arXiv subjects

Ofer Neufeld

Publications and source records attributed to Ofer Neufeld.

At least 19 recordsLinked to original sources

arXiv2026

Attosecond charge migration timescales are dominated by transition dipoles, not correlations

Attosecond charge migration (CM) is an ultrafast process occurring when a molecule is irradiated by ultrashort laser pulses, creating a localized hole. The hole propagates rapidly through the molecule, generating electric currents and transferring charge across the molecular backbone. CM is a key ingredient in solar energy conversion, photosynthesis, and radiation damage. Despite its importance and intensive research, the fundamental physical and chemical mechanisms of CM remain not fully understood. Especially, a deeper insight into the role correlations play in the dynamics, and which chemical attributes determine CM timescales, is needed. Here we study with \textit{ab-initio} time-dependent density functional theory CM in the benchmark molecule, BrC$_4$H. We thoroughly explore CM under different initial conditions at the electronic and structural levels, including with theories of varying degrees of electronic correlations. We uncover a universal behavior where the hole moment (connecting to experimental observables) dominant frequency is roughly independent of all of these characteristics. In contrast, the timescales of the hole density evolution do vary with the chemical conditions and level of correlations. Employing a semi-analytical theory that reconstructs the hole moments in the cationic reference frame, we show that the attosecond timescale of CM is determined by molecular dipoles that filter out specific frequency responses with an analogy to optical selection rules. Our results provide essential insight into CM physics, which should be useful for interpreting attosecond experiments and engineering CM timescales by tailoring transition dipoles.

physics.chem-ph↗

arXiv2026

High-order correlations and ultrafast Wigner negativities in bright-squeezed-vacuum-driven high-harmonic generation

High-harmonic generation (HHG) is a prototypical strong-field process in which intense light drives matter to emit radiation at integer multiples of the driving frequency. Extending HHG into the quantum-optical regime offers new opportunities to probe and control strongly nonlinear light-matter interactions using nonclassical states of light. Yet describing this regime requires a fully quantum treatment of the correlated electron-photon dynamics, which becomes computationally challenging for broadband, strongly squeezed fields. Here we solve the quantum-electrodynamical dynamics of a two-level system driven by bright squeezed vacuum in a converged multimode Hilbert space. Both the driving field and emitted harmonics are fully quantized, with the light-matter interaction treated nonperturbatively. This enables direct access to the multimode quantum state and its higher-order correlations beyond semiclassical sampling or perturbative descriptions. We show that squeezed-vacuum driving produces harmonic emission with qualitatively distinct second- and third-order photon correlations compared with coherent excitation. Moreover, back-action from the driven emitter strongly reshapes the incident squeezed field, generating pronounced Wigner-function negativities that evolve on attosecond timescales. Our results establish a fully quantum framework for broadband strong-field dynamics with squeezed light and provide a route to predicting and interpreting quantum-HHG experiments and their extension to more complex emitters.

physics.optics↗

arXiv2026

Multidimensional attosecond clocking near Dirac cones in graphite

Two-color high-harmonic spectroscopy is widely used to access sub-cycle electron dynamics and to retrieve harmonic timing information, including harmonic phases and attochirp across gases, solids, and liquids. However, the dependence of such timing observables on additional laser-control parameters remains largely unexplored. Here, we introduce driving intensity as an additional dimension of two-color harmonic spectroscopy in highly oriented pyrolytic graphite (HOPG). The retrieved attosecond two-color delays maximizing the 4th and 5th harmonic yields evolve systematically and differently with driving intensity. Semiconductor Bloch-equation calculations reproduce these trends and reveal a pronounced sensitivity of the intensity-dependent delays to the electronic band dispersion. Our results demonstrate multidimensional attosecond clocking near Dirac cones and establish intensity-dependent two-color delays as a sensitive observable for band-dispersion in quantum materials.

physics.optics↗

arXiv2026

Ab-initio study of high harmonic generation from fullerenes: Multi-orbital effects, correlations, and size dependence

We study with ab-initio theory high harmonic generation (HHG) from a series of gas-phase fullerenes (from C$_{20}$ to C$_{60}$, including isomers) under varying laser conditions (linearly polarized at various angles and different ellipticities). We explore the role of multiple orbitals in the dynamics and show that due to the tight energy level spacing in these systems (forming the equivalent of energy bands), multiple orbitals contribute and cause severe spectral interferences. HHG cutoff and plateau ranges are shown relatively similar across species of different sizes. We further explore the role of correlations, which are known to be prevalent in C$_{60}$ and similar species and responsible for plasmonic resonances. We find that the independent-particle approximation, where electrons are not dynamically interacting, fails to quantitatively describe features in the spectrum besides the cutoff energy. This failure is broad across the HHG plateau and perturbative regime in all species. Broadly, correlations are seen to reduce HHG yields and cause sharper emission peaks. Lastly, we study HHG ellipticity and angular dependence across species and show that: (i) HHG angular sensitivity becomes less prevalent for larger fullerenes. This arises from a more isotropic structure of the higher point-group molecules. (ii) HHG yields decay faster with the driving laser ellipticity for smaller systems, owing to their reduced size creating smaller recombination cross sections with re-colliding electron wave packets (with C$_{60}$ posing an anomalous exception to this rule). Our predictions pin-point fullerenes as an ideal system for exploring multi-electron interactions in HHG and strong-field physics, and should motivate experiments.

physics.optics↗

arXiv2026

Ultrafast spectroscopy and role of interlayer coupling in high harmonic generation from layered solids

High harmonic generation (HHG) in solids has recently emerged as a powerful all-optical approach for probing material properties and ultrafast electron dynamics in quantum systems. It has been widely applied for studying two-dimensional and layered solids of various kinds. In these studies, the laser is usually polarized within the layered planes, where most electron dynamics occurs, while out-of-plane hopping is commonly neglected. This is despite of interlayer hopping being ubiquitous in nano-systems. Here we develop theory for HHG in layered solids in presence of interlayer coupling and employ it for studying strong-field driven hexagonal BN, graphite, and the transition metal dichalcogenide WS$_2$. We show that sufficiently intense couplings can alter typical HHG emission characteristics such as angular or ellipticity dependence even when the driving laser is polarized in-plane. We develop an analytic perturbation theory for the laser-driven current expanded in the interlayer coupling parameter and explicitly show that HHG yields follow a 4'th order polynomial form, which is validated numerically. Our work should motivate experiments for probing interlayer coupling via HHG spectroscopy, as well as exploring its modulation as a control parameter for ultrafast dynamics and attopulse generation via laser driving and mechanical strain.

physics.optics↗

arXiv2026

Role of ultrafast electron-optical-phonon interactions in high harmonic generation from graphene

High harmonic generation (HHG) is a widely explored process in solids, where intense lasers drive attosecond-to-femtosecond electron dynamics within bands, causing high-energy emission. While electrons and photons are considered the main players in HHG, solids also host ubiquitous phonons that are typically assumed negligible in HHG due to their longer timescales. We theoretically study HHG in graphene with a formalism including optical phonons in the static limit, where the lattice is frozen on the electronic timescale and HHG is computed by sampling thermally-occupied phonons and ensemble-averaging. We show that in graphene: (i) Optical phonons strongly suppress HHG yields by coupling to interband currents and causing harmonic phase scrambling (destructive interference), explaining the lack of experimental HHG above ~3 eV. (ii) HHG yields become temperature-dependent due to phonon occupations, though in graphene this dependence is weak since phonon energy scales are dominated by zero-point motion. (iii) Optical phonons dephase interband coherences at a rate equivalent to T2~5.7 fs, substantially faster than e-e scattering, suggesting thermal phonons dominate electronic decoherence in strong fields. (iv) Phonons smoothen HHG ellipticity-dependent curves, yielding better agreement with experiments. Remarkably, all effects are timescale-independent, arising in the static picture of electron-phonon interactions, making results transferable to attosecond phenomena. Our results shed light on the dephasing time problem in HHG and the role of phonons on attosecond timescales, with implications for other systems and processes such as Floquet gaps and photocurrents in graphene.

physics.optics↗

arXiv2026

Double circular dichroism high harmonic spectroscopy: An ultrafast probe for topological photocurrents

Understanding optical responses of topological matter is a central problem for enabling optoelectronic applications based on topological physics, which is of fundamental concern for photocurrents control and spectroscopy. Currently, schemes for sensing ultrafast photocurrents and separating their bulk/surface contributions are lacking. We introduce here double circular dichroism (DCD) harmonic spectroscopy as an all-optical probe of ultrafast dynamics in topological materials. In this scheme, pump and probe pulses are circular with helicities that are independently controlled, yielding the circular dichroism of the circular dichroism -- a time-resolved response evaluating how probe-induced dichroism depends on pump helicity. While DCD vanishes in symmetric systems, it survives in broken time-reversal symmetry materials including Chern insulators. We theoretically demonstrate this concept through simulations in a Haldane nanoflake, where a pump laser manipulates chiral current-carrying states, and intense probe pulses drive harmonic emission. We show that DCD originates from both bulk and edge-localized states, but these have opposite signs, similar magnitudes, and a different amplitude scaling. Hence, DCD could allow efficient separation of bulk/edge contributions to photocurrents. Variation of the electronic structure and laser parameters further reveals anomalies that might be useful for probing topological attributes of photocurrents in select harmonics. Overall, our work introduces DCD as a potentially powerful approach for disentangling bulk/boundary photo-responses in broken-symmetry quantum matter, and could also be implemented in other pump-probe spectroscopies based on photoelectrons and absorption, as well as other chiral systems.

cond-mat.other↗

arXiv2026

Second-quantized numerical simulations of tunable entanglement in quantum high harmonic generation

Quantum high-harmonic generation (HHG) is a prominent and growing field of research with potential capabilities of providing high photon-number entangled states of light. However, there is an open debate regarding the theory level required for correctly describing the quantum aspects of HHG, such as squeezing or entanglement. Previous approaches either semi-classically sampled the quantum electromagnetic field distribution, or employed perturbation theory utilizing the semi-classical simulations as a starting point. Both of these schemes miss out key quantum-optical features as self-consistent numerical simulations of the electron-photon wavefunction are not performed at any stage. In this Letter, we develop a full quantum theory for multipartite entanglement in HHG, solving exactly the light-matter interaction Hamiltonian in a given Hilbert space, and employ it for evaluating the quantum correlations of emitted photons. We show that HHG entanglement oscillates with the driving laser power and exhibits multiple local maxima, which allows fine-tuning HHG entanglement. Such features arise for both above-threshold harmonics and between above- and below-threshold harmonics. By analyzing different types of atomic targets, we find that the long-range behavior of driven electrons can qualitatively change the resulting entanglement, potentially leading to non-universal behavior across systems. Lastly, we show that focal averaging over classical degrees of freedom in fact plays a key role in entanglement measures and can change the qualitative behavior of observables. Our work establishes the state-of-the art in exploring entanglement features in HHG, and paves way for analysis and engineering of entangled multi-photon states in the XUV and ultrafast regime for more complex matter systems.

quant-ph↗

arXiv2026

Pulse-duration-sensitive high harmonics and attosecond locally-chiral light from a chiral topological Weyl semimetal

High harmonic generation (HHG) in solids results from an interplay between intraband acceleration and electron-hole recombination driven by a high-intensity laser pulse. Here, we theoretically reveal that the driving pulse duration can play a major role in extending HHG to higher photon energies by promoting higher conduction band excitations. The effect is present in a conventional semiconductor as Si, restricted in a large-gap insulator as MgO, and most prominent in RhSi, a prototypical chiral Weyl semimetal presenting numerous band crossings. Further, we elucidate the HHG selection rules in RhSi required for the synthesis of attosecond locally chiral light. The chiral crystal structure enables the generation of a local 3D electric field exhibiting an asymmetric instantaneous torsion on attosecond timescales. A pronounced circular dichroism emerges when the driving helicity is either aligned with or opposite to the crystal handedness. Our findings motivate future experiments in chiral Weyl semimetals to track high-energy band crossings and in-situ locally chiral light, paving the way for chiral compact light sources and light-wave driven topological electronics.

physics.optics↗

arXiv2026

Two-color harmonic spectroscopy of ultrafast Dirac electron dynamics

High-harmonic generation (HHG), the hallmark effect of attosecond science, is a nonperturbative nonlinear process leading to the emission of high-harmonic light from gases and solids. In gases, extreme driving laser pulse intensities can deplete the ground state, suppressing harmonic emission during the trailing edge of the pulse. Here, we report a similar effect, pronounced ultrafast carrier saturation dynamics and harmonic emission suppression during nonperturbative harmonic generation (NPHG) in a gapless Dirac semimetal -- highly oriented pyrolytic graphite (HOPG). Remarkably, HOPG supports NPHG at laser intensities as low as $\sim 10^{10}$ W cm$^{-2}$, facilitated by its vanishing bandgap. Ultrafast carrier saturation strongly modulates the interplay between interband and intraband currents, a key characteristic of NPHG in Dirac materials. Using two-color spectroscopy, we reveal the excitation dynamics of Dirac electron-hole pairs as it affects the emission of harmonics during the presence of the driving laser pulse. The excitation of out-of-equilibrium hot carriers and the concomitant saturation near the Dirac points leads to a marked suppression of interband harmonics and induces measurable temporal shifts. These observations are supported by simulations based on semiconductor Bloch equations. Our finding reveal that field-driven carrier saturation plays a critical role in gapless solid NPHG. We demonstrate the potential of NPHG and HHG as a sensitive, all-optical probe of ultrafast carrier dynamics, offering novel opportunities for ultrafast optoelectronics in Dirac materials.

physics.optics↗

arXiv2025

In-plane optically tunable magnetic states in 2D materials via tailored femtosecond laser driving

It is well established that light can control magnetism in matter, e.g. via the inverse Faraday effect or ultrafast demagnetization. However, such control is typically limited to magnetization transverse to light's polarization plane, or out-of-plane magnetism in 2D materials, while in-plane magnetic moments have remained largely unexplored. This is due to the difficulty of generating electronic orbital angular momentum components within light's polarization plane. Here we overcome this limitation, demonstrating complete three-dimensional, all-optical control of magnetism in 2D materials. Using first-principles simulations, we show that a tailored, two-color laser field can induce and steer magnetic moments in any direction with the relative angle between the laser polarizations playing a key parameter in coherent control. We analyze the physical mechanism of this process and show that it arises from a simultaneous breaking of time-reversal and spatial-inversion symmetries in the two-color laser. In-plane orbital moments are introduced via non-zero out-of-plane longitudinal photogalvanic currents enabled by broken inversion and mirror symmetries, while time-reversal symmetry breaking enables build-up of spin-rotation processes through spin-orbit coupling, translating the orbital moments to transient magnetism. Our findings demonstrate a full 3D coherent control scheme for transient magnetic states on femtosecond timescales driven by tailored lasers, and can be used to develop novel spectroscopies for magnetism, all-optical magnetic switching for ultrafast spintronics, and novel information storage capabilities.

physics.optics↗

arXiv2025

Photoelectron chiral dichroism induced by lasers without helicity via chiral hole wave-packets

Photoelectron circular dichroism (PECD) is a method where randomly oriented chiral molecules are photoionized due to irradiation by circularly-polarized lasers, yielding large chiral signals in the photoelectron momentum distribution. Recently, PECD was explored with polarization-tailored light such as bi-chromatic and non-collinear drivers, which still produces significant chiral signals. Yet, all known PECD configurations to date exhibit non-zero time-local chirality. That is, they are driven by an intrinsically helical light source. Nonetheless, 'chiral' light can also be non-helical if its chirality manifests on longer timescales (e.g. an optical centrifuge). It remains unknown whether PECD can arise from non-helical coherent light. Here we predict that PECD indeed emerges from non-helical light by employing a train of linearly-polarized intense laser pulses with a rotating polarization axis, which are phase-coherent and time-delayed. We find strong PECD in the model chiral molecule CBrClFH under a wide parameter regime that can be optimized up to ~8% by tuning delays between pulses, suggesting quantum interference. We directly show that the physical mechanism for this type of PECD differs from the standard case, relying on a chiral hole attosecond wave-packet evolving in the molecule, induced by the first linear pulse. Our work shows that multiple mechanisms can give rise to PECD on longer timescales and provides a novel approach for ultrafast chirality spectroscopy and coherent chiral wave-packet manipulation.

physics.optics↗

arXiv2025

Correlation-induced phase shifts and time delays in resonance enhanced high harmonic generation from Cr+

We investigate resonance-enhanced high harmonic generation (rHHG) in Cr+ by comparing a 1D shape-resonant model, time-dependent density functional theory (TDDFT), and independent particle approximation (IPA) simulations. Previous studies linked rHHG to the 3p -> 3d giant resonance, suggesting a modified four-step model where recolliding electrons are first captured in the autoionizing state before recombining into the ground state, potentially leading to an emission delay associated with the resonance lifetime. While both the 1D-model and TDDFT reproduce experimental spectra, TDDFT reveals that rHHG counterintuitively originates from spin-down 3p states, while spin-up 3d electrons negligibly contribute. The IPA fails to reproduce rHHG, highlighting the significance of electron correlations. Furthermore, TDDFT revealed a correlation induced ~480 attosecond time delay, accompanied by a strong phase shift across the resonance, potentially explaining earlier RABBIT measurements. Our work sheds light on long-standing open questions in rHHG and should advance novel ultrafast spectroscopies of electron correlations and resonances.

physics.atom-ph↗

arXiv2025

Attosecond-resolved probing of recolliding electron wave packets in liquids and aqueous solutions

High-harmonic spectroscopy (HHS) in liquids promises real-time access to ultrafast electronic dynamics in the native environment of chemical and biological processes. While electron recollision has been established as the dominant mechanism of high-harmonic generation (HHG) in liquids, resolving the underlying electron dynamics has remained elusive. Here we demonstrate attosecond-resolved measurements of recolliding electron wave packets, extending HHS from neat liquids to aqueous solutions. Using phase-controlled two-colour fields, we observe a linear scaling of the two-colour delay that maximizes even-harmonic emission with photon energy, yielding slopes of 208+/-55 as/eV in ethanol and 124+/-42 as/eV in water, the latter matching ab initio simulations (125+/-48 as/eV). In aqueous salt solutions, we uncover interference minima whose appearance depends on solute type and concentration, arising from destructive interference between solute and solvent emission. By measuring the relative phase of solvent and solute HHG, we retrieve a variation of electron transit time by 113+/-32 as/eV, consistent with our neat-liquid results. These findings establish HHS as a powerful attosecond-resolved probe of electron dynamics in disordered media, opening transformative opportunities for studying ultrafast processes such as energy transfer, charge migration, and proton dynamics in liquids and solutions.

physics.chem-ph↗

arXiv2025

Laser Driven Bulk-to-Layered Phase Transition

Laser-induced phase transitions offer pathways of phase transitions that are inaccessible by conventional stimuli. In this study, we conduct ab initio simulations to numerically demonstrate a novel laser-induced structural transformation: converting a bulk crystal into a layered van der Waals material using intense light pulses. The transition is driven by a nonlinear phononic mechanism, where selectively exciting polar and anti-polar phonon modes with polarized terahertz light breaks targeted interlayer bonds while preserving intralayer ones. We identify that strong anisotropy in bond sensitivity where interlayer bonds are significantly more susceptible to excitation than intralayer bonds is the critical prerequisite. Our findings pave the way for on demand transformations from bulk to 2D materials, facilitate the design of advanced phase-change devices, and suggest a potential optical exfoliation method to expand the range of exfoliable 2D materials.

physics.optics↗

arXiv2025

Probing broken time-reversal symmetry with tailored-light photocurrents

Light-field-driven photocurrents represent a powerful tool for generating photocurrents without external bias in light-matter systems that lack inversion symmetry. While these photocurrents are used in electronic applications, such as current sources, switches, and photovoltaics, their presence can also be used to probe material properties in and out of equilibrium, such as topology. Here we advance this path of light-field-driven photocurrent spectroscopy by utilizing tailored laser fields for ultrafast photocurrent generation to study time-reversal symmetry (TRS) broken phases. We employ combinations of bichromatic linearly-polarized laser beams that individually respect mirror (spatial) and time-reversal symmetry, individually precluding photocurrents, but when combined can break symmetries and generate photocurrents. We show, both theoretically and experimentally, that unique choices of the relative polarization angle and two-color phase imposes a forbidden photocurrent selection rule in TRS-invariant systems, as the tailored light maintains TRS while breaking all other spatial symmetries. We then employ state-of-the-art ab-initio simulations to validate this physical mechanism, and, crucially, predict its breaking in materials with intrinsically-broken TRS, creating a background free signal for magnetism and Chern physics. Our work paves way for probing TRS-broken phases of matter in an ultrafast time-resolved manner, not requiring the application of external magnetic fields or even circularly-polarized electric fields.

physics.optics↗

arXiv2025

Light-Induced Persistent Electronic Chirality in Achiral Molecules Probed with Time-Resolved Electronic Circular Dichroism Spectroscopy

Chiral systems exhibit unique properties traditionally linked to their asymmetric spatial arrangement. Recently, multiple laser pulses were shown to induce purely electronic chiral states without altering the nuclear configuration. Here, we propose and numerically demonstrate a simpler realization of light-induced electronic chirality that is long-lived and occurs well before the onset of nuclear motion and decoherence. A single monochromatic circularly-polarized laser pulse is shown to induce electronic chiral currents in an oriented achiral molecule. Using state-of-the-art ab initio theory, we analyze this effect and relate the chiral currents to induced magnetic dipole moments, detectable via attosecond time-resolved electronic circular dichroism (TR-ECD) spectroscopy, also known as transient absorption ECD. The resulting chiral electronic wavepacket oscillates rapidly in handedness at harmonics of the pump laser's carrier frequency, and the currents persist after the pulse ends. We establish a chiral molecular-current analogue to high harmonic generation, and demonstrate attosecond transient chirality control with potential impact on spintronics and reaction dynamics.

physics.chem-ph↗

arXiv2025

Linearly-polarized few-cycle pulses drive carrier envelope phase-sensitive coherent magnetization injection

Circularly-polarized light is well-known to induce, or flip the direction of, magnetization in solids. At its heart, this arises from time-reversal symmetry breaking by the vector potential, causing inverse-Faraday or analogous physical effects. We show here that very short few-cycle pulses can cause similar phenomena even when they are linearly-polarized and off-resonant. We analyze the new effect with ab-initio calculations and demonstrate that it similarly arises due to broken time-reversal symmetry and is carrier-envelope-phase (CEP) sensitive. Coherently tuning the CEP causes the induced magnetism to oscillate from zero to few percent Bohr magneton within few femtoseconds. By changing the laser angle and intensity, the magnetization sign and magnitude can be controlled. Remarkably, due to the nature of the physical mechanism that relies on spin-orbit interactions and orbital currents, the magnetization survives CEP-averaging and the effect should be accessible even in the absence of CEP stabilization. Our work opens new routes for ultrafast coherent tuning and probing of magnetism and circumvents the need for circularly-polarized driving or external magnetic fields.

physics.optics↗