Near-infrared Laser Field Research Articles

Strong-field ionization dynamics of asymmetric equilateral triatomic model molecules in circularly polarized laser fields

The strong-field-driven ionization dynamics of larger, nondiatomic molecules still constitutes a major challenge for a theoretical and numerical description. While a full numerical treatment of all electronic and nuclear degrees of freedom is computationally prohibitive, several approaches exist to model photoelectron spectra employing certain approximations. Among them are classical trajectory methods or the so-called strong-field approximation (SFA). Here we investigate in detail the strong-field ionization dynamics of simple, noncollinear triatomic model systems. For a detailed characterization of the ionization process of our system, we combine quantum dynamical simulations, modified classical trajectory calculations, and extensions of the SFA in order to analyze the complex pattern of the photoelectron momentum distribution. We aim at disentangling the contributions of excited states and the long-range character of the potential. We show that upon interaction with circularly polarized laser fields, the long-range character merely induces a small shift in the spectra, while the contribution of excited states is in several cases essential: in particular in near-infrared laser fields, compared to mid-infrared drivers, and for systems with larger internuclear distances, when excited electronic states are energetically closer. Key modifications are for our trajectory model to explicitly incorporate the numerically obtained tunnel barrier width directly into the ionization rate, thereby enabling a spatially anisotropic ionization rate, and an approximate treatment of the contribution of excited states via the Stark shift. In contrast, for the SFA, it is important to include electronically excited states for the description of the initial state. We also discuss the effect of the carrier-envelope phase on the ionization dynamics, as well as the influence of molecular averaging.

Dichroism in two-color above-threshold ionization with twisted XUV beams and intense infrared laser fields

We theoretically investigate the two-color above-threshold ionization of atoms and ions by twisted XUV Bessel and Laguerre-Gaussian (LG) beams in the presence of a strong circularly polarized near-infrared (NIR) laser field. The presence of the NIR field modifies the continuum states accessible to the photoelectron. Based on the strong-field approximation, we explore the resulting energy and angular distributions of photoelectron as a function of the beam parameters. In particular, we analyze dichroism signals that arise due to the twisted nature of the XUV beam and the helicity of the NIR field. We focus on the comparison between LG beams and Bessel beams in the paraxial approximation. Here, we find that both beams yield similar results when the paraxial regime is valid. For localized targets, the dichroism signals strongly depend on the size and position of the atoms relative to the beam axis. Moreover, the dichroism signal tends to zero when the XUV LG beam is linear polarized. Detailed computations of the dichroism are performed and discussed for the $4s$ valence-shell photoionization of ${\mathrm{Ca}}^{+}$ ions.

Sub-cycle attosecond control in frustrated double ionization of molecules

We demonstrate sub-cycle control of frustrated double ionization (FDI) of the two-electron triatomic molecule when driven by two orthogonally polarized two-color laser fields, where a weak mid-infrared laser field is employed to probe the FDI process triggered by a strong near-infrared laser field. We use a three-dimensional semi-classical model that fully accounts for the electron and nuclear motion in strong fields. We analyze the FDI probability and the distribution of the momentum of the escaping electron along the polarization direction of the mid-infrared laser field. These observables when considered in conjunction bear clear signatures of sub-cycle control of FDI. We find that the momentum distribution of the escaping electron has a striking hive-shape with features that can accurately be mapped to the time that one of the two electrons tunnel-ionizes at the start of the break-up process. This mapping distinguishes consecutive tunnel-ionization times within a cycle of the mid-infrared laser field but not tunnel-ionization times differing by an integer number of cycles.

An anatomy of strong-field ionization-induced air lasing

It is known that in an intense laser field of a sufficiently high strength combined with a sufficiently long wavelength, i.e., in the regime of the Keldysh parameter γ < 1, photoionization of atoms and molecules can be realized through a quantum tunnel process. The tunnel ionization preferentially occurs from the orbital with the lowest ionization energy, thus the majority of the generated ions will stay on the ground state. It is surprising that tunnel ionization of nitrogen molecules with mid- and near-infrared intense laser fields can initiate strong laser-like emissions, indicating generation of stimulated emissions in molecular nitrogen ions. The physical mechanism behind the observation is still under debate. Here, we review the major progresses we made in the past a few years. The focus is placed on investigations on the lasing action at 391 nm wavelength initiated by either mid-infrared strong laser fields in the wavelength range from 1.2 to 2 µm or near-infrared intense laser fields around 800 nm wavelength. We reveal that the mechanisms of lasing actions are different for the pump lasers in the above two spectral regions. We also show that the coherent wavepackets of molecular nitrogen ions generated in the intense laser fields uniquely allow for efficient nonlinear interaction with light at resonance frequencies.

Effect of two-center interference on molecular ionization in multiphoton ionization regime.

Using solution of the full three-dimensional time-dependent Schrödinger equation (TDSE) in prolate spheroidal coordinates, we investigate the orientation dependence of ionization of H2+ in near-infrared laser fields. It is found that, the ionization probability decreases as a function of the alignment angle in tunneling ionization regime, while it ascends with the increase of orientation angle in multiphoton ionization regime for the internuclear distance R=2 a.u. Furthermore, the result obtained by the length gauge strong-field approximation theory is in qualitative agreement with that calculated by the TDSE but the radiation gauge strong-field approximation and molecular ADK theories fail to reproduce the TDSE result. Analysis indicates that the above intriguing feature can be ascribed to the interference between the partial electron wave packets emitted from different molecular cores, which becomes evident at low laser intensity due to increased width of the initial mechanical momentum of the photoelectron at ionization moment. In addition, when the internuclear distance increases to R=4 a.u., the ionization yields decrease vs alignment angle in both tunneling and multiphoton regimes since the electron wavefunction of the 1σg orbit is more concentrated in the molecular axis than that of R=2 a.u.

Multielectron effects in the photoelectron momentum distribution of noble-gas atoms driven by visible-to-infrared-frequency laser pulses: A time-dependent density-functional-theory approach

We present the photoelectron momentum distributions (PMDs) of helium, neon, and argon atoms driven by a linearly polarized, visible (527-nm) or near-infrared (800-nm) laser pulse (20 optical cycles in duration) based on the time-dependent density-functional theory (TDDFT) under the local-density approximation with a self-interaction correction. A set of time-dependent Kohn-Sham equations for all electrons in an atom is numerically solved using the generalized pseudospectral method. An effect of the electron-electron interaction driven by a visible laser field is not recognizable in the helium and neon PMDs except for a reduction of the overall photoelectron yield, but there is a clear difference between the PMDs of an argon atom calculated with the frozen-core approximation and TDDFT, indicating an interference of its $M$-shell wave functions during the ionization. Furthermore, we find that the PMDs of degenerate $p$ states are well separated in intensity when driven by a near-infrared laser field, so that the single-active-electron approximation can be adopted safely.

Recollision as a probe of magnetic-field effects in nonsequential double ionization

Fully accounting for non-dipole effects in the electron dynamics, double ionization is studied for He driven by a near-infrared laser field and for Xe driven by a mid-infrared laser field. Using a three-dimensional semiclassical model, the average sum of the electron momenta along the propagation direction of the laser field is computed. This sum is found to be an order of magnitude larger than twice the average electron momentum along the propagation direction of the laser field in single ionization. Moreover, the average sum of the electron momenta in double ionization is found to be maximum at intensities smaller than the intensities satisfying previously predicted criteria for the onset of magnetic field effects. It is shown that strong recollisions are the reason for this unexpectedly large value of the sum of the momenta along the direction of the magnetic component of the Lorentz force.

High-order multiphoton laser-assisted elastic electron scattering by Xe in a femtosecond near-infrared intense laser field: Plateau in energy spectra of scattered electrons

High-order multiphoton laser-assisted elastic electron scattering by Xe in a femtosecond near-infrared intense laser field: Plateau in energy spectra of scattered electrons

Dissociative photoionization of 1,2-dichloroethane in intense near-infrared femtosecond laser field

We experimentally demonstrate the dissociative photoionization of 1,2-C2H4Cl2 molecules in femtosecond laser field by time-of-flight mass spectrum and dc-slice imaging technology. Our results show the low kinetic energy components are from the dissociative ionization process of singly charged molecular ions, and the positive charge assignment are greatly influenced by the appearance energy of the fragment ions. The high kinetic energy components result from Coulomb explosion of multi-charged molecular ions, and the different angular distribution of these fragments along CC and CCl bond dissociation can be explained by the potential energy surfaces of the molecular ions.

Unravelling the dynamical origin of below- and near-threshold harmonic generation of H2+ in an intense NIR laser field.

Recently, the study of near- and below- threshold regime harmonics as a potential source of intense coherent vacuum-ultraviolet radiation has received considerable attention. However, the dynamical origin of these lower harmonics, particularly for the molecular systems, is less understood and largely unexplored. Here we perform the first fully ab initio and high precision 3D quantum study of the below- and near-threshold harmonic generation of molecules in an intense 800-nm near-infrared (NIR) laser field. Combining with a synchrosqueezing transform of the quantum time-frequency spectrum and an extended semiclassical analysis, we explore in-depth the roles of various quantum trajectories, including short- and long trajectories, multiphoton trajectories, resonance-enhanced trajectories, and multiple rescattering trajectories of the below- and near- threshold harmonic generation processes. Our results shed new light on the dynamical origin of the below- and near-threshold harmonic generation and various quantum trajectories for diatomic molecules for the first time.

Nonsequential double ionization with mid-infrared laser fields.

Using a full-dimensional Monte Carlo classical ensemble method, we present a theoretical study of atomic nonsequential double ionization (NSDI) with mid-infrared laser fields, and compare with results from near-infrared laser fields. Unlike single-electron strong-field processes, double ionization shows complex and unexpected interplays between the returning electron and its parent ion core. As a result of these interplays, NSDI for mid-IR fields is dominated by second-returning electron trajectories, instead of first-returning trajectories for near-IR fields. Some complex NSDI channels commonly happen with near-IR fields, such as the recollision-excitation-with-subsequent-ionization (RESI) channel, are virtually shut down by mid-IR fields. Besides, the final energies of the two electrons can be extremely unequal, leading to novel e-e momentum correlation spectra that can be measured experimentally.

Core-level attosecond transient absorption spectroscopy of laser-dressed solid films of Si and Zr

We investigated experimentally as well as theoretically the ultrafast response of the wave function of the conduction band (CB) of Si and Zr to a near-infrared laser field using extreme ultraviolet (XUV) absorption spectroscopy in the spectral range of 80--220 eV. The measured dynamics of the XUV transmission demonstrates that the wave function of the CB follows the electric field of the dressing laser pulse. In these terms, laser dressing was earlier mainly studied on gases. Measurements with two-femtosecond and 200-attosecond temporal steps were performed in the vicinity of the $\mathrm{Si}\phantom{\rule{0.16em}{0ex}}{L}_{2,3}$ edge near 100 eV, the $\mathrm{Si}\phantom{\rule{0.16em}{0ex}}{L}_{1}$ edge near 150 eV, and the $\mathrm{Zr}\phantom{\rule{0.16em}{0ex}}{M}_{4,5}$ edge near 180 eV. The observed changes were dependent on the core states being excited by the XUV probe pulse. At the $2p$ to CB transitions of Si, the XUV transmission increased via the effect of the dressing laser pulse, while at the $2s$ to CB transition of Si and the $3d$ to CB transition of Zr, the XUV transmission decreased. Furthermore, beats between the transition from $2{p}_{1/2}$ and $2{p}_{3/2}$ levels of Si and from $3{d}_{3/2}$ and $3{d}_{5/2}$ levels of Zr were observed with 20.7 fs and 3.6 fs periods.

Frustrated double ionization in two-electron triatomic molecules

Using a semi-classical model, we investigate frustrated double ionization (FDI) in $\mathrm{D_3^+}$, a two-electron triatomic molecule, when driven by an intense, linearly polarized, near-infrared (800 nm) laser field. We compute the kinetic energy release of the nuclei and find a good agreement between experiment and our model. We explore the two pathways of FDI and show that, with increasing field strength, over-the-barrier ionization overtakes tunnel ionization as the underlying mechanism of FDI. Moreover, we compute the angular distribution of the ion fragments for FDI and identify a feature that can potentially be observed experimentally and is a signature of only one of the two pathways of FDI.

The spatial distribution in high-order harmonic generation of H2+ with different time delays of the two-color laser fields

We theoretically investigate the high-order harmonic generation (HHG) and the spatial distribution in HHG of the H2+ molecule by introducing a time-delayed two-color laser field which consists of the mid-infrared and near-infrared laser field. For the different time delays of the two-color laser fields, the pulse shapes are changed which result in the variation of the electron recombined with the nuclei along the positive- or negative-z direction. When the time delay is 0fs (1.34fs), a smooth harmonic plateau from the electron recombined with the nuclei along the negative (positive)-z direction can be achieved. An isolated attosecond pulse with a duration of about 99 as is generated when the time delay is 1.34fs. We perform the classical analysis which is consistent with the numerical results from the one-dimensional non-Born-Oppenheimer time dependent Schrödinger equation (TDSE). We also investigate emission time of harmonics in terms of a time-frequency analysis to further understand the underlying physical mechanism.

Alignment dependence of photoelectron momentum distributions of atomic and molecular targets probed by few-cycle circularly polarized laser pulses

We present theoretical photoelectron momentum distributions (PMDs) for ionization from Ar($3p$) and ${\mathrm{H}}_{2}{}^{+}$(${\ensuremath{\sigma}}_{g}$) orbitals by few-cycle, high-intensity, near-infrared laser fields circularly polarized in the $xy$ plane. The three-dimensional time-dependent Schr\odinger equation is solved numerically within the single-active-electron approximation for Ar and within the fixed nuclei approximation for ${\mathrm{H}}_{2}{}^{+}$. The PMDs are investigated for alignment of the probed target orbitals relative to the polarization plane of the laser field. In the atomic case, the PMDs in the polarization plane for aligned $3p$ Ar orbitals are, up to an overall scaling factor, insensitive to alignment of the probed orbital, while the lateral PMDs show a signature of the orbital node when that node is sufficiently close to the polarization plane. For the molecular case of ${\mathrm{H}}_{2}{}^{+}$(${\ensuremath{\sigma}}_{g}$), our results show a significant impact of alignment on the PMDs due to the anisotropic molecular potential and the alignment-dependent coupling between the ground state and excited states.

Exploration of the origin of anomalous dependence for near-threshold harmonics in on the ellipticity of driving laser fields

The anomalous dependence of near-threshold harmonics in the molecular ion on the ellipticity of the driving near-infrared laser field is studied theoretically based on accurate solution of the time-dependent Schrödinger equation in prolate spheroidal coordinates with the help of the generalized pseudospectral method. For these harmonics, the maximum radiation energy corresponds to a non-zero ellipticity of the driving field. Our analysis reveals that the origin of the phenomenon lies in the near-resonant excitation of π-symmetry molecular orbitals. The excited states responsible for the anomalous ellipticity dependence of different near-threshold harmonics are identified. The effect is confirmed at the equilibrium internuclear separation R = 2 a.u. as well as for stretched molecules at R = 3 a.u.

Coulomb explosion of CS2 molecule under an intense femtosecond laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 51132004 and 11474096), and the Science and Technology Commission of Shanghai Municipality, China (Grant No. 14JC1401500). We acknowledge the support of the NYU-ECNU Institute of Physics at NYU Shanghai, China.

We experimentally demonstrate the Coulomb explosion process of CS2 molecule under a near-infrared (800 nm) intense femtosecond laser field by a DC-sliced ion imaging technique. We obtain the DC-sliced images of these fragment ions S+, S2+, CS+, and CS2+ by breaking one C–S bond, and assign their Coulomb explosion channels by considering their kinetic energy release and angular distribution. We also numerically simulate the dissociation dynamics of parent ions by a Coulomb potential approximation, and obtain the time evolution of Coulomb energy and kinetic energy release, which indicates that the dissociation time of parent ions decreases with the increase of the charge number k. These experimental and theoretical results can serve as a useful benchmark for those researchers who work in the related area.

Ultrafast two-photon Rabi oscillations in excited He driven by femtosecond intense laser fields

SynopsisWe demonstrate ultrafast multiphoton coherent excitation in near-infrared intense laser fields (≤10 TW/cm2) using excited helium in the 1s2p (1P) state as the target. Strong two-photon coupling between the 2p state and the 5f and 6f Rydberg states drives ultrafast Rabi oscillations in tens of femtosecond. Theoretical calculations solving the time-dependent Schrodinger equation were carried out to elucidate the strong field effects.

Exploration of the subcycle multiphoton ionization dynamics and transient electron density structures with Bohmian trajectories

An accurate three-dimensional numerical scheme for the De Broglie--Bohm framework of Bohmian mechanics is presented. This method is utilized to explore the subcycle multiphoton ionization dynamics of the hydrogen atom subject to intense near-infrared laser fields on the subfemtosecond time scale. The analysis of the time-dependent electron density reveals that several distinct density portions can be shaped and detached from the core within a half cycle of the laser field. As a complementary perspective, we identify several distinct groups of the Bohmian trajectories which represent the multiple detachments of the electron density at different times. The method presented provides very accurate electron densities and Bohmian trajectories that allow to uncover the origin of the formation of the transient and distinct electron structures seen in the multiphoton ionization processes.

Temporal broadening of attosecond photoelectron wavepackets from solid surfaces

The response of solids to electromagnetic fields is of crucial importance in many areas of science and technology. Many fundamental questions remain to be answered about the dynamics of the photoexcited electrons that underpin this response, which can evolve on timescales of tens to hundreds of attoseconds. How, for example, is the photoexcited electron affected by the periodic potential as it travels in the solid, and how do the other electrons respond in these strongly correlated systems? Furthermore, control of electronic motion in solids with attosecond precision would pave the way for the development of ultrafast optoelectronics. Attosecond electron dynamics can be traced using streaking, a technique in which a strong near-infrared laser field accelerates an attosecond electron wavepacket photoemitted by an extreme ultraviolet light pulse, imprinting timing information onto it. We present attosecond streaking measurements on the wide-bandgap semiconductor tungsten trioxide, and on gold, a metal used in many nanoplasmonic devices. Information about electronic motion in the solid is encoded on the temporal properties of the photoemitted electron wavepackets, which are consistent with a spread of electron transport times to the surface following photoexcitation.