arXiv subjects

Ofer Neufeld

Publications and source records attributed to Ofer Neufeld.

At least 37 records · Page 2Linked to original sources

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↗

arXiv2025

Multi-plateau high-harmonic generation in liquids driven by off-site recombination

Non-perturbative high-harmonic generation (HHG) has recently been observed in the liquid phase, where it was demonstrated to have a different physical mechanism compared to gas and solid phases of matter. The currently best physical picture for liquid HHG eliminates scattered-electron contributions and identifies on-site recombination as the dominant contributor. This mechanism accurately predicts the cut-off energy and its independence of the driving laser wavelength and intensity. However, this implies that additional energy absorbed in the liquid as the driving laser intensity is increased does not result in higher-order non-linearities, which is in contrast to the conventional expectation from most nonlinear media. Here we experimentally observe the formation of a second plateau in HHG from multiple liquids (water, heavy water, propranol, and ethanol), thus explaining the conundrum of the missing higher-order response. We analyze this second plateau with a combination of experimental, state-of-the-art ab-initio numerical (in diverse systems of water, ammonia, and liquid methane), and semi-classical analytical, techniques, and elucidate its physical origin to electrons that recombine on neighboring water molecules rather than at the ionization site, leading to unique HHG ellipticity dependence. Remarkably, we find that the second plateau is dominated by electrons recombining at the second solvation shell, relying on wide hole delocalization. Theory also predicts the appearance of even higher plateaus, indicating a general trend. Our work establishes new physical phenomena in the highly non-linear optical response of liquids, paving the way to attosecond probing of electron dynamics in solutions.

physics.chem-ph↗

arXiv2025

Symmetries and selection rules in photoelectron chiral dichroism from tailored light

Photoelectron circular dichroism (PECD) is a method whereby randomly oriented chiral molecules are irradiated by circularly-polarized light, photoionizing electrons, which are measured in a momentum-resolved manner. This scheme permits chiral light-matter interactions within the electric-dipole approximation (avoiding weak magnetic-dipole interactions), yielding huge chiral signals in the form of a forwards-backwards asymmetry in photoemission. Recently, more intricate realizations of PECD have been explored where the circularly-polarized light is replaced with elaborate polarization-tailored light (e.g. bi-chromatic, non-collinear, etc.), some of which do not even require circularly-polarized components, but still lead to massive chiral signals. However, the connection between generalized symmetries and asymmetries of the laser drive and selection rules in PECD have not yet been derived. Here we formulate this connection analytically from group theory, also predicting two previously unknown selection rules for PECD from fields with dynamical inversion and improper-rotational symmetries. We further propose bi-chromatic bi-elliptical fields for breaking symmetries in typical spectra, yielding potentially more information for ultrafast-resolved measurements. We numerically demonstrate all of our results with state-of-the-art ab-initio simulations in the archetypal chiral molecule, Bromochlorofluoromethane, providing predictions for experiments. Our work presents a roadmap for analyzing PECD from tailored light and resolved a long-standing issue in the field. It should motivate theoretical and experimental investigations in novel PECD set-ups.

physics.chem-ph↗

arXiv2025

Bulk photogalvanic current control and gap spectroscopy in 2D hexagonal materials

Two-dimensional (2D) hexagonal materials have been intensively explored for multiple optoelectronic applications such as spin current generation, all-optical valleytronics, and topological electronics. In the realm of strong-field and ultrafast light-driven phenomena, it was shown that tailored laser driving such as polychromatic or few-cycle pulses can drive robust bulk photogalvanic (BPG) currents originating from the K/K' valleys. We here explore the BPG effect in 2D systems in the strong-field regime and show that monochromatic elliptical pulses also generically generate such photocurrents. The resultant photocurrents exhibit both parallel and transverse (Hall-like) components, both highly sensitive to the laser parameters, providing photocurrent control knobs. Interestingly, we show that the photocurrent amplitude has a distinct behavior vs. the driving ellipticity that can be indicative of material properties such as the gap size at K/K', which should prove useful for novel forms of BPG-based spectroscopies. We demonstrate these effects also in benchmark ab-initio simulations in monolayer hexagonal boron-nitride. Our work establishes new paths for controlling photocurrent responses in 2D systems that can also be used for multi-dimensional spectroscopy of ultrafast material properties through photocurrent measurements.

cond-mat.mtrl-sci↗

arXiv2025

Light's symmetry, asymmetry, and their role in nonlinear optics and ultrafast phenomena

The analysis of symmetries is extremely useful across science. In Physics, symmetries are used to derive conservation laws and selection rules for transitions in interacting systems. In the early days of nonlinear optics (NLO), symmetries were used to formulate a set of rules for photonic processes according to the medium's symmetries that are reflected in the NLO coefficient tensor. While this approach was believed to be complete and closed, the field has recently reignited as multi-color ultrashort laser pulses with tailored polarization and spatiotemporal structures become standard in NLO. A more complete theory has been recently emerging, which aims to incorporate all possible dynamical degrees of freedom of light: spin and orbital angular momentum, spatial structure, time-dependent polarizations, temporal envelopes, etc., in addition to the symmetries of the medium. This theoretical development is also accompanied by experimental advances that rely on tailored light beams that can now be generated with ever-increasing complexity, including topologies in real and a variety of synthetic dimensions, carrying poly-chromatic carrier waves, time-dependent varying angular momenta, local-chirality, and more. The nonlinear interactions between light fields with unique symmetries (or asymmetries) and matter is especially appealing, since that holds the key for developing new ultrafast spectroscopies with sub-femtosecond resolution, for exerting exact control over matter, and improving our fundamental understanding of how light and matter interact. We review these recent advances in this expanding field, focusing on the theory, its implications, and seminal experiments. We aim to establish a comprehensive database of symmetries and selection rules governing NLO light-matter interactions within the emerging new formalism, and invite the scientific community to contribute to this effort.

physics.optics↗

arXiv2025

Quantum interference and occupation control in high harmonic generation from monolayer $WS_2$

Two-dimensional hexagonal materials such as transition metal dichalcogenides exhibit valley degrees of freedom, offering fascinating potential for valley-based quantum computing and optoelectronics. In nonlinear optics, the K and K' valleys provide excitation resonances that can be used for ultrafast control of excitons, Bloch oscillations, and Floquet physics. Under intense laser fields, however, the role of coherent carrier dynamics away from the K/K' valleys is largely unexplored. In this study, we observe quantum interferences in high harmonic generation from monolayer $WS_2$ as laser fields drive electrons from the valleys across the full Brillouin zone. In the perturbative regime, interband resonances at the valleys enhance high harmonic generation through multi-photon excitations. In the strong-field regime, the high harmonic spectrum is sensitively controlled by light-driven quantum interferences between the interband valley resonances and intraband currents originating from electrons occupying various points in the Brillouin zone, also away from K/K' valleys such as $Γ$ and M. Our experimental observations are in strong agreement with quantum simulations, validating their interpretation. This work proposes new routes for harnessing laser-driven quantum interference in two-dimensional hexagonal systems and all-optical techniques to occupy and read-out electronic structures in the full Brillouin zone via strong-field nonlinear optics, advancing quantum technologies.

physics.optics↗

arXiv2025

Optical control of electrons in a Floquet topological insulator

Light-dressed materials hold enormous potential for generating new electronic properties. The band structure resulting from light-dressing can exhibit starkly different quantum and topological phenomena. So far, optical control of charge within a light-dressed band structure has been elusive. Here, we demonstrate optical control of electrons in light-dressed graphene. By focusing circularly polarized femtosecond laser pulses at 1550 nm on monolayer graphene, we generate a Floquet topological insulator (FTI). With a phase-locked second harmonic field, we dynamically control electrons in this FTI state. For the first time, we observe photocurrent circular dichroism, the all-optical anomalous Hall effect, and FTI valley-polarized currents. The photocurrents show strong sub-cycle phase-sensitivity, opening the door to ultrafast control within topologically protected electronics (topotronics), spectroscopy, and attosecond physics in novel quantum materials.

physics.optics↗

arXiv2025

The 2025 Roadmap to Ultrafast Dynamics: Frontiers of Theoretical and Computational Modelling

The exploration of ultrafast phenomena is a frontier of condensed matter research, where the interplay of theory, computation, and experiment is unveiling new opportunities for understanding and engineering quantum materials. With the advent of advanced experimental techniques and computational tools, it has become possible to probe and manipulate nonequilibrium processes at unprecedented temporal and spatial resolutions, providing insights into the dynamical behavior of matter under extreme conditions. These capabilities have the potential to revolutionize fields ranging from optoelectronics and quantum information to catalysis and energy storage. This Roadmap captures the collective progress and vision of leading researchers, addressing challenges and opportunities across key areas of ultrafast science. Contributions in this Roadmap span the development of ab initio methods for time-resolved spectroscopy, the dynamics of driven correlated systems, the engineering of materials in optical cavities, and the adoption of FAIR principles for data sharing and analysis. Together, these efforts highlight the interdisciplinary nature of ultrafast research and its reliance on cutting-edge methodologies, including quantum electrodynamical density-functional theory, correlated electronic structure methods, nonequilibrium Green's function approaches, quantum and ab initio simulations.

cond-mat.mtrl-sci↗

arXiv2024

Efficient time dependent Wannier functions for ultrafast dynamics

Time-dependent Wannier functions were initially proposed as a means for calculating the polarization current in crystals driven by external fields. In this work, we present a simple gauge where Wannier states are defined based on the maximally localized functions at the initial time, and are propagated using the time-dependent Bloch states obtained from established first-principles calculations, avoiding the costly Wannierization at ech time step. We show that this basis efficiently describes the time-dependent polarization of the laser driven system through the analysis of the motion of Wannier centers. We use this technique to analyze highly nonlinear and non-perturbative responses such as high harmonic generation in solids, using the hexagonal boron nitride as an illustrative example, and we show how it provides an intuitive picture for the physical mechanisms.

cond-mat.mtrl-sci↗

arXiv2024

Quantum electrodynamics in high harmonic generation: multi-trajectory Ehrenfest and exact quantum analysis

High-harmonic generation (HHG) is a nonlinear process in which a material sample is irradiated by intense laser pulses, causing the emission of high harmonics of the incident light. HHG has historically been explained by theories employing a classical electromagnetic field, successfully capturing its spectral and temporal characteristics. However, recent research indicates that quantum-optical effects naturally exist, or can be artificially induced, in HHG. Even though the fundamental equations of motion for quantum electrodynamics (QED) are well-known, a unifying framework for solving them to explore HHG is missing. So far, numerical solutions employed a wide range of basis-sets and untested approximations. Based on methods originally developed for cavity polaritonics, here we formulate a numerically accurate QED model consisting of a single active electron and a single quantized photon mode. Our framework can in principle be extended to higher electronic dimensions and multiple photon modes to be employed in ab initio codes. We employ it as a model of an atom interacting with a photon mode and predict a characteristic minimum structure in the HHG yield vs. phase-squeezing. We find that this phenomenon, which can be used for novel ultrafast quantum spectroscopies, is partially captured by a multi-trajectory Ehrenfest dynamics approach, with the exact minima position sensitive to the level of theory. On the one hand, this motivates using multi-trajectory approaches as an alternative for costly exact calculations. On the other hand, it suggests an inherent limitation of the multi-trajectory formalism, indicating the presence of entanglement. Our work creates a road-map for a universal formalism of QED-HHG that can be employed for benchmarking approximate theories, predicting novel phenomena for advancing quantum applications, and for the measurements of entanglement and entropy.

physics.optics↗

arXiv2024

The Application of Tailored Fields for Studying Chirality and Symmetry

Ultrashort laser pulses pose unique tools to trigger and probe the fastest charge dynamics in matter, allowing the investigation of fundamental physical phenomena with unprecedented resolution in space, time, and energy. One of the most fascinating opportunities that ultrashort pulses offer is the possibility of modulating and investigating symmetries by tailoring the properties of the laser beam in the spatial and polarization domains, effectively controlling symmetry breaking on multiple levels. In particular, this allows probing chiral matter and ultrafast chiral dynamics. In recent years, the development of highly sensitive approaches for studying chirality has been a hot topic in physics and chemistry that has developed largely separately from the field of tailored light. This perspective discusses the individual and joint evolution of these fields with an emphasis on how the fields have already cross-fertilized, opening new opportunities in science. We outline a future outlook of how the topics are expected to fully merge and mutually evolve, emphasizing outstanding open issues.

physics.optics↗

arXiv2023

New photonic conservation laws in parametric nonlinear optics

Conservation laws are one of the most generic and useful concepts in physics. In nonlinear optical parametric processes, conservation of photonic energy, momenta and parity often lead to selection rules, restricting the allowed polarization and frequencies of the emitted radiation. Here we present a new scheme to derive conservation laws in optical parametric processes in which many photons are annihilated and a single new photon is emitted. We then utilize it to derive two new such conservation laws. Conservation of reflection-parity (RP) arises from a generalized reflection symmetry of the polarization in a superspace, analogous to the superspace employed in the study of quasicrystals. Conservation of space-time-parity (STP) similarly arises from space-time reversal symmetry in superspace. We explore these new conservation laws numerically in the context of high harmonic generation and outline experimental set-ups where they can be tested.

physics.optics↗

arXiv2023

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

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

physics.optics↗

arXiv2023

Are there universal signatures of topological phases in high harmonic generation? Probably not

High harmonic generation (HHG) has developed in recent years as a promising tool for ultrafast materials spectroscopy. At the forefront of these advancements, several works proposed to use HHG as an all-optical probe for topology of quantum matter by identifying its signatures in the emission spectra. However, it remains unclear if such spectral signatures are indeed a robust and general approach for probing topology. To address this point, we perform here a fully ab-initio study of HHG from prototypical two-dimensional topological insulators in the Kane-Mele quantum spin-Hall and anomalous-Hall phases. We analyze the spectra and previously proposed topological signatures by comparing HHG from the topological and trivial phases. We demonstrate and provide detailed microscopic explanations of why in these systems none of the thus far proposed observables uniquely and universally probe material topology. Specifically, we find that the: (i) HHG helicity, (ii) anomalous HHG ellipticity, (iii) HHG elliptical dichroism and (iv) temporal delays in HHG emission, are all unreliable signatures of topological phases. Our results suggest that extreme care must be taken when interpreting HHG spectra for topological signatures, and that contributions from the crystal symmetries and chemical nature might be dominant over those from topology. They hint that a truly universal topological signature in nonlinear optics is unlikely, and raise important questions regarding possible utilization and detection of topology in systems out-of-equilibrium.

cond-mat.mtrl-sci↗

arXiv2023

Mapping light-dressed Floquet bands by highly nonlinear optical excitations and valley polarization

Ultrafast nonlinear optical phenomena in solids have been attracting major interest as novel methodologies for femtosecond spectroscopy of electron dynamics and control of material properties. Here, we theoretically investigate strong-field nonlinear optical transitions in a prototypical two-dimensional material, hBN, and show that the k-resolved conduction band charge occupation patterns induced by an elliptically-polarized laser can be understood in a multi-photon resonant picture; but remarkably, only if using the Floquet light-dressed states instead of the undressed matter states. Consequently, our work establishes a direct measurable signature for band-dressing in nonlinear optical processes in solids, and opens new paths for ultrafast spectroscopy and valley manipulation.

physics.optics↗

arXiv2022

An ab initio supercell approach for high-harmonic generation in liquids

Many important ultrafast phenomena take place in the liquid phase. However, there is no practical theory to predict how liquids respond to intense light. Here, we propose an $ab~initio$ accurate method to study the non-perturbative interaction of intense pulses with a liquid target to investigate its high-harmonic emission. We consider the case of liquid water, but the method can be applied to any other liquid or amorphous system. The liquid water structure is reproduced using Car-Parrinello molecular dynamics simulations in a periodic supercell. Then, we employ real-time time-dependent density functional theory to evaluate the light-liquid interaction. We outline the practical numerical conditions to obtain a converged response. Also, we discuss the impact of nuclei ultrafast dynamics on the non-linear response of system. In addition, by considering two different ordered structures of ice, we show how harmonic emission responds to the loss of long-range order in liquid water.

cond-mat.mtrl-sci↗

arXiv2022

High-harmonic generation in liquids with few-cycle pulses: effect of laser-pulse duration on the cut-off energy

High-harmonic generation (HHG) in liquids is opening new opportunities for attosecond light sources and attosecond time-resolved studies of dynamics in the liquid phase. In gas-phase HHG, few-cycle pulses are routinely used to create isolated attosecond pulses and to extend the cut-off energy. Here, we study the properties of HHG in liquids, including water and several alcohols, by continuously tuning the pulse duration of a mid-infrared driver from the multi- to the sub-two-cycle regime. Similar to the gas phase, we observe the transition from discrete odd-order harmonics to continuous extreme-ultraviolet emission. However, the cut-off energy is shown to be entirely independent of the pulse duration. This observation is confirmed by ab-initio simulations of HHG in large clusters. Our results support the notion that the cut-off energy is a fundamental property of the liquid, independent of the driving-pulse properties. Combined with the recently reported wavelength-independence of the cutoff, these results confirm the direct sensitivity of HHG to the mean-free paths of slow electrons in liquids. Our results additionally imply that few-cycle mid-infrared laser pulses are suitable drivers for generating isolated attosecond pulses from liquids.

physics.optics↗

arXiv2022

Probing the low-energy electron-scattering dynamics in liquids with high-harmonic spectroscopy

High-harmonic spectroscopy (HHS) is a nonlinear all-optical technique with inherent attosecond temporal resolution, which has been applied successfully to a broad variety of systems in the gas phase and solid state. Here, we extend HHS to the liquid phase, and uncover the mechanism of high-harmonic generation (HHG) for this phase of matter. Studying HHG over a broad range of wavelengths and intensities, we show that the cut-off ($E_c$) is independent of the wavelength beyond a threshold intensity, and find that $E_c$ is a characteristic property of the studied liquid. We explain these observations within an intuitive semi-classical model based on electron trajectories that are limited by scattering to a characteristic length, which is connected to the electron mean-free path. Our model is validated against rigorous multi-electron time-dependent density-functional theory calculations in, both, supercells of liquid water with periodic boundary conditions, and large clusters of a variety of liquids. These simulations confirm our interpretation and thereby clarify the mechanism of HHG in liquids. Our results demonstrate a new, all-optical access to effective mean-free paths of slow electrons ($\leq$10 eV) in liquids, in a regime that is inaccessible to accurate calculations, but is critical for the understanding of radiation damage to living tissue. Our work also establishes the possibility of resolving sub-femtosecond electron dynamics in liquids, which offers a novel, all-optical approach to attosecond spectroscopy of chemical processes in their native liquid environment.

physics.chem-ph↗