Strong-field Processes Research Articles

Drastically enhanced high-order harmonic generation from endofullerenes

Dynamically rich nature of the high-order harmonic generation process lends itself to a variety of ways to increase photon yield and extend the harmonic cut-off frequency. We show here that high-harmonic generation from an atom confined inside an attractive potential shell can show a dramatic increase in the photon yield in certain cases. We consider an endohedrally confined hydrogen atom inside a C$_{60}$ cage as an example, and consider three distinct physical situations in which the initial state is (1) entirely confined inside the C$_{60}$, (2) partially outside, and (3) mainly localized on the cage wall. We demonstrate that when the atom-cage system starts in a state with a classical turning point outside the C$_{60}$ shell, the high-harmonic photon yield can be enhanced up to 4 orders of magnitude when compared with a free atom in the same initial state. We explain the underlying physical mechanisms in each case using fully three-dimensional quantum simulations. This gives a prime example of how directly coupling an atom to a nanostructure can alter strong field processes in atoms in interesting ways.

In-line Spectral Interferometry in Shortwave-Infrared Laser Filaments in Air.

We investigate the nonlinear propagation of intense, two-cycle, carrier-envelope phase (CEP) stable laser pulses at 1.7 μm center wavelength in air. We observe CEP-dependent spectral interference in the visible part of the forward-propagating white light generated on propagation. The effect is robust against large fluctuations of the input pulse energy. This robustness is enabled by rigid clamping of both the peak optical field and the phase of the propagating waveform, which has been revealed by numerical simulations. The CEP locking can enhance the yield of the CEP-dependent strong-field processes in gaseous media with long-wavelength drivers, while the observed spectral interference enables single-shot, stand-off CEP metrology in the atmosphere.

Disentangling conical intersection and coherent molecular dynamics in methyl bromide with attosecond transient absorption spectroscopy

Attosecond probing of core-level electronic transitions provides a sensitive tool for studying valence molecular dynamics with atomic, state, and charge specificity. In this report, we employ attosecond transient absorption spectroscopy to follow the valence dynamics of strong-field initiated processes in methyl bromide. By probing the 3d core-to-valence transition, we resolve the strong field excitation and ensuing fragmentation of the neutral σ* excited states of methyl bromide. The results provide a clear signature of the non-adiabatic passage of the excited state wavepacket through a conical intersection. We additionally observe competing, strong field initiated processes arising in both the ground state and ionized molecule corresponding to vibrational and spin-orbit motion, respectively. The demonstrated ability to resolve simultaneous dynamics with few-femtosecond resolution presents a clear path forward in the implementation of attosecond XUV spectroscopy as a general tool for probing competing and complex molecular phenomena with unmatched temporal resolution.

Laser-solid interaction and its potential for probing radiative corrections in strong-field quantum electrodynamics

The present work discusses the possibility of probing radiative corrections in strong-field quantum electrodynamics for a future experiment. Therefore, the framework of self-consistent and fully relativistic particle-in-cell simulations is employed. In particular, the interaction of a solid target irradiated at normal incidence with an ultraintense laser pulse is considered. It is shown that one can form an adequate environment for a 100 GeV class electron beam to reach a completely novel super strong-field sector of quantum electrodynamics. In this regime, the influence of radiative corrections to strong-field processes can no longer be treated perturbatively. Since so far there is no reliable theory describing this strongly coupled regime, the present paper is dedicated to draw attention to this unstudied issue by pointing out a feasible experimental setup.

Inclusion of Coulomb effects in laser-atom interactions

We investigate the role of the Coulomb interaction in strong field processes. We find that the Coulomb field of the ion makes its presence known even in highly intense laser fields, in contrast to the assumptions of the strong field approximation. The dynamics of the electron after ionization is analyzed with four models for an arbitrary laser polarization: the Hamiltonian model in the dipole approximation, the strong field approximation, the Coulomb-corrected strong field approximation and the guiding center. These models illustrate clearly the Coulomb effects, in particular Coulomb focusing and Coulomb asymmetry. We show that the Coulomb-corrected strong field approximation and the guiding center are complementary, in the sense that the Coulomb-corrected strong field approximation describes well short time scale phenomena (shorter than a laser cycle) while the guiding center is well suited for describing long time scale phenomena (longer than a laser cycle) like Coulomb-driven recollisions and Rydberg state creation.

Effect of laser polarization on strong-field ionization and fragmentation of nitrous oxide molecules**Project supported by the National Natural Science Foundation of China (Grant Nos. 11534004, 11874179, and 11704149) and the Natural Science Foundation of Jilin Province, China (Grant No. 20180101289JC).

Ionization of molecules in femtosecond laser fields is the most fundamental and important step of various strong-field physical processes. In this study, we experimentally investigate strong field ionization of linear N2O molecules using a time-of-flight mass spectrometer in 800-nm laser fields. Yields of the parent ion and different fragment ions are measured as a function of laser intensity in the range of 2.0×1013 W/cm2 to 3.6×1014 W/cm2. We also investigate the dependence of strong field ionization and dissociation of N2O on laser ellipticity and polarization direction. The significant role of laser induced electron re-collision in the formation of highly charged fragment ions is proved. The physical mechanism of strong field ionization and fragmentation is discussed, based on our experimental results.

Fiber-amplifier-pumped, 1-MHz, 1-µJ, 2.1-µm, femtosecond OPA with chirped-pulse DFG front-end.

High-repetition-rate, high-power, few-cycle mid-infrared lasers with carrier-envelope phase (CEP) stabilization are ideal driving sources for studying strong-field nonlinear processes, such as strong-field driven electron emission, solid-state high-harmonic generation, and nonlinear microscopy. Here, we report on a 1-MHz, 1-μJ, femtosecond, 2.1-µm optical parametric amplifier (OPA), pumped by a Yb-doped fiber chirped-pulse amplifier (CPA) and seeded by a chirped-pulse difference-frequency generation (DFG) front-end providing positively chirped 2.1-μm signal pulses. The home-built multi-stage 1030-nm Yb-doped fiber CPA pump laser generates >55-μJ near-transform-limited (245-fs) pulses at 1 MHz repetition rate using a novel 4-pass all-fiber stretcher/front-end for careful dispersion/spectral management. The chirped-pulse DFG scheme is achieved by wave-mixing the 1030 nm pump pulse with a dispersive wave at 645-735 nm generated in a photonic crystal fiber, allowing passive CEP stability of the 2.1-µm pulses. The 2.1-μm pulse is amplified to 1 μJ in a two-stage dispersion-managed optical parametric amplifier (OPA) with a pump energy of ~21 μJ resulting in 95-fs pulses with nice beam profile without additional pulse compression. Multi-μJ, sub-30 fs pulses can be obtained at full pump energy and additional dispersion compensation. The fiber-amplifier-based mid-infrared OPA can be directly applied to high-harmonic generation in solids and optical-field-driven nanophotonic devices and is a compact front-end for a future high-power, high-repetition-rate, long wavelengths CEP-stabilized source for gas-phase high-order harmonic generation.

Direct probing of tunneling time in strong-field ionization processes by time-dependent wave packets.

We propose a straightforward approach to directly probe the tunneling time by observing the transition of photoelectron wave packets in strong-field ionization processes, where Coulomb potentials do not affect the results. A circularly polarized laser pulse is used to avoid the impact of scattering electrons on the direct ionization electrons, and a pure transmission photoelectron wave packet can be obtained. Then, a positive tunneling time is extracted. The results demonstrate that the tunneling time is dominated mainly by the laser frequency in some laser intensity range. At the same time, we also investigate the tunneling time by analyzing the instantaneous ionization rate, and the consentaneous results are obtained.

Electron correlation effects in enhanced ionization of diatomic molecules in near-infrared fields

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of $\mathrm{H}_2$, $\mathrm{LiH}$, and $\mathrm{HF}$ molecules by intense near-infrared laser pulses using a 3D description of the systems with the time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron correlation of the quantum many-electron system under consideration. Our correlated description of diatomic molecules shows that enhanced ionization occurs at certain critical internuclear separations and electron correlation systematically improves the ionization probability in this process until convergence is reached. We demonstrate the failure of the single-active-electron and the configuration-interaction singles approximations to produce the correct internuclear position and probability of the strong-field enhanced-ionization process. We elucidate the role of low-lying electronic excited states in the enhanced ionization process of diatomic molecules. There is clear evidence that an accurate description of low-lying electronically excited states is important to describe the non-perturbative enhanced ionization phenomenon in the ultrashort intense near infrared laser pulses.

Dissociative frustrated multiple ionization of hydrogen chloride in intense femtosecond laser fields

The dissociative frustrated multiple ionization of hydrogen chloride (HCl) molecules in intense femtosecond laser fields is investigated by employing the coincidence measurement technique in cold target recoil ion momentum spectroscopy. Various dissociative frustrated multiple ionization channels caused by populating one or two electrons to highly excited Rydberg states can be clearly distinguished for HCl in linearly polarized intense laser fields. The excited charged fragmentations, depending on their occupied principle quantum numbers, can be either directly identified forming a ``knee'' structure in the kinetic energy release spectrum, or singly ionized by the weak electric field in the spectrometer, or by blackbody radiation after the conclusion of the laser pulse. The inherent feature of the potential energy curves of HCl plays an important role in the strong-field dissociative frustrated multiple ionization process.

Ultrafast Multiharmonic Plasmon Generation by Optically Dressed Electrons.

Interactions between electrons and photons are a source of rich physics from atomic to astronomical scales. Here, we examine a new kind of electron-photon interaction in which an electron, modulated by light, radiates multiple harmonics of plasmons. The emitted plasmons can be femtosecond in duration and nanometer in spatial scale. The extreme subwavelength nature of the plasmons lowers the necessary input light intensity by at least 4 orders of magnitude relative to state-of-the-art strong-field processes involving bound or free electrons. The results presented here reveal a new means of ultrafast (10-1000fs) interconversion between photonic and plasmonic energy, and a general scheme for generating spatiotemporally shaped ultrashort pulses in optical materials. More generally, our results suggest a route towards realizing analogues of fascinating physical phenomena like nonlinear Compton scattering in plasmonics and nanophotonics with relatively low intensities, slow electrons, and on nanometer length scales.

Quantum coherent control of H3 + formation in strong fields.

Quantum coherent control (QCC) has been successfully demonstrated experimentally and theoretically for two- and three-photon optical excitation of atoms and molecules. Here, we explore QCC using spectral phase functions with a single spectral phase step for controlling the yield of H3 + from methanol under strong laser field excitation. We observe a significant and systematic enhanced production of H3 + when a negative 34 π phase step is applied near the low energy region of the laser spectrum and when a positive 34 π phase step is applied near the high energy region of the laser spectrum. In some cases, most notably the HCO+ fragment, we found the enhancement exceeded the yield measured for transform limited pulses. The observation of enhanced yield is surprising and far from the QCC prediction of yield suppression. The observed QCC enhancement implies an underlying strong field process responsible for polyatomic fragmentation controllable by easy to reproduce shaped pulses.

Accurate insitu Measurement of Ellipticity Based on Subcycle Ionization Dynamics.

Elliptically polarized laser pulses (EPLPs) are widely applied in many fields of ultrafast sciences, but the ellipticity (ϵ) has never been insitu measured in the interaction zone of the laser focus. In this Letter, we propose and realize a robust scheme to retrieve the ϵ by temporally overlapping two identical counterrotating EPLPs. The combined linearly electric field is coherently controlled to ionize Xe atoms by varying the phase delay between the two EPLPs. The electron spectra of the above-threshold ionization and the ion yield are sensitively modulated by the phase delay. We demonstrate that these modulations can be used to accurately determine ϵ of the EPLP. We show that the present method is highly reliable and is applicable in a wide range of laser parameters. The accurate retrieval of ϵ offers a better characterization of a laser pulse, promising a more delicate and quantitative control of the subcycle dynamics in many strong field processes.

Central frequency of few-cycle laser pulses in strong-field processes

We analyze the role of the difference between the central frequencies of the spectral distributions of the vector potential and the electric field of a short laser pulse. The frequency shift arises when the electric field is determined as the derivative of the vector potential to ensure that both quantities vanish at the beginning and end of the pulse. We derive an analytical estimate of the frequency shift and show how it affects various light induced processes, such as excitation, ionization and high harmonic generation. Since observables depend on the frequency spectrum of the electric field, the shift should be taken into account when setting the central frequency of the vector potential to avoid potential misinterpretation of numerical results for processes induced by few-cycle pulses.

Coulomb-Volkov S-matrix theory of ionisation

In this paper we outline the recently derived Coulomb-Volkov (CV) S-matrix theory and present the results of calculations of the energy and the momentum spectra of the electrons emitted from ionisation of Ar atoms by an intense long-wavelength laser field. The results are compared with the corresponding calculations using the usual (plane-wave) SFA S-matrix theory, up to the second order of the series. We introduce a modified stationary phase method with a parametric integration to evaluate the high-dimensional integrals that occur in the second order of the ionisation amplitudes. The final-state Coulomb effect is found to be dominant at the lower energies, where the two theories differ greatly. In contrast, at high energies, the two theories produce qualitatively similar (in the logarithmic scale) distributions. It is found that despite the similarity of the positions of the ‘forward rescattering’ (or ‘soft collision’) peaks shown by the second order CV- and the SFA-amplitude, the line-shapes produced by them are qualitatively different. For example, the CV-spectrum shows a peak at 0.1 Up with a characteristic line-shape that is convex on one side and steeply rising on the other side; this is qualitatively similar to the line-shape of the peak observed experimentally earlier at the same energy. In contrast, the SFA-spectrum shows at the same energy a pointed peak with a line-shape that is concave on both sides. The characteristic line-shape of the low energy LES-peak(s) thus appears to suggest itself as a signature of the final-state Coulomb interaction in strong-field ionisation processes.

Ab initio study of time-dependent dynamics in strong-field triple ionization

An ab initio analysis of strong-field three-electron ionization in a restricted-dimensionality model reveals the dynamics of the ionization process and the dominant channels for double (DI) and triple ionization (TI). Simulations using wave functions that respect the Pauli principle show that the most likely channel is a sequence of single ionization (SI) and DI, while direct TI has a much lower probability. The dominant DI process has the highest probability for a singlet of up- and down-spin electrons. The results demonstrate the significance of the Pauli principle for the selection of dominant pathways in ionization and possibly other many-electron processes in strong fields.

Enhanced high-harmonic generation from an all-dielectric metasurface

The recent observation of high-harmonic generation from solids creates a new possibility for engineering fundamental strong-field processes by patterning the solid target with subwavelength nanostructures. All-dielectric metasurfaces exhibit high damage thresholds and strong enhancement of the driving field, making them attractive platforms to control high-harmonics and other high-field processes at nanoscales. Here we report enhanced non-perturbative high-harmonic emission from a Si metasurface that possesses a sharp Fano resonance resulting from a classical analogue of electromagnetically induced transparency. Harmonic emission is enhanced by more than two orders of magnitude compared to unpatterned samples. The enhanced high harmonics are highly anisotropic with excitation polarization and are selective to excitation wavelength due to its resonant feature. By combining nanofabrication technology and ultrafast strong-field physics, our work paves the way for designing new compact ultrafast photonic devices that operate under high intensities and short wavelengths.

Cobalt-Induced Performance Instabilities of Mn–Zn Ferrite Cores

Stability and reliability of transformers and inductors in power applications strongly depend on the quality of the employed ferrite cores. Given the critical role of cores, Mn–Zn ferrites’ sensitivity to their magnetic history and environmental storage conditions is investigated. To this end, Co-doped and Co-free Mn–Zn ferrites are prepared and primarily characterized in terms of structural, microstructural, and magnetic properties. The produced cores are subjected to endurance tests of technical interest comprising the response to a saturating magnetic bias and longtime storage under elevated temperature or humidity levels. With the exception of the damp heat test, prior imposition of a strong field or thermal aging process are found to induce significant instabilities to the power loss density of the Co-containing Mn–Zn ferrite cores in particular. Depending on the excitation, power loss may notably increase in the low- or high-frequency regime, whereas the effects can be either reversed with time or time independent. The proposed underlying mechanisms are based on the magnetically and thermally induced reorientation of Co2+ cations, which affects magnetocrystalline anisotropy and domain wall kinetics. Studies of different stress tests are supported by analysis of complex permeability spectra and electrical characteristics.

Interplay between Coulomb-focusing and non-dipole effects in strong-field ionization with elliptical polarization

We study strong-field ionization and rescattering beyond the long-wavelength limit of the dipole approximation with elliptically polarized mid-IR laser pulses. Full three-dimensional photoelectron momentum distributions (PMDs) measured with velocity map imaging and tomographic reconstruction revealed an unexpected sharp ridge structure in the polarization plane (2018 Phys. Rev. A 97 013404). This thin line-shaped ridge structure for low-energy photoelectrons is correlated with the ellipticity-dependent asymmetry of the PMD along the beam propagation direction. The peak of the projection of the PMD onto the beam propagation axis is shifted from negative to positive values when the sharp ridge fades away with increasing ellipticity. With classical trajectory Monte Carlo simulations and analytical analysis, we study the underlying physics of this feature. The underlying physics is based on the interplay between the lateral drift of the ionized electron, the laser magnetic field induced drift in the laser propagation direction, and Coulomb focusing. To apply our observations to emerging techniques relying on strong-field ionization processes, including time-resolved holography and molecular imaging, we present a detailed classical trajectory-based analysis of our observations. The analysis leads to the explanation of the fine structure of the ridge and its non-dipole behavior upon rescattering while introducing restrictions on the ellipticity. These restrictions as well as the ionization and recollision phases provide additional observables to gain information on the timing of the ionization and recollision process and non-dipole properties of the ionization process.

Virtual detector theory for strong-field atomic ionization

A virtual detector (VD) is an imaginary device located at a fixed position in space that extracts information from the wave packet passing through it. By recording the particle momentum and the corresponding probability current at each time, the VDs can accumulate and build the differential momentum distribution of the particle, in a way that resembles real experiments. A mathematical proof is given for the equivalence of the differential momentum distribution obtained by the VD method and by Fourier transforming the wave function. In addition to being a tool for reducing the computational load, VDs have also been found useful in interpreting the ultrafast strong-field ionization process, especially the controversial quantum tunneling process.