Carlos Hernández-García

• PhD in Physics
• Professor (Assistant) at University of Salamanca

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...

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 in...

High harmonic generation (HHG) stands as one of the most complex processes in strong-field physics, as it enables the conversion of laser light from the infrared to the extreme-ultraviolet or even the soft x-rays, enabling the synthesis and control of pulses lasting as short as tens of attoseconds. Accurately simulating this nonlinear and non-pertu...

High-order harmonic spectroscopy is a robust method for probing electron dynamics under the influence of a driving field, capturing phenomena as brief as attoseconds. It relies on the extreme non-linear process of high-harmonic generation (HHG), where intense laser pulses are directed at a material, causing it to emit high-energy photons in harmoni...

We report an unexpected result of the anisotropy of the nonlinear optical response of carbon nanotubes, inherent to their chirality. Using a model based on the resolution of the semiconductor Bloch equations, we theoretically demonstrate that, upon irradiation with an intense linearly polarized laser pulse along the axial direction, chiral nanotube...

In the rapidly evolving field of structured light, 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 time scale, are naturally p...

Isolated attosecond pulse (IAP) generation usually involves the use of short-medium gas cells operated at high pressures. In contrast, long-medium schemes at low pressures are commonly perceived as inherently unsuitable for IAP generation due to the nonlinear phenomena that challenge favourable phase-matching conditions. Here we provide clear exper...

The landscape of ultrafast structured light pulses has significantly advanced thanks to the ability of high-order harmonic generation (HHG) to translate the spatial properties of infrared laser beams to the extreme-ultraviolet (EUV) spectral range. In particular, the up-conversion of orbital angular momentum (OAM) has enabled the generation of high...

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 natu...

Spatiotemporal optical vortices (STOV) are structured light pulses with a unique topology that couples the spatial and temporal domains. Up to now, their generation has been limited to low-order topological charges in the visible and infrared regions of the spectrum. During the last decade, it was shown that through the process of high-order harmon...

In this work, we exploit HHG in a noble gas to merge the azimuthally twisted wavefront of a vortex beam and the spatially varying polarization of a vector beam, yielding EUV vector-vortex beams (VVB) that are tailored simultaneously in their SAM and OAM. Employing a high-resolution EUV Hartmann wavefront sensor (EUV HASO, Imagine Optic), we perform...

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...

High-order harmonic generation (HHG) arising from the nonperturbative 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...

Linearly polarized vector beams are structured lasers whose topology is characterized by a well-defined Poincaré index, which is a topological invariant during high-order harmonic generation. As such, harmonics are produced as extreme-ultraviolet vector beams that inherit the topology of the driver. This holds for isotropic targets such as noble ga...

We theoretically demonstrate a compact and robust scheme for the direct generation of extreme-ultraviolet isolated attosecond pulses from high-order harmonics driven by self-compressed sub-cycle waveforms produced in a gas-filled hollow capillary fiber.

We use artificial intelligence to simulate high-order harmonic generation driven by structured laser pulses. Our method couples time-dependent Schrödinger simulations with electromagnetic field propagation, accelerating calculations and revealing hidden signatures in the far-field harmonic emission.

The rotational symmetries of a light beam are linked with angular momenta. Thus, the spin and orbital angular momentum are respectively related to the rotational invariance of the polarization vector and of the spatial distribution of the light field. They may take values which are integer multiple of ℏ. In some cases, a generalized angular momentu...

We theoretically and experimentally demonstrate the generation of high-topological charge, extreme-ultraviolet (EUV) spatiotemporal optical vortices (STOV) from high-order harmonic generation. EUV-STOVs are unique structured light tools for exploring ultrafast topological laser-matter interactions.

Attosecond light pulses structured in their spatial intensity, phase and polarization profiles can be designed thanks to high-order harmonic generation. We report the latest advances including generation of attosecond vortex pulses with controlled angular momenta.

Spatiotemporal (STOV) and spatiospectral (SSOV) optical vortices are unique structured light tools for exploring ultrafast laser-matter interactions. We theoretically and experimentally study the high-topological charge extreme-ultraviolet/attosecond regime STOV and SSOV generation through high-order harmonic generation.

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 s...

Understanding high-order harmonic generation (HHG) from solid targets holds the key of potential technological innovations in the field of high-frequency coherent sources. Solids present optical nonlinearities at lower driving intensities, and harmonics can be efficiently emitted due to the increased electron density in comparison with the atomic a...

High harmonic generation (HHG) is one of the richest processes in strong-field physics. It allows to up-convert laser light from the infrared domain into the extreme-ultraviolet or even soft x-rays, that can be synthesized into laser pulses as short as tens of attoseconds. The exact simulation of such highly non-linear and non-perturbative process...

The technological refinements on high-power laser systems of Petawatt class unveil scenarios for light-matter interaction beyond the laser-plasma perspective. In this contribution we explore the possibility of assisting high-harmonic generation (HHG) with the strong magnetic field associated with one of those intense sources. Recently, there has be...

Exotic light fields combining non-trivial spin and angular momentum may not be eigenstates of either the spin or orbital angular momenta operators. For these fields, it is relevant to define a Generalized Angular Momentum operator of which they are eigenvectors. Their associated eigenvalues can take, depending on the case, non-integer values. We re...

Fully polarized light, cylindrical vector beams, and beams with opposite orbital angular momentum (OAM) and their superpositions are respectively represented as points on the Poincaré sphere (PS), the higher-order Poincaré sphere (HOPS) and the OAM Poincaré sphere (OAMPS). Here, we study the mapping of inner points between these spheres, which we r...

Ultrafast laser pulses provide unique tools to manipulate magnetization dynamics at femtosecond timescales, where the interaction of the electric field usually dominates over the magnetic field. Recent proposals using structured laser beams have demonstrated the possibility to produce regions where intense oscillating magnetic fields are isolated f...

One of the main constraints for reducing the temporal duration of attosecond pulses is the attochirp inherent to the process of high-order harmonic generation (HHG). Though the attochirp can be compensated in the extreme-ultraviolet using dispersive materials, this is unfeasible toward x-rays, where the shortest attosecond or even sub-attosecond pu...

Symmetries and conservation laws of energy, linear momentum, and angular momentum play a central role in nonlinear optics. Recently, paraxial light fields with nontrivial topology have been attracting a keen interest. Despite not being eigenstates of the orbital and spin angular momenta (OAM and SAM), they are eigenstates of the generalized angular...

Structured light beams have enabled a multitude of novel applications [1]. An emblematic example of structured light is the optical vortex beam exhibiting an azimuthally varying transverse phase. Optical vortices are known to carry Orbital Angular Momentum (OAM) [2] and are characterized by an annular intensity profile resulting from the on-axis ph...

Structured waves are ubiquitous for all areas of wave physics, both classical and quantum, where the wavefields are inhomogeneous and cannot be approximated by a single plane wave. Even the interference of two plane waves, or a single inhomogeneous (evanescent) wave, provides a number of nontrivial phenomena and additional functionalities as compar...

We demonstrate a new scheme for high harmonic generation driven by a 2- lobe Hermite-Gaussian mode. The passive approach allows for combining high harmonic spectroscopy and interferometry to retrieve the nonlinear dynamics of the process.

We observe experimentally a secondary plateau in UV-driven high harmonic generation in the soft X-ray region, extending up to 300 eV, due to a simultaneous double-recombination of highly-correlated electrons in helium atoms.

The concept of critical ionization fraction has been essential for high-harmonic generation, because it dictates the maximum driving laser intensity while preserving the phase matching of harmonics. In this work, we reveal a second, nonadiabatic critical ionization fraction, which substantially extends the phase-matched harmonic energy, arising bec...

In this work, the density matrix formalism that describes any standard polarization state (fully or partially polarized) is applied to describe vector beams and spatial modes with orbital angular momentum (OAM). Within this framework, we provide a comprehensive description of the mapping between the corresponding Poincaré spheres (PSs); namely: the...

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,...

We analyse the high harmonic emission from single-layer graphene driven by infrared vector beams. We demonstrate that graphene’s anisotropy offers a privileged scenario to explore non-trivial light spin-orbit couplings, which substantially extends the possibilities for the generation of high-harmonic structured beams currently studied in atomic and...

Coherent extreme-ultraviolet (EUV)/x-ray laser sources, structured in their temporal/spectral, spatial and angular momentum properties are emerging as unique tools to probe the nanoworld. One of the key ingredients for the emergence of such sources is the extraordinary coherence in the up-conversion of infrared laser sources through the highly nonl...

After more than two decades of attosecond physics, the generation and control of the shortest laser pulses available remains as a complex task. One of the main limitations of reducing the temporal duration of attosecond pulses emitted from high-order harmonic generation (HHG) is the attochirp. In this contribution, we demonstrate that HHG assisted...

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 i...

Vigorous efforts to harness the topological properties of light have enabled a multitude of novel applications. Translating the applications of structured light to higher spatial and temporal resolutions mandates their controlled generation, manipulation, and thorough characterization in the short-wavelength regime. Here, we resort to high-order ha...

Symmetries and conservation laws of energy, linear momentum and angular momentum play a central role in physics, in particular in nonlinear optics. Recently, light fields with non trivial topology, such as polarization Möbius strips or torus-knot beams, have been unveiled. They cannot be associated to well-defined values of orbital and spin angular...

Present mass production of large-area single-layer graphene relies fundamentally on chemical vapor deposition methods. The generation of grain boundaries, which divides the sample into a set of crystalline domains, is inherent to these fabrication methods. Recent studies have demonstrated a strong anisotropy in the ultrafast non-linear response of...

Vigorous efforts to harness the topological properties of light have enabled a multitude of novel applications. Translating the applications of structured light to higher spatial and temporal resolutions mandates their controlled generation, manipulation, and thorough characterization in the short-wavelength regime. Here, we resort to high-order ha...

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 pro...

The nonlinear process of high harmonic generation (HHG) stands as a highly coherent tool to up-convert the properties of infrared/visible light into the extreme ultraviolet regime (XUV), or even into the soft x-rays. Thanks to HHG, we experimentally and theoretically report the generation of XUV scalar and vectorial vortices with very high topologi...

Extending the photon energy range of bright high-harmonic generation to cover the entire soft X-ray region is important for many applications in science and technology. The concept of critical ionization fraction has been essential, because it dictates the maximum driving laser intensity that can be used while preserving bright harmonic emission. I...

Using multiplexed broadband ptychography, we investigate high-order harmonic vector beams. The method retrieves highly structured beam profiles and wavefronts in addition to spectral resolution without grating dispersion.

Recent studies in high-order harmonic generation (HHG) in solid targets reveal new scenarios of extraordinary rich electronic dynamics, in comparison to the atomic and molecular cases. For the later, the main aspects of the process can be described semiclassically in terms of electrons that recombine when the trajectories revisit the parent ion. HH...

The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wave function and, in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse nec...

Structured light in the short-wavelength regime opens exciting avenues for the study of ultrafast spin and electronic dynamics. Here, we demonstrate theoretically and experimentally the generation of vector-vortex beams (VVB) in the extreme ultraviolet through high-order harmonic generation (HHG). The up-conversion of VVB, which are spatially tailo...

Recent developments of high harmonic generation (HHG) have enabled the production of structured extreme-ultraviolet (EUV) ultrafast laser beams with orbital angular momentum (OAM). Precise manipulation and characterization of their spatial structure is paramount for their application in state-of-the-art ultrafast studies. In this work, we report th...

Using multiplexed broadband ptychography, we characterize the EUV light from high-order harmonic generation. The method allows for spectrally resolved complex beam profiles to be imaged for different harmonic outputs without grating dispersion.

The advent of intense coherent XUV sources opens up a novel regime of extreme nonlinear light-mater interactions. We theoretically identify attosecond resonant dynamics in high harmonic generation driven by XUV pulses, leading to efficient X-ray emission.

We demonstrate that polarization control and characterization of high-harmonic generation in non-collinear geometry performs as an excellent ellipsometry that can fully retrieve the amplitude and phase of ultrafast dipole response, advancing high harmonic spectroscopy.

The nonlinear optical process of high harmonic generation (HHG) provides an appealing opportunity to up-convert the properties of visible or infrared (IR) beams into the extreme ultraviolet regime (XUV), or even into the x-ray regime. This is particularly relevant in the context of the emerging field of structured light 1 , since a precise control...

Light beams structured in their phase and polarization have proven their usefulness for a wide range of applications. On the one hand, optical vortex beams carry orbital angular momentum (OAM) and are characterized by their azimuthally twisting wavefront. On the other hand, vector beams exhibit a spatially varying polarization. In this work, we uti...

Structured light in the short-wavelength regime is emerging as a paramount tool to explore ultrafast spin and electronic dynamics. In this work, we demonstrate experimentally and theoretically the up-conversion of Vector-Vortex Beams (VVB) from the IR to the XUV regime, introducing a unique configuration of XUV coherent radiation spatially structur...

High-harmonic generation (HHG) is a unique tabletop light source with femtosecond-to-attosecond pulse duration and tailorable polarization and beam shape. Here, we use counter-rotating femtosecond laser pulses of 0.8 µm and 2.0 μm to extend the photon energy range of circularly polarized high-harmonics and also generate single-helicity HHG spectra....

The coherent manipulation of the electron wavefunction at the atomic spatial and temporal scales is the fundamental breakthrough underlying far-reaching ultrafast phenomena as high-order harmonic radiation and attosecond pulse generation. In this work, we present a next step in the coherent control of matter waves by translating the concept of Talb...

The extreme nonlinear optical process of high-harmonic generation (HHG) makes it possible to map the properties of a laser beam onto a radiating electron wavefunction, and in turn, onto the emitted x-ray light. Bright HHG beams typically emerge from a longitudinal phased distribution of atomic-scale quantum antennae. Here, we form a transverse neck...

The visualization of collagen-based tissues imaged with second harmonic generation (SHG) microscopy strongly depends on the polarization state of the scanning laser beam. This has been often explored using spatially homogeneous states (mainly linear or circular). However, vector beams (i.e. laser beams with spatially non-uniform polarization states...

Over the past decades, the use of ultrafast laser beams at a highly varied range of applications (e.g., biophotonics, material science, telecommunications, physics, chemistry, etc.) has evolved, demanding more complex light structures not only in time (i.e., ultrafast pulses), but also spatiotemporally and in polarization (e.g., vector beams, time...

We report the generation, and intensity, wavefront, modal content characterization of optical vortices with topological charges as high as 100 in the extreme-ultraviolet spectral range. Furthermore, we complement the experimental observations with advanced simulations.

We theoretically and experimentally introduce a novel structured EUV beam—a vector-vortex—which combines the helical phase and inhomogeneous polarization of vortex and vector beams. These beams are emitted as an azimuthally polarized attosecond light-spring.

Polarization engineering and characterization of coherent high-frequency radiation are essential to investigate and control the symmetry properties of light–matter interaction phenomena at their most fundamental scales. This work demonstrates that polarization control and characterization of high-harmonic generation provides an excellent ellipsomet...

The efficiency of high-harmonic generation (HHG) from a macroscopic sample is strongly linked to the proper phase matching of the contributions from the microscopic emitters. We develop a combined micro+macroscopic theoretical model that allows us to distinguish the relevance of high-order harmonic phase matching in single-layer graphene. For a Gau...

By driving high-harmonics with necklace laser beams, we produce combs with tunable frequency content and spacing, up to the soft x-ray region. The emitted harmonics also exhibit distinct spatial profiles and lower divergence than Gaussian-driven harmonics.

We demonstrate multi-modal transmission ptychography on high-order harmonic vector beams. The process retrieves highly structured beam profiles and wavefronts in addition to spectral resolution without grating dispersion.

The synchronous control of spin and orbital angular momentum in high-harmonic generation allows us to introduce experimentally and theoretically a novel XUV structured beam with spatially-varying polarization and phase, high topological charge, and robust propagation.

Ultrafast laser pulses generated at the attosecond timescale represent a unique tool to explore the fastest dynamics in matter. An accurate control of their properties, such as polarization, is fundamental to shape three-dimensional laser-driven dynamics. We introduce a technique to generate attosecond pulse trains whose polarization state varies f...

In this paper, we review the development and application of coherent short wavelength light sources implemented using the high harmonic generation (HHG) process. The physics underlying HHG brought quantum physics into the domain of attosecond time-scales for the first time. The observation and manipulation of electron dynamics on such short time-sc...

The use of structured ultrashort pulses with coupled spatiotemporal properties is emerging as a key tool for ultrafast manipulation. Ultrafast vector beams are opening exciting opportunities in different fields such as microscopy, time-resolved imaging, nonlinear optics, particle acceleration or attosecond science. Here, we implement a technique fo...

Few-electron atoms interacting with electromagnetic fields provide for privileged scenarios for disentangling elemental correlation mechanisms. We demonstrate that high-order harmonic generation (HHG) from neutral helium presents a distinctive trace of correlation back reaction in the electron dynamics prior to ionization. We identify a mechanism i...

High-order harmonic generation from neutral He presents a distinctive trace of correlation back-reaction: a secondary plateau extending the emission towards higher frequencies. We identify a novel mechanism prior to ionization in which the field interacts with one of the electrons, while the other is excited to a Rydberg level through the Coulomb i...

We study high-order harmonic generation (HHG) in armchair-type single-wall carbon nanotubes (SWNTs) driven by ultrashort, mid-infrared laser pulses. For a SWNT with chiral indices (n, n), we demonstrate that HHG is dominated by bands |m| = n − 1 and that the cut-off frequency saturates with intensity, as it occurs in the case of single layer graphe...

We demonstrate that the standard picture of strong-field tunnel-ionization from molecules should be reformulated. The extended nature of the molecular potential implies the separation of some of the molecular sites from the edge of the ionization barrier. We show that the dependence of the tunnel probability with the distance to the barrier is tran...

We perform a complete measurement of ultrashort pulsed beams where the polarization evolves spatially and temporally. We apply our technique to characterize ultrafast shaped vector beams which combine temporal polarization gating and radial polarization.

We demonstrate that polarization control and characterization of high-harmonic generation in non-collinear geometry perform as excellent ellipsometry that can precisely retrieve the non-perturbative dipole response, both amplitude and phase, in the extremely nonlinear up-conversion process.

We study the macroscopic build-up of the high-harmonic signal in single-layer graphene. Our results show that the emission is dominated by a phase-matched ring.

We demonstrate that polarization control and characterization of high-harmonic generation in non-collinear geometry performs as an excellent ellipsometry that can fully retrieve the amplitude and phase of ultrafast dipole response, advancing attosecond metrology.