arXiv subjects

Carlos Hernández-García

Publications and source records attributed to Carlos Hernández-García.

At least 19 recordsLinked to original sources

arXiv2026

Attosecond pulse trains from graphene via macroscopic phase-matching in high harmonic generation

Attosecond pulse generation in solids remains challenging due to the complex phase structure arising from multiple electronic pathways in high harmonic generation (HHG). Here, we identify the macroscopic conditions that enable the synthesis of attosecond pulse trains in graphene via HHG. Using numerical simulations that combine microscopic and macroscopic HHG in single-layer graphene, we show that harmonic contributions associated with different emission times acquire distinct far-field beam-divergence properties, analogous to the short- and long-trajectory contributions in gas-phase HHG. Our results identify how transverse phase-matching can be engineered in solid-state HHG to select short-time or long-time electronic contributions through proper tailoring of the driving-field waveform. In particular, the macroscopic suppression of long-time contributions associated with delayed electron-hole recombinations or imperfect recollisions (usually removed in semiconductor Bloch equation calculations by introducing artificial decoherence times) leads to clean, positively chirped, attosecond pulse trains with a temporal quality comparable to that achieved in gas-phase HHG. These results establish a general framework for controlling attosecond emission in solid-state HHG and provide a route toward compact solid-state attosecond sources.

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↗

arXiv2025

Attosecond Light Skyrmion Pulses via High Harmonic Generation

Paraxial light skyrmions are topological configurations that map a spatial domain of the field onto the full Poincaré sphere of polarization states. While optical skyrmions have been explored in continuous-wave regimes, their realization in the ultrafast domain remains open. Here we demonstrate that attosecond skyrmion pulses can be generated via high-harmonic generation. Advanced simulations combining single-atom strong-field theory and macroscopic propagation reveal that an infrared linearly polarized vector beam with fractional orbital angular momentum produces extreme-ultraviolet harmonic fields with nearly identical skyrmion polarization distributions across a broad spectral range. Using 1.2 $μ$m driving fields and experimentally feasible spectral filtering, we show that the coherent superposition of consecutive harmonics centered at 70 eV yields a train of skyrmion pulses with $\sim500$ attoseconds duration. Our results open opportunities to use structured attosecond light with topological polarization textures in fields as ultrafast control, imaging and spectroscopy.

physics.optics↗

arXiv2025

Compact, intense attosecond sources driven by hollow Gaussian beams

High-order harmonic generation (HHG) enables the up-conversion of intense infrared or visible femtosecond laser pulses into extreme-ultraviolet attosecond pulses. However, the highly nonlinear nature of the process results in low conversion efficiency, which can be a limitation for applications requiring substantial pulse energy, such as nonlinear attosecond time-resolved spectroscopy or single-shot diffractive imaging. Refocusing of the attosecond pulses is also essential to achieve a high intensity, but difficult in practice due to strong chromatic aberrations. In this work, we address both the generation and the refocusing of attosecond pulses by sculpting the driving beam into a ring-shaped intensity profile with no spatial phase variations, referred to as a Hollow Gaussian beam (HGB). Our experimental and theoretical results reveal that HGBs efficiently redistribute the driving laser energy in the focus, where the harmonics are generated on a ring with low divergence, which furthermore decreases with increasing order. Although generated as a ring, the attosecond pulses can be refocused with greatly reduced chromatic spread, therefore reaching higher intensity. This approach enhances the intensity of refocused attosecond pulses and enables significantly higher energy to be delivered in the driving beam without altering the focusing conditions. These combined advantages open pathways for compact, powerful, tabletop, laser-driven attosecond light sources.

physics.optics↗

arXiv2025

Enhanced Control of High Harmonic Generation in Mixed Argon-Helium Gaseous Media

High harmonic generation (HHG) in gaseous media provides a robust method for producing coherent extreme-ultraviolet (EUV) radiation and attosecond pulses. However, the spectral and temporal properties of these pulses -- such as bandwidth and chirp -- are fundamentally limited by the underlying generation mechanisms. Typically, tailoring the EUV emission involves modifying the properties of the intense infrared femtosecond driving pulse, and/or the macroscopic laser-matter configuration. Here, we focus on controlling the HHG process through the gas specie, introducing mixed-gas targets as a practical approach to enhance control over the EUV harmonic radiation. Through advanced simulations assisted by artificial intelligence that take into account both the quantum microscopic and macroscopic aspects of HHG, we demonstrate how mixtures of argon and helium modulate the emitted EUV harmonics. A simple model reveals that these modulations arise from coherent interference between harmonics emitted by different species at the single-atom level, and that they can be tuned by adjusting the macroscopic relative concentrations. Furthermore, by spatially separating the gas species into two distinct jets in a symmetric configuration, we gain additional control over the whole harmonic bandwidth. This strategy provides a realistic and versatile pathway to tailor EUV light and attosecond sources via HHG, while also enabling the identification of species-specific contributions to the process.

physics.optics↗

arXiv2025

Self-interfering high harmonic beam arrays driven by Hermite-Gaussian beams

The use of structured light to drive highly nonlinear processes in matter not only enables imprinting spatially-resolved properties onto short-wavelength radiation, but also opens alternative avenues for exploring the dynamics of nonlinear laser-matter interactions. In this work, we experimentally and theoretically explore the unique properties of driving high-order harmonic generation (HHG) with Hermite-Gaussian beams. HHG driven by Laguerre-Gauss modes results in harmonics that inherit the azimuthal Laguerre-Gauss modal structure, with their topological charge scaling according to orbital angular momentum conservation. In contrast, when HHG is driven by Hermite-Gauss beams, the harmonic modes do not show a direct correspondence to the driving modal profile. Our experimental measurements using HG$_{0,1}$ and HG$_{1,1}$ modes, which are in excellent agreement with our numerical simulations, show that the lobes of the Hermite-Gauss driving beams effectively produce a set of separate phase-locked harmonic beamlets which can interfere downstream. This self-interference, which can be adjusted through the relative position between the gas target and the driving beam focus, can be exploited for precision extreme-ultraviolet interferometry. We demonstrate a simple application to calibrate the dispersion of an extreme-ultraviolet diffraction grating. In addition, we show through simulations that the array of harmonic beamlets can be used as an illumination source for single-shot extreme-ultraviolet ptychography.

physics.optics↗

arXiv2024

Filamentation-Assisted Isolated Attosecond Pulse Generation

Isolated attosecond pulses (IAPs) generated by few-cycle femtosecond lasers are essential for capturing ultrafast dynamics in atoms, molecules, and solids. Nonetheless, the advancement of attosecond science critically depends on achieving stable, high-temporal-contrast IAPs. Our study reveals a universal scenario in which self-compression of the infrared driver in high harmonic generation in extended gas media leads to high-contrast high-frequency IAP generation. Our experimental and theoretical results reveal that filamentation in a semi-infinite gas cell not only shapes the infrared driving pulse spatially and temporally, but also creates a stable propagation region where high harmonic generation is phase-matched, leading to the production of bright IAPs. In an argon-filled gas cell, filamentation notably reduces the pulse duration of Yb-based 1030 nm pulses from 4.7 fs to 3.5 fs, while simultaneously generating high-contrast 200-attosecond IAPs at 70 eV. We demonstrate the universality of filamentation-assisted IAP generation, showing that post-compressed Yb-based laser filaments in neon and helium yield even shorter IAPs: 69-attoseconds at 100 eV, and 65-attoseconds IAPs at 135 eV, respectively. This spatiotemporal reshaping of few-cycle pulses through filamentation possesses immediate impacts on both post-compression techniques and attosecond-based technologies.

physics.optics↗

arXiv2024

Circularly polarized high harmonic beams carrying self-torque or time-dependent orbital angular momentum

In the rapidly evolving field of structured light, the self-torque has been recently defined as an intrinsic property of light beams carrying time-dependent orbital angular momentum. In particular, extreme-ultraviolet (EUV) beams with self-torque -- exhibiting a topological charge that continuously varies on the subfemtosecond timescale -- are naturally produced in high-order harmonic generation (HHG) when driven by two time-delayed intense infrared vortex beams with different topological charges. Until now, the polarization state of such EUV beams carrying self-torque has been restricted to linear states due to the drastic reduction in the harmonic up-conversion efficiency with increasing the ellipticity of the driving field. In this work, we theoretically demonstrate how to control the polarization state of EUV beams carrying self-torque, from linear to circular. The extremely high sensitivity of HHG to the properties of the driving beam allows us to propose two different driving schemes to circumvent the current limitations to manipulate the polarization state of EUV beams with self-torque. Our advanced numerical simulations are complemented with the derivation of selection rules of angular momentum conservation, which enable precise tunability over the angular momentum properties of the harmonics with self-torque. The resulting high-order harmonic emission, carrying time-dependent orbital angular momentum with a custom polarization state, can expand the applications of ultrafast light-matter interactions, particularly in areas where dichroic or chiral properties are crucial, such as magnetic materials or chiral molecules.

physics.optics↗

arXiv2024

Generation of intense, polarization-controlled magnetic fields with non-paraxial structured laser beams

The ability to spatially separate the electric and magnetic fields of a light beam enables the inspection of laser-matter interactions driven solely by optical magnetic fields. However, magnetic field excitations are commonly orders of magnitude weaker than those driven by the electric field. Several studies have already demonstrated the isolation of an intense, linearly polarized magnetic field using structured light. In this work, we report the generation of isolated high intensity magnetic fields with controlled polarization state in the non-paraxial regime using structured laser beams. Our theoretical findings highlight a significant enhancement in the amplitude of the longitudinal magnetic field carried by an azimuthally polarized laser under tight-focusing conditions. Furthermore, by implementing a multiple-beam configuration, we achieve precise control over the polarization state and amplitude of the spatially isolated magnetic field. We report the generation of polarization-controlled magnetic fields reaching up to tens of Tesla, even from moderately intense laser beams of $\sim 10^{12} \, \mathrm{W}/\mathrm{cm}^2$. Our study paves the way for ultraintense interactions with circularly polarized magnetic fields from a feasible experimental setup point of view, particularly interesting to probe ferromagnetic materials and chiral media.

physics.optics↗

arXiv2024

Attosecond Rabi Oscillations in High Harmonic Generation Resonantly Driven by Extreme Ultraviolet Laser Fields

High-order harmonic generation driven by intense extreme ultraviolet (EUV) fields merges quantum optics and attosecond science, giving rise to an appealing route for the generation of coherent EUV and soft X-ray light for high-resolution imaging and spectroscopies. We theoretically investigate ultrafast resonant dynamics during the interaction of He atoms with strong extreme ultraviolet pulses. At high driving intensities, we identify record fast attosecond Rabi oscillations imprinting observable signatures in the high harmonic spectrum. At field strengths suppressing the Coulomb potential barrier for all the bounded states, we demonstrate the survival of the attosecond two-level dynamics for several Rabi cycles. Consequently, this intense EUV laser-atom interaction reveals a new strong-field scenario where the resonant coupling of two-level bound-bound transitions prevails, contrasting with the dominance of bound-continuum transitions in the conventional strong-field infrared regimes. These findings set an interesting perspective for extreme attosecond nonlinear optics with intense short-wavelength fields.

physics.atom-ph↗

arXiv2024

Bright Coherent Ultrahigh Harmonics in the keV X-Ray Regime from Mid-Infrared Femtosecond Lasers

High harmonic generation traditionally combines ~100 near-infrared laser photons, to generate bright, phase matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here we show that by guiding a mid-infrared femtosecond laser in a high pressure gas, ultrahigh harmonics can be generated up to orders > 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to > 1.6 keV, allowing in-principle the generation of pulses as short as 2.5 attoseconds. The multi-atmosphere gas pressures required for bright, phase matched emission also supports laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density, the recolliding electrons responsible for high harmonic generation encounter other atoms during the emission process.

physics.optics↗

arXiv2024

Attosecond vortex pulse trains

The landscape of ultrafast structured light pulses has recently evolved driven by the capability of high-order harmonic generation (HHG) to up-convert orbital angular momentum (OAM) from the infrared to the extreme-ultraviolet (EUV) spectral regime. Accordingly, HHG has been proven to produce EUV vortex pulses at the femtosecond timescale. Here we demonstrate the generation of attosecond vortex pulse trains, i.e. a succession of attosecond pulses with a helical wavefront, resulting from the synthesis of a comb of EUV high-order harmonics with the same OAM. By driving HHG with a polarization tilt-angle fork grating, two spatially separated circularly polarized high-order harmonic beams with order-independent OAM are created. Our work opens the route towards attosecond-resolved OAM light-matter interactions.

physics.optics↗

arXiv2024

All-optical non-linear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields

Ultrafast laser pulses provide unique tools to manipulate magnetization dynamics at femtosecond timescales, where the interaction of the electric field -- such as excitation of spin carriers to non-equilibrium states, generation of localized charge currents, demagnetization, or inverse Faraday effect -- dominates over the magnetic field. Recent proposals using structured laser beams have enlightened the possibility to generate intense femtosecond magnetic fields, spatially isolated from the electric field. Here we demonstrate the relevance of this novel scenario to femtomagnetism, unveiling the purely precessional, non-linear, chiral response of the magnetization when subjected to circularly polarized magnetic fields. This fundamental result not only opens an avenue in the study of laser-induced ultrafast magnetization dynamics, but also sustains technological implications as a route to promote all-optical non-thermal magnetization switching both at shorter timescales -- towards the attosecond regime -- and at THz frequencies.

cond-mat.mes-hall↗

arXiv2024

Optical Magnetic Field Enhancement using Ultrafast Azimuthally Polarized Laser Beams and Tailored Metallic Nanoantennas

Structured light provides unique opportunities to spatially tailor the electromagnetic field of laser beams. This includes the possibility of a sub-wavelength spatial separation of their electric and magnetic fields, which would allow isolating interactions of matter with pure magnetic (or electric) fields. This could be particularly interesting in molecular spectroscopy, as excitations due to electric and -- usually very weak -- magnetic transition dipole moments can be disentangled. In this work, we show that the use of tailored metallic nanoantennas drastically enhances the strength of the longitudinal magnetic field carried by an ultrafast azimuthally polarized beam (by a factor of $\sim65$), which is spatially separated from the electric field by the beam's symmetry. Such enhancement is due to favorable phase-matching of the magnetic field induced by the electronic current loops created in the antennas. Our particle-in-cell simulation results demonstrate that the interaction of moderately intense ($\sim10^{11}$ W/cm$^2$) and ultrafast azimuthally polarized laser beams with conical, parabolic, Gaussian, or logarithmic metallic nanoantennas provide spatially isolated magnetic field pulses of several tens of Tesla.

physics.optics↗

arXiv2023

Robust isolated attosecond pulse generation with self-compressed sub-cycle drivers from hollow capillary fibers

High-order harmonic generation (HHG) arising from the non-perturbative interaction of intense light fields with matter constitutes a well-established tabletop source of coherent extreme-ultraviolet and soft X-ray radiation, which is typically emitted as attosecond pulse trains. However, ultrafast applications increasingly demand isolated attosecond pulses (IAPs), which offer great promise for advancing precision control of electron dynamics. Yet, the direct generation of IAPs typically requires the synthesis of near-single-cycle intense driving fields, which is technologically challenging. In this work, we theoretically demonstrate a novel scheme for the straightforward and compact generation of IAPs from multi-cycle infrared drivers using hollow capillary fibers (HCFs). Starting from a standard, intense multi-cycle infrared pulse, a light transient is generated by extreme soliton self-compression in a HCF with decreasing pressure, and is subsequently used to drive HHG in a gas target. Owing to the sub-cycle confinement of the HHG process, high-contrast IAPs are continuously emitted almost independently of the carrier-envelope phase (CEP) of the optimally self-compressed drivers. This results in a CEP-robust scheme which is also stable under macroscopic propagation of the high harmonics in a gas target. Our results open the way to a new generation of integrated all-fiber IAP sources, overcoming the efficiency limitations of usual gating techniques for multi-cycle drivers.

physics.optics↗

arXiv2023

Interplay of crystal symmetries and light's topology in high harmonic spectroscopy

Structured ultrafast laser beams offer unique opportunities to explore the interplay of the angular momentum of light with matter at the femtosecond scale. Linearly polarized vector beams are paradigmatic examples of structured beams whose topology is characterized by a well-defined Poincaré index. It has been demonstrated that the Poincaré index is a topological invariant during high-order harmonic generation from isotropic targets, such as noble gases. As a result, harmonics are produced as extreme-ultraviolet vector beams, with the same topology as the driver. We demonstrate that this simple conservation rule does not apply to crystalline solids, characterized by their anisotropic non-linear response to the driving excitation. In this context, we identify the topological properties of the harmonic field as unique probes, sensitive to both the microscopic and macroscopic features of the target's complex non-linear response. Our simulations, performed in single-layer graphene but extendable to other solid targets, show that the harmonic field is split into a multi-beam structure whose topology -- different from that of the driver -- encodes information about laser-driven electronic dynamics. Our work opens the route towards using the topological analysis of the high-order harmonic field as a novel spectroscopic tool to reveal the coupling of light and target symmetries in the non-linear response of matter.

physics.optics↗

arXiv2023

Tunable Tesla-scale magnetic attosecond pulses through ring-current gating

Coherent control over electron dynamics in atoms and molecules using high-intensity circularly-polarized laser pulses gives rise to current loops, resulting in the emission of magnetic fields. We propose and demonstrate with ab-initio calculations ``current-gating" schemes to generate direct or alternating-current magnetic pulses in the infrared spectral region, with highly tunable waveform and frequency, and showing femtosecond-to-attosecond pulse duration. In optimal conditions, the magnetic pulse can be highly isolated from the driving laser and exhibits a high flux density ($\sim1$ Tesla at few hundred nanometers from the source, with a pulse duration of 787 attoseconds) for application in forefront experiments of ultrafast spectroscopy. Our work paves the way toward the generation of attosecond magnetic fields to probe ultrafast magnetization, chiral responses, and spin dynamics.

physics.optics↗

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↗