Luis Plaja

• Associate Professor (Profesor Titular) at University of Salamanca

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...

We present a numerical experiment that demonstrates the possibility to capture topological phase transitions via an x-ray absorption spectroscopy scheme. We consider a Chern insulator whose topological phase is tuned via a second-order hopping. We perform time-dynamics simulations of the out-of-equilibrium laser-driven electron motion that enables...

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...

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...

We present a numerical experiment that demonstrates the possibility to capture topological phase transitions via an x-ray absorption spectroscopy scheme. We consider a Chern insulator whose topological phase is tuned via a second-order hopping. We perform time-dynamics simulations of the out-of-equilibrium laser-driven electron motion that enables...

A comprehensive theoretical investigation of the process of high-order harmonic generation induced by intense few-cycle infrared laser pulses in one-dimensional single-wall carbon nanotubes is presented. The resulting emission spectra exhibit a non-perturbative plateau at high intensities. However, unlike more conventional systems such as atoms, mo...

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...

Structured light provides unique opportunities to spatially tailor the electromagnetic field of laser beams. These include 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...

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 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.

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.

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...

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...

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...

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...

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.

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...

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...

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...

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...

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...

We demonstrate the production of EUV vortex beams, and vector-vortex beams merging the helical phase of a vortex and the spatially variant polarization of a vector beam, both carrying large orbital angular momentum per photon.

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...

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.

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...

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...

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.

Real-time observations of the coherent electron dynamics induced by ultrashort light pulses in condensed-matter systems have remained elusive due to limitations in existing attosecond devices. Here we show that attosecond transient absorption resulting from an IR-pump/x-ray probe numerical experiment performed in graphene unambiguously encodes the...

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.

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...

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 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.

We describe back-reaction as a novel correlation mechanism in the two-electron dynamics of helium atoms exposed to intense laser fields. The electron-electron correlation information is encoded as a high-energy secondary plateau in high-harmonic spectroscopy.

Pulses with a twist and torque Structured light beams can serve as vortex beams carrying optical angular momentum and have been used to enhance optical communications and imaging. Rego et al. generated dynamic vortex pulses by interfering two incident time-delayed vortex beams with different orbital angular momenta through the process of high harmo...

The nature of high-order harmonic generation process limits the harmonics emission to linear polarization. In this letter, we review the recent progress to generate elliptically or circularly polarized high-harmonic EUV pulses. We further demonstrate how complete control of polarization state of isolated high-harmonic pulse can be realized today by...

High-order harmonic generation stands as a unique nonlinear optical up-conversion process, mediated by a laser-driven electron recollision mechanism, which has been shown to conserve energy, linear momentum, and spin and orbital angular momentum. Here, we present theoretical simulations that demonstrate that this process also conserves a mixture of...

High harmonic generation in atomic or molecular targets stands as a robust mechanism to produce coherent ultrashort pulses with controllable polarization in the extreme-ultraviolet. However, the production of elliptically or circularly-polarized harmonics is not straightforward, demanding complex combinations of elliptically or circularly-polarized...

Optical interactions are governed by both spin and angular momentum conservation laws, which serve as a tool for controlling light–matter interactions or elucidating electron dynamics and structure of complex systems. Here, we uncover a form of simultaneous spin and orbital angular momentum conservation and show, theoretically and experimentally, t...

Twisted light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics and microparticle rotation. Here we introduce and experimentally validate a new class of light beams, whose unique property is associated with a temporal OAM variation along a pulse: the s...

High-harmonics—and attosecond pulses—with controllable spin and orbital angular momenta (SAM and OAM) are generated for the first time. The coupled SAM-OAM conservation laws enable exquisite control over the polarization of attosecond pulse trains.

Extreme-ultraviolet, attosecond beams with custom spin and orbital angular momentum are produced via high-harmonic generation, for the first time. Entwined angular momentum conservation rules yield exquisite control over their polarization, divergence, and vortex charge.

We report a novel scheme to generate laser-induced, ultrafast, intense (Tesla scale) magnetic fields. Three-dimensional particle-in-cell simulations show that a femtosecond azimuthally-polarized infrared vector beam, aimed to a conducting circular aperture, produces an intense axially polarized tip-shaped femtosecond magnetic field, extending over...

We report a novel scheme to generate laser-induced, ultrafast, intense (Tesla scale), spatially isolated, magnetic fields. Three-dimensional particle-in-cell simulations show that a femtosecond azimuthally-polarized infrared vector beam, aimed to a conducting circular aperture, produces an intense axially polarized tip-shaped femtosecond magnetic f...

High-order harmonic generation stands as a unique nonlinear optical up-conversion process, mediated by a laser-driven electron recollision mechanism, which has been shown to conserve energy, momentum, and spin and orbital angular momentum. Here we present theoretical simulations which demonstrate that this process also conserves a mixture of the la...

High-harmonic generation driven by femtosecond lasers makes it possible to capture the fastest dynamics in molecules and materials. However, thus far, the shortest isolated attosecond pulses have only been produced with linear polarization, which limits the range of physics that can be explored. Here, we demonstrate robust polarization control of i...

We study the high-harmonic spectrum emitted by a single-layer graphene, irradiated by an ultrashort intense infrared laser pulse. We show the emergence of the typical non-perturbative spectral features, harmonic plateau and cut-off, for mid-infrared driving fields, at fluences below the damage threshold. In contrast to previous works, using THz dri...

We present a theoretical analysis of the time-gated phase matching (ionization gating) mechanism in high-order harmonic generation for the isolation of attosecond pulses at near-infrared and mid-infrared driver wavelengths, for both few-cycle and multi-cycle driving laser pulses. Results of our high harmonic generation and three-dimensional propaga...

In this work we study the impact of chromatic focusing of few-cycle laser pulses on high-order harmonic generation (HHG) through analysis of the emitted extreme ultraviolet (XUV) radiation. Chromatic focusing is usually avoided in the few-cycle regime, as the pulse spatio-temporal structure may be highly distorted by the spatiotemporal aberrations....

Plasmon resonances are known to amplify the electromagnetic fields near metallic nanostructures. Therefore, they are considered to provide a promising scheme to generate extreme-ultraviolet harmonics, using low power drivings. During high-order harmonic generation (HHG), the driving and harmonic fields accumulate a phase difference as they propagat...

Bright, circularly polarized, extreme ultraviolet (EUV) and soft x-ray high-harmonic beams can now be produced using counter-rotating circularly polarized driving laser fields. Although the resulting circularly polarized harmonics consist of relatively simple pairs of peaks in the spectral domain, in the time domain, the field is predicted to emerg...

We present observations of the emission of XUV continua in the 20–37-eV region by high-harmonic generation with 4–7-fs pulses focused onto a Kr gas jet. The underlying mechanism relies on coherent control of the relative delays and phases between individually generated attosecond pulses, achievable by adjusting the chirp of the driving pulses and t...

High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function,...

High-order harmonic generation (HHG) driven by beams carrying orbital angular momentum has been recently demonstrated as a unique process to generate spatio-temporal coherent extreme ultraviolet (XUV)/x-ray radiation with attosecond helical structure. We explore the details of the mapping of the driving vortex to its harmonic spectrum. In particula...

We present theoretical analysis and numerical calculations for the generation of high harmonics and ultrashort pulses driven by midinfrared lasers. Our calculations are based on a framework, in which the process of harmonic generation is computed via an extension of the strong-field approximation on the microscopic level and macroscopically, the ga...