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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.

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., Comment: Main text (6 pages) and supplementary materials (5 pages)

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., Comment: 32 pages, 9 figures (4 in main text, 5 in supplemental materials)

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., Comment: Main text (1-14 pages) and supplemental material (14-16 pages)

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., Comment: 10 pages, 7 figures

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., Comment: 18 pages, 5 figures

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 Poincar\'e index. It has been demonstrated that the Poincar\'e 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.

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., Comment: Main text (7 pages including references, 3 figures), Supplementary material (8 pages, 3 figures)

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., Comment: This is the version of the article before peer review or editing, as submitted by an author to Journal of Optics. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it

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 M\"obius strips or torus-knot beams, have been unveiled. They cannot be associated to well-defined values of orbital and spin angular momenta (OAM and SAM), but are invariant under coordinated rotations, i.e. rotational symmetries that are generated by the generalized angular momentum (GAM) operator, a mixture of the OAM and SAM operators. The discovery of the GAM, which at variance with integer-valued OAM and SAM, can have arbitrary value, and raises the question of its conservation in nonlinear optical processes. By driving high harmonic generation with a polarization M\"obius strip and implementing novel OAM characterization methods in the XUV range, we experimentally observe the conservation of the GAM, each harmonic carrying a precise half-integer GAM charge equal to that of the fundamental field multiplied by the harmonic order. The GAM is thus revealed as the appropriate quantum number to describe nonlinear processes driven by light fields containing topological polarization singularities., Comment: 4 figures in the main text and 13 in the supplemental document

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 harmonic generation (HHG) in a noble gas to upconvert near-infrared (IR) vector, vortex, and vector-vortex driving beams that are tailored respectively in their Spin Angular Momentum (SAM), Orbital Angular Momentum (OAM), and simultaneously in their SAM and OAM. We show that HHG enables the controlled generation of extreme-ultraviolet (EUV) vector beams exhibiting various spatially-dependent polarization distributions, or EUV vortex beams with a highly twisted phase. Moreover, we demonstrate the generation of EUV vector-vortex beams (VVB) bearing combined characteristics of vector and vortex beams. We rely on EUV wavefront sensing to unambiguously affirm the topological charge scaling of the HHG beams with the harmonic order. Interestingly, our work shows that HHG allows for a synchronous controlled manipulation of SAM and OAM. These EUV structured beams bring in the promising scenario of their applications at nanometric spatial and sub-femtosecond temporal resolutions using a table-top harmonic source., Comment: 13 pages, 6 figures

Electron sources are essential to an array of electron accelerator supporting research in high-energy physics and beyond. This report summarizes the "Snowmass 2021 Electron Source Workshop" which reviewed the current state-of-the art research and identified some possible research directions.

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.

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. In this work, we reveal a second, nonadiabatic critical ionization fraction that substantially extends the maximum phase-matched high-harmonic photon energy, that arises due to strong reshaping of the intense driving laser field. We validate this understanding through a systematic comparison between experiment and theory, for a wide range of pulse durations and driving laser wavelengths. In particular, high harmonics driven by intense few-cycle pulses experience the most pronounced spectral reshaping, significantly extending the bright photon energy range. We also present an analytical model that predicts the spectral extension that can be achieved for different driving lasers. This reveals an increasing role of nonadiabatic critical phase matching when driven by few-cycle mid-infrared lasers. These findings are important for the development of high-brightness soft X-ray sources for applications in spectroscopy and imaging., Comment: 18 pages, 4 figures

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 necklace-shaped phased array of HHG emitters, where orbital angular momentum conservation allows us to tune the line spacing and divergence properties of extreme-ultraviolet and soft X-ray high harmonic combs. The on-axis HHG emission has extremely low divergence, well below that obtained when using Gaussian driving beams, which further decreases with harmonic order. This work provides a new degree of freedom for the design of harmonic combs, particularly in the soft X-ray regime, where very limited options are available. Such harmonic beams can enable more sensitive probes of the fastest correlated charge and spin dynamics in molecules, nanoparticles and materials., Comment: Submitted to Science Advances

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 for the full characterization of structured time-dependent polarization light waveforms with spatiotemporal resolution, using a compact twofold spectral interferometer, based on in-line bulk interferometry and fibre-optic coupler assisted interferometry. We measure structured infrared femtosecond vector beams, including radially polarized beams and complex-shaped beams exhibiting both temporal and spatial evolving polarization. Our measurements confirm that light waveforms with polarization evolving at the micrometer and femtosecond scales can be achieved through the use of structured waveplates and polarization gates. This new scale of measurement achieved will open the way to predict, check and optimize applications of structured vector beams at the femtosecond and micrometer scales., Comment: Information: 37 pages, 15 figures, final version. This work was partially supported by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Action with grant agreement No. 798264, project FastMeasure entitled 'Development and industry transfer of new techniques: full characterization of vector ultrashort pulsed laser beams'

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 self-torque of light. Self-torque is a phenomenon that can arise from matter-field interactions in electrodynamics and general relativity, but to date, there has been no optical analog. In particular, the self-torque of light is an inherent property, which is distinguished from the mechanical torque exerted by OAM beams when interacting with physical systems. We demonstrate that self-torqued beams in the extreme-ultraviolet (EUV) naturally arise as a necessary consequence of angular momentum conservation in non-perturbative high-order harmonic generation when driven by time-delayed pulses with different OAM. In addition, the time-dependent OAM naturally induces an azimuthal frequency chirp, which provides a signature for monitoring the self-torque of high-harmonic EUV beams. Such self-torqued EUV beams can serve as unique tools for imaging magnetic and topological excitations, for launching selective excitation of quantum matter, and for manipulating molecules and nanostructures on unprecedented time and length scales., Comment: 24 pages, 4 figures

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 field, extending over micrometer distances and being isolated from the electric field. Our results are backed-up by an analytic model, demonstrating the underlying physics and guiding for optimal parameters. In particular, we find the conditions under which the magnetic nanoprobe is substantially enhanced, reaching 4 T when driven by a $10^{11}$ W/cm$^2$ laser field, which reflects a selective enhancement by a factor of $\sim$6. Our scheme offers a promising tool to control, probe and tailor magnetic nanodomains in femtosecond timescales through pure magnetic interaction by using structured laser beams.

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 latter, the torus-knot angular momentum $J_\gamma$, by producing high-order harmonics with driving pulses that are invariant under coordinated rotations. We demonstrate that the charge $J_\gamma$ of the emitted harmonics scales linearly with the harmonic order, and that this conservation law is imprinted onto the polarization distribution of the emitted spiral of attosecond pulses. We also demonstrate how the nonperturbative physics of high-order harmonic generation affect the torus-knot angular momentum of the harmonics, and we show that this configuration harnesses the spin selection rules to channel the full yield of each harmonic into a single mode of controllable orbital angular momentum., Comment: Accepted Manuscript

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. Here, however, we demonstrate it as an additional control parameter to modify the generated XUV radiation. We present experiments where few-cycle pulses are focused by a singlet lens in a Kr gas jet. The chromatic distribution of focal lengths allows us to tune HHG spectra by changing the relative singlet-target distance. Interestingly, we also show that the degree of chromatic aberration needed to this control does not degrade substantially the harmonic conversion efficiency, still allowing for the generation of supercontinua with the chirped-pulse scheme, demonstrated previously for achromatic focussing. We back up our experiments with theoretical simulations reproducing the experimental HHG results depending on diverse parameters (input pulse spectral phase, pulse duration, focus position) and proving that, under the considered parameters, the attosecond pulse train remains very similar to the achromatic case, even showing cases of isolated attosecond pulse generation for near single-cycle driving pulses., Comment: 8 pages, 7 figures

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 propagate through the target. In a typical set-up -a laser focused into a gas jet or cell- the propagation distances amount to several wavelengths, and the cumulative phase-mismatch affects strongly the efficiency and properties of the harmonic emission. In contrast, HHG in metallic nanostructures is considered to overcome this limitations, as the common sources of phase mismatch -optical density and focusing geometry- are negligible for subwavelength propagation distances. Nevertheless, in this letter we demonstrate that phase matching still plays a relevant role in HHG from nanostructures. Our computations attribute this to the non-perturbative character of HHG, that links the phase of the harmonics to the intensity distribution of the driving field. We also show that widely used applications of phase matching control, such as quantum path selection and the increase of contrast in attosecond pulse generation, are also feasible at the nanoscale., Comment: 9 pages, 7 figures

This paper summarizes the discussions, presentations, and activity of the Future Light Sources Workshop 2012 (FLS 2012) working group dedicated to Electron Sources. The focus of the working group was to discuss concepts and technologies that might enable much higher peak and average brightness from electron beam sources. Furthermore the working group was asked to consider methods to greatly improve the robustness of operation and lower the costs of providing electrons., Comment: 11 pages, 7 figures, summary paper from working group Future Light Sources 2012 Workshop at Newport News, Virginia, USA (http://www.jlab.org/conferences/FLS2012/)