arXiv subjects

Miguel A. Porras

Publications and source records attributed to Miguel A. Porras.

At least 37 records · Page 2Linked to original sources

arXiv2022

Roadmap on spatiotemporal light fields

Spatiotemporal sculpturing of light pulse with ultimately sophisticated structures represents the holy grail of the human everlasting pursue of ultrafast information transmission and processing as well as ultra-intense energy concentration and extraction. It also holds the key to unlock new extraordinary fundamental physical effects. Traditionally, spatiotemporal light pulses are always treated as spatiotemporally separable wave packet as solution of the Maxwell's equations. In the past decade, however, more generalized forms of spatiotemporally nonseparable solution started to emerge with growing importance for their striking physical effects. This roadmap intends to highlight the recent advances in the creation and control of increasingly complex spatiotemporally sculptured pulses, from spatiotemporally separable to complex nonseparable states, with diverse geometric and topological structures, presenting a bird's eye viewpoint on the zoology of spatiotemporal light fields and the outlook of future trends and open challenges.

physics.optics↗

arXiv2022

Enhanced quantum tunneling in quantum Zeno dynamics freezing momentum direction

Quantum tunneling is a fundamental quantum mechanical effect involved in plenty of physical phenomena. Its control would impact these phenomena and the technologies based on them. We show that the quantum tunneling probability through a potential barrier can be increased to approach unity in a quantum Zeno dynamics undergone by the tunneling particle in which the direction of the momentum is frequently monitored. We first model the measurements of the momentum direction as selective von Neumann projections, and then as nonselective, direction-sensitive interactions of the particle with probe particles. Nonselective measurements are more efficient than selective measurements in enhancing the quantum tunneling probability.

quant-ph↗

arXiv2021

Broadband X-waves with orbital angular momentum

We describe the minimal diffraction-free and dispersion-free, superluminal wave packets (X-waves and generic superluminal localized waves) that can carry a given amount of units $m$ of orbital angular momentum (OAM) per photon. Even if their frequency spectrum is wide enough to synthesize a unipolar pulse, OAM imposes a temporal pulse shape with as many zeroes as units of OAM, and therefore $|m|/2$ temporal oscillations. All the frequencies in the broadband spectrum are displayed, like in a rainbow, from the bluer ones in the vicinity of the vortex to the redder ones in the wave periphery. At the same time, the whole wave paclet experiences a blue shift proportional to $|m|$. As a result of the radial red shift and the OAM blue shift, the colour of the brilliant ring around the central vortex is independent of OAM, and solely determined by the broadband source spectrum. Given the very peculiar properties of X-waves with OAM, their generation and use, instead of standard Laguerre-Gauss modes, could improve the performance of OAM-based communication and quantum cryptographic systems, as well as the efficiency of the generation of high harmonics and attosecond pulses with OAM.

physics.optics↗

arXiv2021

Spatiotemporal optical vortex solitons: Dark solitons with transverse and tilted phase line singularities

Vortex solitons (dark solitons) are described in self-defocusing Kerr media whose phase line singularity is not parallel to the propagation direction, but is perpendicular or tilted almost arbitrary angles, depending on the medium linear dispersion. These solitons broaden the so-called spatiotemporal optical vortices recently observed in experiments to the nonlinear realm, opening new perspectives in practical applications of optical vortices.

physics.optics↗

arXiv2021

Exploding paraxial beams, vortex beams, and cylindrical beams of light with finite power in linear media, and their enhanced longitudinal field

We present a set of paraxial light beams with cylindrical symmetry, smooth and localized transversal profile carrying finite power, that develop intensity singularities when they are focused in a linear medium, such as vacuum. They include beams with orbital angular momentum and with radial polarization, in which case they develop punctual phase and polarization singularities surrounded by infinitely bright rings, along with singular longitudinal fields. In practice, these effects are manifested in focal intensities and spot sizes, vortex bright ring intensities and radii, and strengths of the longitudinal field, that strongly change with the lens aperture radius. Continuous control of these focal properties is thus exercised without changing the light incident on the lens, with substantially the same collected power, and while maintaining paraxial focusing conditions. As solutions of the Schrödinger equation, these exploding beams have analogues in other areas of physics where this equation is the fundamental dynamical model.

physics.optics↗

arXiv2020

Rotating azimuthons in dissipative Kerr media excited by superpositions of Bessel beams

We report the existence of persistently rotating azimuthons in media with self-focusing Kerr and absorption nonlinearities. The nonlinear loss is balanced by power influx from the peripheral reservoir stored in slowly decaying tails of the azimuthons. These modes are excited by a superposition of two Bessel beams with opposite vorticities, $\pm s$, and slightly different conicities. The excited azimuthon exhibits opposite vorticity in its center to that of the input Bessel-beam superposition due to spontaneous inversion of the topological charge in the course of the azimuthon formation. Unlike azimuthons in loss-free media, number $N$ of rotating intensity maxima and $s$ are not mutually independent, being related by $N=2s$. The robustness of the rotating azimuthons is enhanced in comparison to similar static dissipative patterns. They can be excited in almost any transparent material, in the range of intensities for which the nonlinear absorption, induced by multiphoton absorption, is relevant. Close to the ionization threshold, the rotating azimuthons reproduce recently observed helical filaments of light in air and CS$_{2}$.

physics.optics↗

arXiv2020

General laws of the propagation of ultrafast vortices in free space

We conduct a theoretical study of the propagation of few-cycle, ultrafast vortices (UFVs) carrying orbital angular momentum (OAM) in free space. Our analysis reveals much more complex temporal dynamics than that of few-cycle fundamental Gaussian-like beams, particularly when approaching the single-cycle regime and the magnitude of the topological charge $l$ is high. The recently described lower bound $\sqrt{|l|}$ to the number of oscillations of UFVs with propagation-invariant temporal shape (isodiffracting UFVs) is found to hold on average also for UFVs of general type, with variations along the propagation direction above and below that bound, even vanishing locally. These variations are determined by the so-called Porras factor or $g_0$-factor characterizing the dependence of the Rayleigh distance of the spectral constituents with frequency. With a given available bandwidth, UFVs must widen temporally with increasing magnitude of the topological charge, and must widen or may shrink temporally during propagation as a result of the axially varying, $g_0$-dependent lower bound. Under very restrictive conditions in their generation, an UFV can be shrunk below the lower bound $\sqrt{|l|}$ at a focus into a kind of locally compressed state of OAM, but it broadens well-above $\sqrt{|l|}$ and distorts in a tiny fraction of the depth of focus because of the dispersions introduced by Gouy's phase and wave front mismatch. These propagation phenomena have implications and should be taken into account in experiments and applications of UFVs, such as the generation of high-harmonics and attosecond pulses with high OAM, or in OAM-based ultrafast communications systems, as well as in other areas of physics such as acoustics or electron waves.

physics.optics↗

arXiv2020

Nervous Excitability dynamics in a multisensory syndrome and its similitude with a normal state. Scaling Laws

In the context of increased number of works published on multisensory and cross-modal effects, we review a cortical multisensory syndrome (called central syndrome) associated with a unilateral parieto-occipital lesion in a rather unspecific (or multisensory) zone of the cortex. The patients with this syndrome suffered from bilateral and symmetric multisensory disorders dependent on the extent of nervous mass lost and the intensity of the stimulus. They also presented cross-modal effects. A key point is the similitude of this syndrome with a normal state, since this syndrome would be the result of a scale reduction in brain excitability. The first qualities lost when the nervous excitation diminishes are the most complex ones, following allometric laws proper of a dynamic system. The inverted perception (visual, tactile, auditive) in this syndrome is compared to other cases of visual inversion reported in the literature. We focus on the capability of improving perception by intensifying the stimulus or by means of another type of stimulus (cross-modal), muscular effort being one of the most efficient and least known means. This capability is greater when nervous excitability deficit (lesion) is greater and when the primary stimulus is weaker. Thus, in a normal subject, this capability is much weaker although perceptible for functions with high excitability demand. We also review the proposed scheme of functional cortical gradients whereby the specificity of the cortex is distributed with a continuous variation leading to a brain dynamics model accounting for multisensory or cross-modal interactions. Perception data (including cross-modal effects) in this syndrome are fitted using Stevens' power law which we relate to the allometric scaling power laws dependent on the active neural mass, which seem to be the laws governing many biological neural networks.

q-bio.NC↗

arXiv2020

Couplings between the temporal and orbital angular momentum degrees of freedom in ultrafast vortices with propagation-invariant temporal shape

In any form of wave propagation, strong spatiotemporal coupling appears when non-elementary, three-dimensional wave-packets are composed by superimposing pure plane waves, or spontaneously generated by light-matter interaction and nonlinear processes. Ultrashort pulses with orbital angular momentum (OAM), or ultrashort vortices, furnish a critical paradigm in which the analysis of the spatiotemporal coupling in the form of temporal-OAM coupling can be carried out accurately by analytical tools. By generalizing and unifying previously reported results, we show that universal and spatially heterogeneous space-time correlations occur in propagation-invariant temporal pulses carrying OAM. In regions with high intensity, the pulse duration has a lower bound fixed by the topological charge of the vortex and such that the duration must increase with the topological charge. In regions with low intensity in the vicinity of the vortex, a large blue-shift of the carrier oscillations and an increase of the number of them is predicted for strongly twisted beams. We think that these very general findings highlight the existence of a structural coupling between space and time. These results have also applications in free-space communications, spectroscopy, and high-harmonic generation.

physics.optics↗

arXiv2020

Effects of orbital angular momentum on the pulse shape at the most intense ring of ultrafast vortices

It has been recently shown that the orbital angular momentum (OAM) and temporal degrees of freedom in ultrafast (few-cycle) vortices are coupled. This coupling manifests itself with different effects in different parts of the vortex, as has been shown for the ring surrounding the vortex where the pulse energy is maximum, and also in the immediate vicinity of the vortex center. In may applications, however, the ring of maximum energy is not of primary interest, but that where the pulse peak intensity is maximum, which is particularly true in nonlinear optics applications such as the experiments with ultrafast vortices exciting high harmonics and attosecond pulses carrying also OAM. This article describes the effects of the OAM-temporal coupling at the ring of maximum pulse peak intensity, which does not always coincide with the ring of maximum pulse energy. We find that there exists an upper bound to the magnitude of the topological charge that an ultrafast vortex with prescribed pulse shape at its most intense ring can carry, and vice versa, a lower bound to the duration of the pulse at the most intense ring for a given magnitude of the topological charge. These bounds imply that with a given laser source spectrum, the duration of the synthesized ultrafast vortex increases with the magnitude of the topological charge. Explicit analytical expressions of ultrafast vortices containing these OAM-temporal couplings are given that can be of interest in a variety of applications, particularly in the study of their propagation and interaction with matter.

physics.optics↗

arXiv2019

Attosecond helical pulses

We find a solution of the wave equation in the paraxial approximation that describes the attosecond pulses with spatiotemporal helical structure in the phase and in the intensity recently generated by means of highly nonlinear optical processes driven by visible or infrared femtosecond vortex pulses. Having a simple analytical model for these helical pulses will greatly facilitate the study of their predicted applications, particularly their interaction with matter after their generation. It also follows from our analysis that the topological charge dispersion inherent to helical pulses allows to beat the minimum duration to which a pulsed vortex without charge dispersion is limited.

physics.optics↗

arXiv2019

Effects of orbital angular momentum on few-cycle pulse shape: Coupling between the temporal and angular momentum degrees of freedom

We describe the coupling between orbital angular momentum (OAM) and temporal degrees of freedom in pulsed Laguerre-Gauss beams. The effects of this coupling are to increase significantly the duration of few-cycle pulses when the OAM carried by the pulse is large, and to enhance spatiotemporal couplings. Contrary to other detrimental but retrievable effects such as spatial, group velocity and topological charge dispersions in these pulses, OAM-temporal coupling effects are unavoidable, and have therefore an impact in applications such as OAM-based optical communications, or in the generation of vortex-carrying attosecond pulses.

physics.optics↗

arXiv2018

Spontaneous localization in self-focusing of ultrashort light pulses

This is a summary directed to PhD students of the research work conducted on the problem of the production of "light bullets", or multidimensional wave packets that propagate without distortion in unbounded, homogeneous, nonlinear media, and on the actual nature of the self-localized light wave packets spontaneously generated in self-focusing experiments with ultrashort pulses.

physics.optics↗

arXiv2018

An upper limit to the orbital angular momentum of a vortex-carrying ultrashort pulse

The magnitude of the topological charge of the vortex of a ring-shaped pulse is found to be limited by the squared mean frequency of the pulse spectrum at the ring relative to its variance. This limitation implies a upper bound to the orbital angular momentum carried by a pulse, and a lower bound to the duration for a pulse to carry any orbital angular momentum.

physics.optics↗

arXiv2018

Self-trapped pulsed beams with finite power in pure cubic Kerr media excited by time-diffracting, space-time beams

We study the nonlinear propagation of diffraction-free, space-time wave packets, also called time-diffracting beams because their spatiotemporal structure reproduces diffraction in time. We report on the spontaneous formation of propagation-invariant, spatiotemporally compressed pulsed beams carrying finite power from exciting time-diffracting Gaussian beams in media with cubic Kerr nonlinearity at powers below the critical power for collapse, and also with other collapse-arresting nonlinearities above the critical power. Their attraction property makes the experimental observation of the self-trapped pulsed beams in cubic Kerr media feasible. The structure in the temporal and transversal dimensions of the self-trapped wave packets is shown to be the same as the structure in the axial and transversal dimensions of the self-focusing and (arrested) collapse of monochromatic Gaussian beams.

physics.optics↗

arXiv2018

Optical vortex trapping and annihilation by means of nonlinear Bessel beams in nonlinearly absorbing media

In nonlinear Kerr media at intensities such that multiphoton absorption is significant, a vortex of topological charge m in the center of a high-order nonlinear Bessel beam (NBB) can be stable and subsist endlessly. We show that the m-charged NBB is not only stable but is formed spontaneously from any other n-charged NBB and N "foreign" vortices of total charge s randomly nested in the beam cross section if n + s = m. All nested vortices merge in the center of the original NBB, which undergoes a mode conversion to the NBB that preserves the topological charge and the inward-directed power current that sustains the diffraction-free and attenuation-free propagation in the medium with nonlinear absorption. We foresee different applications such as the creation of stable, multiply charged vortices without tight alignment requirements but by spontaneous vortex combination, mixing waves or particles that the vortices can guide, fast annihilation of vortex dipoles, and cleaning of speckled beams by massive annihilation of vortices.

physics.optics↗

arXiv2018

Time-diffracting beams: On their nature, diffraction-free propagation as needles of light, and nonlinear generation

We investigate on the properties of the recently introduced time-diffracting (TD) beams in free space. They are shown to be paraxial and quasi-monochromatic realizations of localized waves, spatiotemporal localized waves travelling undistorted at arbitrary speeds. The paraxial and quasi-monochromatic regime is shown to be necessary to observe what can properly be named diffraction in time. TD beams of finite energy travelling at quasi-luminal velocities are seen to form substantially longer foci or needles of light than the so-called abruptly focusing and defocusing needle of light, or limiting TD beam of infinite speed. Exploring the properties of TD beams under Lorentz transformations and transformation by paraxial optical systems, we realize that the relativistically moving nonlinear polarization of material media induced by a strongly localized fundamental pump wave generates a TD beam at its second harmonic, whose diffraction-free behavior as a needle of light in free space can be optimized with a standard $4f$-imager system.

physics.optics↗

arXiv2017

Nonlinear lossy light bullets in self-focusing media with nonlinear absorption

We review the properties of nonlinear, multidimensional localized waves whose stationary propagation is sustained by a dynamic equilibrium between self-focusing and nonlinear losses. Their finite-energy versions preserve light bullet behavior well-beyond the characteristic diffraction or dispersion distances, and rebuild after obstacles. There exists a preferential lossy light bullet with maximum intensity and losses, defined solely by the optical properties of the medium, which is the most stable, non-conical localized wave supported by a medium with self-focusing nonlinearity and nonlinear losses. This preferential lossy light bullet acts as an attractor during self-focusing of Gaussian-like wave packets when collapse is halted by nonlinear absorption, a fact that can explain relevant characteristics of the observed light filament dynamics in media with anomalous dispersion.

physics.optics↗