Ultrashort Intense Laser Fields Research Articles

Direct manipulation of atomic excitation with intense extreme-ultraviolet laser fields

The coherent excitation and manipulation of a two-level system with ultrashort intense extreme-ultraviolet laser fields is investigated theoretically, based on numerically solving the time-dependent Schr\"odinger equation. We are particularly interested in the dynamical phase excursion of the energy states over the course of the interaction and the resulting spectral modifications. Fitting the absorption line shapes with the Fano profile quantifies the asymmetry parameter and the corresponding dipole phase offset, capturing the phase difference of the state coefficients after the interaction. Nonperturbative analytical calculations using rectangular driving pulses are employed, yielding physical insights into the dependence of the dipole phase shift on the external field. The validity of the formulas is validated by comparing their predictions with numerical results, which proves to be robust against the variation of laser parameters. The present investigation of strong-field dressing effects complements recent attosecond transient absorption studies assuming weak excitation, and marks a ubiquitous phenomenon that should be generally considered for the interaction of matter with intense laser fields.

Asymmetric Dissociative Tunneling Ionization of Tetrafluoromethane in ω - 2ω Intense Laser Fields.

Dissociative ionization of tetrafluoromethane (CF4) in linearly polarized ω-2ω ultrashort intense laser fields (1.4 × 1014 W/cm2, 800 and 400 nm) has been investigated by three-dimensional momentum ion imaging. The spatial distribution of produced by CF4 → + F + e− exhibited a clear asymmetry with respect to the laser polarization direction. The degree of the asymmetry varies by the relative phase of the ω and 2ω laser fields, showing that 1) the breaking of the four equivalent C-F bonds can be manipulated by the laser pulse shape and 2) the C-F bond directed along the larger amplitude side of the ω-2ω electric fields tends to be broken. Weak-field asymptotic theory (WFAT) shows that the tunneling ionization from the 4t 2 second highest-occupied molecular orbital (HOMO-1) surpasses that from the 1t 1 HOMO. This predicts the enhancement of the tunneling ionization with electric fields pointing from F to C, in the direction opposite to that observed for the asymmetric fragment ejection. Possible mechanisms involved in the asymmetric dissociative ionization, such as post-ionization interactions, are discussed.

Ionization and dissociation dynamics of H2O in ultrashort intense near-IR laser fields by the time-dependent adiabatic state method and the time-dependent configuration interaction method

In order to investigate dissociation dynamics of water molecules in a near-IR femtosecond intense laser pulse, we performed ab initio molecular dynamics calculations combined with the time-dependent adiabatic state approach. We showed that H+ is produced from H2O2+ not only by the two-body dissociation but also by the three-body dissociation and that the kinetic energy distributions of H+ for the two-body dissociation are in good agreement with the corresponding experimental data. We also performed time-dependent configuration interaction calculations to estimate the ionization probability of H2O2+ and confirmed that H2O2+ is ionized to H2O3+ by the charge-resonance enhanced ionization mechanism.

Femtosecond Laser-Assisted Electron Scattering for Ultrafast Dynamics of Atoms and Molecules

The recent progress in experimental studies of laser-assisted electron scattering (LAES) induced by ultrashort intense laser fields is reviewed. After a brief survey of the theoretical backgrounds of the LAES process and earlier LAES experiments started in the 1970s, new concepts of optical gating and optical streaking for the LAES processes, which can be realized by LAES experiments using ultrashort intense laser pulses, are discussed. A new experimental setup designed for measurements of LAES induced by ultrashort intense laser fields is described. The experimental results of the energy spectra, angular distributions, and laser polarization dependence of the LAES signals are presented with the results of the numerical simulations. A light-dressing effect that appeared in the recorded LAES signals is also shown with the results of the numerical calculations. In addition, as applications of the LAES process, laser-assisted electron diffraction and THz-wave-assisted electron diffraction, both of which have been developed for the determination of instantaneous geometrical structure of molecules, are introduced.

Coincidence momentum imaging of four- and three-body Coulomb explosion of formaldehyde in ultrashort intense laser fields

Ion-coincidence momentum imaging of Coulomb explosion of formaldehyde (H2CO) in femtosecond intense laser fields (800 nm, 1.3 × 1015 W/cm2) is performed with two different laser pulse durations (7 fs and 35 fs). In the 7-fs laser fields, the full fragmentation pathway from H2CO4+ → H+ + H+ + C+ + O+ is identified. The angles between the fragment momenta are well reproduced by a simple Coulomb explosion model from the geometry of neutral formaldehyde, showing that the molecular structure is virtually frozen along the bending coordinates during the multiple ionization. Three-body Coulomb explosion pathways from triply charged states, H2CO3+ → H+ + H+ + CO+ and H+ + CH+ + O+, are observed in both the 7-fs and 35-fs laser fields. Significant changes in the momentum angle distribution and asymmetric energy partitioning between two H+ ions are observed in the 35-fs case, which are attributed to structural deformation prior to the Coulomb explosion in the longer pulse.

Generation of isolated attosecond pulses by polarization gating of high-order harmonic emission from in intense ultrashort laser fields

This study presents a quantum-mechanical investigation of single attosecond pulse generation by gating high-order harmonic emission from H molecules in intense laser pulses with time-dependent ellipticity. The high-order harmonic generation from H molecules under left- and right-hand circularly polarized Gaussian pulses in superposition, is studied, and the effect of time duration and carrier-envelope phase of laser field on single attosecond pulse generation is investigated. Using a laser field formed by combination of left- and right-hand elliptically polarized Gaussian pulses, the effect of ellipticity of field on the duration and intensity of the attosecond pulses generated is studied. Numerical calculations show that, with the conventional polarization gating, a single attosecond pulse can be isolated from the pulse train emitted by the H molecule in a laser field.

Ultracold-atom quantum simulator for attosecond science

A quantum simulator based on ultracold optically trapped atoms for simulating the physics of atoms and molecules in ultrashort intense laser fields is introduced. The slowing down by about 13 orders of magnitude allows to watch in slow motion the tunneling and recollision processes that form the heart of attosecond science. The extreme flexibility of the simulator promises a deeper understanding of strong-field physics, especially for many-body systems beyond the reach of classical computers. The quantum simulator can experimentally straightforwardly be realized and is shown to recover the ionization characteristics of atoms in the different regimes of laser-matter interaction.

Double Ionization Dynamics of Molecular Hydrogen in Ultrashort Intense Laser Fields**Supported by the National Basic Research Program of China under Grant No 2013CB922201, and the National Natural Science Foundation of China under Grant Nos 11304365, 11374329 and 11334009.

We experimentally investigate the double ionization of molecular hydrogen subjected to ultrashort intense laser pulses. The total kinetic energy release of the two coincident H+ ions, which provides a diagnosis of different processes to double ionization of H2, is measured for two different pulse durations, i.e., 25 and 5 fs, and various laser intensities. It is found that, for the long pulse duration (i.e., 25 fs), the double ionization occurs mainly via two processes, i.e., the charge resonance enhanced ionization and recollision-induced double ionization. Moreover, the contributions from these two processes can be significantly modulated by changing the laser intensity. In contrast, for a few-cycle pulse of 5 fs, only the recollsion-induced double ionization survives, and in particular, this process could be solely induced by the first-return recollision at appropriate laser intensities, providing an efficient way to probe the sub-laser-cycle molecular dynamics.

Coincidence momentum imaging of asymmetric Coulomb explosion of CO2 in phase-locked two-color intense laser fields

Coulomb explosion of CO2, CO2→CO22++2e−→CO++O++2e−, in phase-locked two-color ultrashort intense laser fields (800nm and 400nm, ∼1014W/cm2) has been investigated by coincidence momentum imaging. The momentum images of the O+ and CO+ fragments show asymmetric distributions along the laser polarization direction depending on the relative phase of the two-color laser fields. The fragment asymmetry becomes most prominent (∼4%) at the relative phases providing the largest asymmetry of the electric field amplitude, with the O+ fragments preferentially ejected to the stronger field side. The mechanism of the asymmetric bond breaking of the two equivalent CO bonds is discussed in terms of the nuclear dynamics in CO22+.

Femtosecond two-photon Rabi oscillations in excited He driven by ultrashort intense laser fields

Two-photon Rabi oscillations are observed in He on a timescale of 10 fs by utilizing the strong-field phenomenon known as Freeman resonance. The importance of ponderomotive shifts of both the Rydberg states and the ionization limit is highlighted. Coherent light–matter interaction provides powerful methods for manipulating quantum systems1,2,3. Rabi oscillation is one such process. As it enables complete population transfer to a target state, it is thus routinely exploited in a variety of applications in photonics, notably quantum information processing4,5. The extension of coherent control techniques to the multiphoton regime offers wider applicability, and access to highly excited or dipole-forbidden transition states. However, the multiphoton Rabi process is often disrupted by other competing nonlinear effects such as the a.c. Stark shift, especially at the high laser-field intensities necessary to achieve ultrafast Rabi oscillations6. Here we demonstrate a new route to drive two-photon Rabi oscillations on timescales as short as tens of femtoseconds, by utilizing the strong-field phenomenon known as Freeman resonance7. The scenario is not specific to atomic helium as investigated in the present study, but broadly applicable to other systems, thus opening new prospects for the ultrafast manipulation of Rydberg states8.

Structural determination of argon trimer

Rare gas clusters are model systems to investigate structural properties at finite size. However, their structures are difficult to be determined with available experimental techniques because of the strong coupling between the vibration and the rotation. Here we experimentally investigated multiple ionization and fragmentation dynamics of argon trimer by ultrashort intense laser fields and reconstructed their structures with Coulomb explosion technique. The measured structure distribution was compared with our finite-temperature ab initio calculations and the discrepancy was discussed. The present study provides a guidance for the development of theoretical methods for exploring the geometric structure of rare gas clusters.

Effects of nuclear vibration on the ionization process of H2+ in ultrashort intense laser field

By numerically solving the time-dependent Schrödinger equation, we calculate the ionization probability of a vibrating H2+ exposed to ultrashort intense laser fields. The results show that the ionization probability increases by time and gets a saturation value. We also find that with some first vibration levels, the ionization probability from a higher vibration level is larger than that from a lower one. However, with higher vibration levels, at a certain level the ionization probability will take maximum and decrease with next levels.

High harmonic generation from pre-ionized H2 in ultrashort intense laser fields

Effects of the laser pulse wavelength and intensity on the high harmonic generation (HHG) production from the ionic and homolytic pre-ionization transient states of the two-electron H2 system exposed to ultrashort intense laser pulses are studied by solving the time-dependent Schrödinger equation. It is found that for the populated homolytic species in each half cycle of the laser pulse having enough strength, a pair of strong and weak HHG radiations are produced simultaneously. For the populated ionic species divided into two regions, in each half cycle, one of the ionic regions has a strong radiation and the other ionic region has a weak HHG radiation. The HHG spectra of the homolytic and ionic species are almost the same, except that in some limited parts of the HHG spectrum, one of them dominates the other.

High-order-harmonic generation of Ar atoms in intense ultrashort laser fields: An all-electron time-dependent density-functional approach including macroscopic propagation effects

We present an ab initio approach for the study of macroscopic high-order-harmonic generation (HHG) of rare-gas atoms (Ar) in intense ultrashort laser fields. The single-atom response is calculated by means of the self-interaction-free time-dependent density-functional theoretical (TDDFT) approach with proper long-range potential, and the macroscopic propagation effects are taken into account by solving Maxwell's equations using the induced dipole moments of the single-atom multiple electron shell. We perform all-electron nonperturbative investigation of macroscopic HHG of rare-gas atoms with the emphasis on the role of multiple valence electron subshells. In particular, we found that the valence electron subshell features a destructive interference distribution in multiphoton resonant HHG. In addition, our results show the Cooper minimum in macroscopic HHG from Ar atoms is located at 53 eV, in good agreement with the experimentally measured minimum position.

Hydrogen scrambling in H3+generation from ethane induced by ultrashort intense laser fields

Two-body Coulomb explosion processes of ethane (CH3 CH3 ) and its isotopomers (CD3 CD3 and CH3 CD3 ) induced by an intense laser field are investigated by a coincidence momentum imaging method. From the yield ratios of H3 + , H2 D+ , HD2 + , and D3 + ejected from CH3 CD3 + induced by an ultrashort-pulsed intense laser field, nearly statistical randomization of H and D atoms called hydrogen atom scrambling was identified.

Decrypting the charge-resolved kinetic-energy spectrum in the Coulomb explosion of argon clusters

Ion emissions from clusters in intense ultrashort laser fields have been studied predominantly using time-of-flight (TOF) spectroscopy so far. Assuming atomic ion emission, arrival time signal is converted to a charge-integrated kinetic-energy spectrum. We present here a Thomson parabola spectrum that decrypts the charge-integrated energy distribution to the charge-resolved kinetic-energy spectra (CRKES). TOF measurements compare well with the spectrum generated by encrypting back the CRKES. A quantitative measure of ionization probabilities of Ar${}_{36\phantom{\rule{0.16em}{0ex}}000}$ clusters to varied charge states at $7\ifmmode\times\else\texttimes\fi{}{10}^{15}$ W cm${}^{\ensuremath{-}2}$ is compared with three-dimensional microscopic particle-in-cell simulations. A good agreement between these detailed measurements and the simulations shows the possibility for the retrieval of charge distribution within a nanocluster.

A magnetic-bottle multi-electron-ion coincidence spectrometer

A novel multi-electron-ion coincidence spectrometer developed on the basis of a 1.5 m-long magnetic-bottle electron spectrometer is presented. Electrons are guided by an inhomogeneous magnetic field to a detector at the end of the flight tube, while a set of optics is used to extract counterpart ions to the same detector, by a pulsed inhomogeneous electric field. This setup allows ion detection with high mass resolution, without impairing the high collection efficiency for electrons. The performance of the coincidence spectrometer was tested with double ionization of carbon disulfide, CS(2) → CS(2)(2+) + e(-) + e(-), in ultrashort intense laser fields (2.8 × 10(13) W/cm(2), 280 fs, 1030 nm) to clarify the electron correlation below the rescattering threshold.

Erratum: Structure and dynamics of cluster plasmas created by ultrashort intense laser fields [Phys. Rev. A73, 031201 (2006)

Erratum: Structure and dynamics of cluster plasmas created by ultrashort intense laser fields [Phys. Rev. A<b>73</b>, 031201 (2006)

Single-active-electron approximation for molecules in strong laser fields : Test application to H2

A new method for solving the electronic three-dimensional time-dependent Schrödinger equation (TDSE) for molecules in ultrashort intense laser fields was developed. In this method the molecules are described within the single-active-electron (SAE) approximation using density-functional theory (DFT). The method and its implementation is tested for H2 for which a full six-dimensional two-electron solution is obtained via a time-dependent configuration-interaction approach. For larger molecular systems (for which no full solution is possible) the novel SAE method can, e.g., be used to test the validity of simplified SAE-based models like the molecular strong-field approximation (MO-SFA) or the molecular-orbital Ammosov-Delone-Krainov (MO-ADK) model.

Theoretical study of orientation-dependent multiphoton ionization of polyatomic molecules in intense ultrashort laser fields: A new time-dependent Voronoi-cell finite difference method

We present a new grid-based time-dependent method to investigate multiphoton ionization (MPI) of polyatomic molecules in intense ultrashort laser fields. The electronic structure of polyatomic molecules is treated by the density-functional theory (DFT) with proper long-range potential and the Kohn–Sham equation is accurately solved by means of the Voronoi-cell finite difference method on non-uniform and highly adaptive molecular grids utilizing geometrical flexibility of the Voronoi diagram. This method is generalized to the time-dependent problems with the split-operator time-propagation technique in the energy representation, allowing accurate and efficient non-perturbative treatment of attosecond electronic dynamics in strong fields. The new procedure is applied to the study of MPI of N 2 and H 2O molecules in intense linearly-polarized and ultrashort laser fields with arbitrary field–molecule orientation. Our results demonstrate that the orientation dependence of MPI is determined not just by the highest-occupied molecular orbital (HOMO) but also by the symmetries and dynamics of other contributing molecular orbitals. In particular, the inner orbitals can show dominant contributions to the ionization processes when the molecule is aligned in some specific directions with respect to the field polarization. This feature suggests a new way to selectively probe individual orbitals in strong-field electronic dynamics.