Intense Few-cycle Laser Fields Research Articles

Dynamic interference minimum affected by permanent dipole in high-harmonic generation from CO molecules

We investigate the high-harmonic generation from CO molecules subjected to intense few-cycle laser fields by numerically solving the time-dependent Hartree–Fock equations. A dynamic minimum due to the interference of the HOMO and HOMO-2 orbitals is observed in the harmonic spectrum and further studies reveal that the HOMO-1 orbital plays an indispensable role in the formation of the minimum. Because the HOMO-1 orbital has a permanent dipole moment, it affects the coupling between HOMO and HOMO-2 orbitals during the time evolution, which can modify the phase difference between harmonic emission of HOMO and HOMO-2 orbitals in the propagation and recombination processes and relative contributions from two orbitals to the harmonic spectrum. This work sheds new insight into the study of multi-orbital effect in high-harmonic generation from polar molecules.

Absolute carrier-envelope-phase dependences of single and double ionization of methanol in a near-IR few-cycle laser field.

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH3OH) in an intense near-IR few-cycle laser field (2.1 × 1014 W/cm2) by the asymmetry in the ejection direction of CH3 + for the non-hydrogen migration channels and CH2 + for the hydrogen migration channels created through the C-O bond breaking after the ionization. Based on the absolute CEP values at the laser-molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1-053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition.

Asymmetry flip in photoelectron angular distribution of rare gas atoms in intense circularly-polarized few-cycle laser fields

An analytical formula representing the photoelectron kinetic energy at which the ejection direction of photoelectrons generated by an intense circularly-polarized few-cycle laser pulse flips was derived and was used for determining the laser pulse duration.

Spin-Orbit State-Selective C-I Dissociation Dynamics of the CH3I+ X̃ Electronic State Induced by Intense Few-Cycle Laser Fields.

Studies of ultrafast molecular dynamics induced by intense laser fields can reveal new approaches to manipulating chemical reactions in the strong-field regime. Here, we show that intense few-cycle laser pulses can induce the spin-orbit state-selective C-I dissociation of the iodomethane cation (CH3I+) in the X̃ electronic state. Irradiation of CH3I by 6 fs laser pulses with peak intensities of 1.9 × 1014 W/cm2 followed by femtosecond extreme ultraviolet probing of the iodine 4d core-level transitions reveals dissociation of the CH3I+ X̃ 2E1/2 state with a time constant of 0.76 ± 0.16 ps. By contrast, the X̃ 2E3/2 spin-orbit ground state does not exhibit any appreciable dissociation on the picosecond time scale. The observed spin-orbit state-selective dissociation of the X̃ state is rationalized in terms of the laser-induced coupling to the à state. Our results suggest that the intense-laser control of photodissociation channels can be potentially extended to spin-orbit split states.

Sub-cycle steering of the deprotonation of acetylene by intense few-cycle mid-infrared laser fields.

Directional breaking of the C-H/C-D molecular bond is manipulated in acetylene (C2H2) and deuterated acetylene (C2D2) by waveform controlled few-cycle mid-infrared laser pulses with a central wavelength around 1.6 μm at an intensity of about 8 × 1013 W/cm2. The directionality of the deprotonation of acetylene is controlled by changing the carrier-envelope phase (CEP). The CEP-control can be attributed to the laser-induced superposition of vibrational modes, which is sensitive to the sub-cycle evolution of the laser waveform. Our experiments and simulations indicate that near-resonant, intense mid-infrared pulses permit a higher degree of control of the directionality of the reaction compared to those obtained in near-infrared fields, in particular for the deuterated species.

High-harmonic generation in graphene: Interband response and the harmonic cutoff

We numerically examine the high-harmonic generation in graphene caused by an intense few-cycle terahertz laser field by solving the time-dependent Dirac equation. The observed spectra feature a complex interplay of interband and intraband electron dynamics. At high harmonic frequencies, we observe a plateau region with a cut-off frequency linearly proportional to the laser field. The linear dependence of the cutoff on the field resembles the behavior for electrons driven in bulk crystals but differs from the case of atoms or molecules. The unique features of the graphene band structure allow for a transparent decomposition of interband and intraband contributions and for an analytic estimate of the harmonic cut-off frequency in excellent agreement with the simulations.

Local-microfield enhancement of high-order harmonic generation and the associated electron dynamics

The high-order harmonic generation (HHG) by local-microfield enhancement is investigated using the simple diatomic model H2+, which is initially prepared in coherent superposition of the ground state and the first excited state. The high-order harmonic spectroscopy is calculated by solving time-dependent schrodinger equation (TDSE) using the split-operator technique in a large internuclear distance. Numerically we use asymmetric-spatial mask functions, multi-trajectories contributions are revealed in the harmonic spectra, which contribute to different frequencies of harmonic spectroscopy. Also we numerically solve the Newton equation to get the classical analysis, the maximum return energies show great agreement with the cut-off energies of high-order harmonics from different trajectrories. Due to the tunneling ionization has a sensitive response to the instantaneous electric field, all the trajectories can be controlled by the carrier-envelop phase of intense few-cycle laser field. It is shown that electrons migrating directly from one nucleus to the neighboring one without typical tunneling ionization, contribute dominantly to the lower order harmonics, and the intensities are dramatically enhanced along with the enhancement of the local-microfield. The migration of electrons between nuclei in multi-nuclei model will mimic the dynamics of electrons in solids, which will shed light on HHG from solids especially with local impurity.

Ionization dynamics ofC2H2in intense laser fields: Time-dependent Hartree-Fock approach

We investigate the multielectron ionization dynamics of C2H2 subjected to intense few-cycle laser fields with the time-dependent Hartree-Fock approach, in which one-center method is employed with the finite-element discrete-variable representation and B-spline functions. It is found that, when the photon energy is close to the energy gap between the two inner orbitals, an inner-orbital single-photon resonant transition phenomenon occurs and plays an important role in the ionization process of the molecule. Furthermore, the ionization of inner electrons surpasses that of the electrons in the highest occupied molecular orbital (HOMO) at high laser intensities, which may be attributed to the effect of the spatial distributions of molecular orbitals. Moreover, this phenomenon will be enhanced by a reduction of the screen for electrons in the HOMO due to the ionization of inner electrons.

Carrier-envelope-phase dependence of asymmetric C[sbnd]D bond breaking in C2D2 in an intense few-cycle laser field

The carrier envelope phase (CEP) dependence in the CD bond breaking of C2D2 induced by an intense few-cycle laser pulse is investigated. The ejection direction of D+ ions generated from the Coulomb explosion, C2D22+→C2D+ and D+, exhibits the CEP dependent asymmetry, while its CEP dependence is out of phase by π with respect to the CEP dependence of the recoil momentum of C2D2+. From a recollisional double ionization model, the asymmetry in the ejection direction of D+ was ascribed to the laser-assisted CD bond weakening in the few-cycle laser field.

Nonlinear response of hydrogen atom induced by intense few-cycle laser field and its influence on refractive index

Nonlinear response of hydrogen atom in intense few-cycle laser field is investigated through numerical integration of Schrödinger equation. According to the calculation results, in order to investigate the saturation of instantaneous response and higher-order Kerr effect (HOKE), the electron transitions between bound-excited states of hydrogen atom are needed to be taken in account. The pulse-defocusing induced by HOKE is not obvious due to the quick rise of plasma effect when the pulse carrier wavelength is 800nm. If the pulse wavelength is tuned from 800nm to 1800nm, there is an enlarging intensity window in which HOKE can play the defocusing role and can result in ionization-free filamentation, and beyond this window the plasma effect will dominate. With the increasing of pulse duration, this intensity window will become narrower because of the accumulation of plasma density. As a result, short pulses with wavelength longer than 800nm should be used to observe the ionization-free filamentation.

Carrier–envelope phase-tagged imaging of the controlled electron acceleration from SiO2 nanospheres in intense few-cycle laser fields

Waveform-controlled light fields offer the possibility of manipulating ultrafast electronic processes on sub-cycle timescales. The optical lightwave control of the collective electron motion in nanostructured materials is key to the design of electronic devices operating at up to petahertz frequencies. We have studied the directional control of the electron emission from 95 nm diameter SiO2 nanoparticles in few-cycle laser fields with a well-defined waveform. Projections of the three-dimensional (3D) electron momentum distributions were obtained via single-shot velocity-map imaging (VMI), where phase tagging allowed retrieving the laser waveform for each laser shot. The application of this technique allowed us to efficiently suppress background contributions in the data and to obtain very accurate information on the amplitude and phase of the waveform-dependent electron emission. The experimental data that are obtained for 4 fs pulses centered at 720 nm at different intensities in the range (1–4) × 1013 W cm−2 are compared to quasi-classical mean-field Monte-Carlo simulations. The model calculations identify electron backscattering from the nanoparticle surface in highly dynamical localized fields as the main process responsible for the energetic electron emission from the nanoparticles. The local field sensitivity of the electron emission observed in our studies can serve as a foundation for future research on propagation effects for larger particles and field-induced material changes at higher intensities.

Quantum phase-space analysis of electronic rescattering dynamics in intense few-cycle laser fields

We present a quantum phase-space analysis of the ionization and rescattering dynamics of a one-electron atom in an intense few-cycle laser field. Snapshots of the Wigner function W(x, p) will be analysed for both soft-core Coulomb and short-ranged potentials tuned to yield identical ground state energies and similar energies for the first lowest electronic states. The influence of the long-range Coulomb potential on the rescattering wavepacket in the continuum can be disentangled from that of the induced bound-state polarization. Short- and long-range atomic potentials entail differences in the rescattering dynamics, manifesting themselves in high-order harmonic generation spectra.

Controlled near-field enhanced electron acceleration from dielectric nanospheres with intense few-cycle laser fields

A demonstration of attosecond control of the motion and directed emission of electrons from individual silica nanoparticles using few-cycle laser fields opens new possibilities to manipulate electronic processes in nanoscale systems.

Selective Steering of Molecular Multiple Dissociative Channels with Strong Few-Cycle Laser Pulses

We report that multiple dissociative channels of carbon monoxide (CO) molecules are selectively controlled using intense phase-stabilized few-cycle laser fields (4.2 fs, 740 nm, 6×10(14) W/cm(2)). The controllable emission direction of C(2+) from charge asymmetrical dissociation and ionization of CO dications is out of phase in a linear polarized laser field. The strong coupling between the channels is explained as the competition of recollision excitation and recollision ionization in a recollision process, leading to the opposite asymmetrical property. The experimental result provides insight into high degree controlling molecular multiple dissociative processes in the time scale of electronic motion.

Sub-cycle strong-field interferometry

A nonlinear interferometry scheme is described theoretically to induce and resolve electron wavefunction beating on time scales shorter than the optical cycle of the time-delayed pump and probe pulses. By employing two moderately intense few-cycle laser fields with a stable carrier-envelope phase, a large range of the entire electronic level structure of a quantum system can be retrieved. In contrast to single-photon excitation schemes, the retrieved electronic states include levels that are both dipole- and non-dipole-accessible from the ground electronic state. The results show that strong-field interferometry can reveal both high-resolution and broad-band spectral information at the same time with important consequences for quantum-beat spectroscopy on attosecond time scales.

Molecular dynamics of CO in few-cycle laser fields

We experimentally study the ionization, fragmentation and Coulomb explosion of CO using 6 fs laser pulses. Different from previous observations in tens or hundreds of femtoseconds laser pulses, strong charge asymmetric dissociation and CO 2+ are observed in the current intense few-cycle laser field.

Imaging of carrier-envelope phase effects in above-threshold ionization with intense few-cycle laser fields

Sub-femtosecond control of the electron emission in above-threshold ionization of the rare gases Ar, Xe and Kr in intense few-cycle laser fields is reported with full angular resolution. Experimental data that were obtained with the velocity-map imaging technique are compared to simulations using the strong-field approximation (SFA) and full time-dependent Schrödinger equation (TDSE) calculations. We find a pronounced asymmetry in both the energy and angular distributions of the electron emission that critically depends on the carrier-envelope phase (CEP) of the laser field. The potential use of imaging techniques as a tool for single-shot detection of the CEP is discussed.

Time-resolved sequential double ionization ofD2molecules in an intense few-cycle laser pulse

The kinetic energy spectra of D{sup +} ions resulting from the sequential double ionization of D{sub 2} molecules in an intense few-cycle laser field are calculated. The D{sub 2} molecule is first ionized at its equilibrium distance, followed by further ionization of D{sub 2}{sup +} at larger internuclear separations. The time evolution of the nuclear wave packet is calculated including loss from ionization by the time-dependent laser field. The calculated D{sup +} energy spectra are compared with the recent experiment, and the time duration between the two ionization events is deduced to subfemtosecond accuracy.