving laser pulses. We use analytical expressions for focused STOVs to perform macroscopic calculations of HHG that are directly compared to the experimental results. As STOV beams are not eigenmodes of propagation, we characterize the highly-charged EUV STOVs both in the near and far fields, to show that they represent conjugated spatiotemporal and spatiospectral vortex pairs. Our work provides high-frequency light beams topologically coupled at the nanometer/attosecond scales domains with transverse OAM, that could be suitable to explore electronic dynamics in magnetic materials, chiral media, and nanostructures.

physics.optics↗

arXiv2024

Clarification of the transverse orbital angular momentum of spatiotemporal optical vortices

Advances in the generation and the application of spatiotemporal optical vortices (STOV) are proceeding fast, but fundamental aspects of their nature remain obscure. Phys. Rev. A 107, L031501 (2023) (PRA) and Prog. Electromagn. Res. 177, 95 (2023) (PIER) provide contradictory results on the transverse orbital angular momentum (OAM) carried by STOVs. We show that the results by Porras in PIER and by Bliokh in PRA refer to different STOVs and are all correct. In PIER, STOVs are elliptical at given cross section and time, or in space-time, but not in three-dimensional space. In PRA, STOVs are elliptical in space but not in space-time. This is evidenced from two dual, equivalent theories on the transverse OAM where a wave packet is seen in space-time evolving with propagation distance or in space evolving in time, that accounts for all values of the total, intrinsic and extrinsic OAM in PIERS and PRA. However, the intrinsic OAM with respect to the photon wave function center in PRA is not generally conserved, which advocates for the energy center in PIER as the STOV center. We argue that STOVs are generated in experiments to purportedly have elliptical symmetry in space-time. The values provided in PIER should then be taken as the reference for elliptical STOVs, and the theory therein to evaluate the transverse OAM of other wave packets. Hancock et al. in Phys. Rev. Lett. 127, 193901 (2021) and Phys. Rev. X. 14, 011031 (2024) erroneously attribute the transverse OAM of elliptical STOVs in space to the elliptical STOVs in space-time they consider theoretically and can generate in their experiments.

physics.optics↗

arXiv2024

Comment on "Spatiotemporal torquing of light"

The recent paper Phys. Rev. X. 14, 011031 (https://doi.org/10.48550/arXiv.2307.01019) includes an appendix that casts doubts on the validity the theory of the transverse orbital angular momentum of spatiotemporal optical vortices (STOVs) in Prog. Electromagn. Res. 177, 95 (https://doi.org/10.48550/arXiv.2301.09105). The argumentation in that appendix mixes a mechanical approach where STOVs are seen in space evolving in time with an optical approach where STOVs are seen in space-time evolving with propagation distance, which leads to wrong conclusions. We consistently carry out the analysis from the mechanical approach, and the results confirm the theory in Prog. Electromagn. Res. 177, 95 consistently performed within the optical approach.

physics.optics↗

arXiv2024

Comment on "Mode Structure and Orbital Angular Momentum of Spatiotemporal Optical Vortex (STOV) Pulses"

We report a mathematical error and a misinterpretation in arXiv:2103.03263v4 [Phys. Rev. Lett. 127, 193901 (2021)] that has led to a debate about the nature of the transverse orbital angular momentum (OAM) of spatiotemporal optical vortices (STOVs). The transverse OAM of STOVs evaluated theoretically in that Letter is actually only the intrinsic contribution, while the operators used to evaluate the intrinsic and extrinsic contributions are not Hermitian operators as they may lead to complex-valued expectation values.

physics.optics↗

arXiv2024

Transverse orbital angular momentum imparted upon focusing spatio-temporally coupled ultrashort pulses

A focusing system such as a single lens or a spherical mirror imparts intrinsic transverse orbital angular momentum (OAM) to spatiotemporal (ST) coupled fields the ST intensity distribution of which presents ST covariance. This fact may greatly simplify the experimental setups used to date to impart transverse OAM. We evaluate analytically the imparted transverse OAM as a function of the focal length and the covariance. The focused fields with transverse OAM include elliptical ST vortices and rotating pulses without any ST phase singularity such as the "lighthouse" pulse. We provide closed-form, analytical expressions for these fields valid at any propagation distance from the focusing system, which are of interest in applications such the interaction of these fields with matter. In general, focusing of ST coupled fields with intensity covariance generates mixed fields with ST vortices and rotating pulse-fronts, where one or another feature dominates depending on the input field.

physics.optics↗

arXiv2023

Control of vortex orientation of ultrashort optical pulses using spatial chirp

Introducing a spatial chirp into a pulse with a longitudinal vortex, such as a standard pulsed Laguerre-Gauss beam, results in a vortex pulse with an arbitrary orientation of the line phase singularity between longitudinal and transverse, depending on the amount of chirp. Analytical expressions are given for such pulses with arbitrary topological charge valid at any propagation distance.

physics.optics↗

arXiv2023

Transverse orbital angular momentum of spatiotemporal optical vortices

Spatiotemporal optical vortices (STOVs) are electromagnetic wave packets that transport a phase line singularity perpendicular to their propagation direction. We address the problem of the transverse orbital angular momentum (OAM) ``per photon" actually transported by STOVs propagating in free space or non-dispersive media, the most frequent experimental situation. Unlike longitudinal vortices in monochromatic light beams, STOVs do not carry any net transverse OAM about a fixed transverse axis crossing its center. However, STOVs transport an intrinsic transverse OAM per photon about a moving, transverse axis through its center, and an opposite extrinsic transverse OAM. Their applications would thus preclude setting particles at rest into rotation, but STOVs could transmit their intrinsic transverse OAM to photons of other waves. The intrinsic transverse OAM per photon of an elliptically symmetric STOV of frequency $ω_0$ and topological charge $l$ is $γl/2ω_0$, where $γ$ is the STOV ellipticity. Thus circularly symmetric STOVs ($γ=1$) carry half the intrinsic longitudinal OAM of circularly symmetric monochromatic light beams with a vortex of the same $l$ and $ω_0$. We show that the formula $(γ+1/γ)l/2ω_0$ for the intrinsic transverse OAM in Phys. Rev. A 107, L031501 (2023) yields infinite values and is not conserved on propagation for a particular STOV. When STOVs lose their elliptical symmetry upon propagation, they preserve the intrinsic transverse OAM $γl/2ω_0$ despite the phase singularity may split, the split singularities may disappear, or even change the sign of their topological charges.

physics.optics↗

arXiv2023

Teleportation of a quantum particle in a potential via quantum Zeno dynamics

We report on the possibility of teleportation of a quantum particle, a distinctly different phenomenon from the teleportation of a quantum state through entanglement. With the first meaning, teleportation is theoretically possible by placing the particle initially at rest (with a certain uncertainty) out of any equilibrium point of a potential well or barrier and by frequently monitoring whether the particle remains at rest. This quantum Zeno dynamics inhibits acceleration, and features disappearance from the classical turning point and appearance in other turning point, if there is any other, with a probability that approaches unity by increasing the frequency of the measurements. This phenomenon has all the ingredients attributed in science fiction to teleportation: The particle is always at rest, cannot be found in the path between the two turning points, and saves travel time. We discuss the feasibility, in principle, of teleportation of electrons, protons and other particles, and conclude its increasing impracticability as the particle gets heavier.

quant-ph↗