Ionization Regime Research Articles

Filamentation of long-wave infrared pulses in the atmosphere [Invited

Filamentation of a high-power laser beam in air opens the possibility of the diffraction-compensated propagation of a laser beam over long distances and as such is being considered for remote stand-off detection, lightning control, free-space communications, and long-range projection of high-energy pulses. Switching to long-wave infrared (LWIR) range for filamentation, as shown in recent experiments, allows for generation of a single centimeter-diameter channel in air that, in comparison with a short-wavelength laser filament, has 4 orders of magnitude larger cross section and guides many joules of pulse energy over multiple Rayleigh distances at a clamped intensity of ∼1012W/cm2. Self-guiding of LWIR pulses in air arises from the balance between self-focusing, diffraction, and defocusing caused by free carriers generated via many-body Coulomb-induced ionization which effectively decrease the molecular polarizability during the long-wavelength laser pulse. Understanding where this new ionization regime plays a role, below the threshold of the well-adopted single-atom tunnel ionization in gases, could become a new frontier in strong-field interactions. This paper offers an overview of the atmospheric filamentation research at long-wave infrared wavelengths.

Waves in plasma formed by above-threshold ionization of gas atoms

In the regime of above-threshold ionization of gas atom in the field of laser radiation, plasma with photoelectron distribution consisting of peaks at discrete energy values is formed. It is shown that the number of longitudinal waves in such plasma coincides with the number of peaks in the distribution function. When peaks practically don't overlap, the dispersion law of each wave in the region of short waves is determined by electrons from the corresponding peak. In this case the phase and group velocities of the waves are close to the electron velocity, which corresponds to the peak maximum. It is possible to talk about such waves as an electronic sound, since the perturbations of the electron density mainly arise due to pressure perturbations. When the peaks are narrow, but having a finite width, the Cherenkov damping of waves is exponentially small. Numerical calculations the dispersion laws for of the two and four waves in photoionized xenon plasma, in which the electron distribution function consists of two or four narrow peaks are given.

Stabilization of Optical Pulse Propagation in a Regime of Ionization and Stimulated Raman Self-Scattering

Dynamics of the propagation of soliton-like pulses in a regime of tunnel ionization and stimulated Raman self-scattering is described analytically. It is shown that the mutual compensation of these effects stabilizes signal parameters. The corresponding formulas for pulse duration are obtained.

High-energy direct photoelectron spectroscopy in strong-field ionization

Recently, in a strong Coulomb field regime of tunneling ionization an unexpected large enhancement of photoelectron spectra due to the Coulomb field of the atomic core has been identified by numerical solution of time-dependent Schr\"odinger equation [Phys. Rev. Lett. \textbf{117}, 243003 (2016)] in the upper energy range of the tunnel-ionized direct electrons. We investigate the origin of the enhancement employing a classical theory with Monte Carlo simulations of trajectories, and a quantum theory of Coulomb-corrected strong field approximation based on the generalized eikonal approximation for the continuum electron. Although the quantum effects at recollisions with a small impact parameter yield an overall enhancement of the spectrum relative to the classical prediction, the high energy enhancement itself is shown to have a classical nature and is due to momentum space bunching of photoelectrons released not far from the peak of the laser field. The bunching is caused by a large and nonuniform, with respect to the ionization time, Coulomb momentum transfer at the ionization tunnel exit.

Attosecond pulse generation via multicolor beam superposition in the over-barrier ionization regime

Based on coherent superposition of multicolor continuous wave beams, high order harmonics and attosecond pulse generation from a hydrogen molecular ion (HMI), with different internuclear distances, R, are studied. Driving pulse intensity was taken to be 400 TW cm−2, which can ionize an ion with R > Rb = 2.4 a.u. over the barrier. It is seen that cutoff harmonic with respect to R decreases until the over barrier ionization threshold, Rb, then becomes almost uniform to the end of R = 10 a.u. whereas it is almost constant with respect to the spectral width of the beams, Δω. Besides attosecond pulse intensity increases versus R, and decreases with respect to Δω so that the most intense attosecond pulses can be generated from HMIs with R ≥ 5 a.u. via the pulses by Δω ≤ 0.03 a.u. Attosecond pulse durations were insensitive to both Δω and R.

Multiphoton photoelectron circular dichroism of limonene with independent polarization state control of the bound-bound and bound-continuum transitions.

Photoionization of randomly oriented chiral molecules with circularly polarized light leads to a strong forward/backward asymmetry in the photoelectron angular distribution. This chiroptical effect, referred to as Photoelectron Circular Dichroism (PECD), was shown to take place in all ionization regimes, from single photon to tunnel ionization. In the Resonance Enhanced Multiphoton Ionisation (REMPI) regime, where most of the table-top PECD experiments have been performed, understanding the role of the intermediate resonances is currently the subject of experimental and theoretical investigations. In an attempt to decouple the role of bound-bound and bound-continuum transitions in REMPI-PECD, we photoionized the (+)-limonene enantiomer using two-color laser fields in [1 + 1'] and [2 + 2'] ionization schemes, where the polarization state of each color can be controlled independently. We demonstrate that the main effect of the bound-bound transition is to break the sample isotropy by orientation-dependent photoexcitation, in agreement with recent theoretical predictions. We show that the angular distribution of PECD strongly depends on the anisotropy of photoexcitation to the intermediate state, which is different for circularly and linearly polarized laser pulses. On the contrary, the helicity of the pulse that drives the bound-bound transition is shown to have a negligible effect on the PECD.

Improvement of the ionization factor in the quantitative rescattering theory for simulating macroscopic high-order harmonic generation

We propose to correct the ionization factor in the quantitative rescattering (QRS) theory for a higher precision calculation of macroscopic high harmonic generation (HHG) in a gaseous medium. Compared to the first-order strong-field approximation (SFA-1) and the Ammosov-Delone-Krainov (ADK) ionization models, the Perelomov-Popov-Terent'ev (PPT) model achieves the best agreement with the macroscopic HHG spectra of Ar based on the solution of the time-dependent Schr\odinger equation (TDSE). The investigation of the ionization probability dependence on laser intensity indicates that the PPT model is consistent with the TDSE, both in the tunneling and multiphoton ionization regimes, and the SFA-1 or the ADK mostly works in the tunneling regime. This explains the difference in the total macroscopic harmonic spectra, mainly due to off-axis harmonic emissions generated by small laser intensities. The examples for Ne and Xe by using three ionization models in the QRS theory also show the significance of ionization correction by the PPT model to precisely simulate the macroscopic HHG spectra. This study will help further increase the accuracy of simulating measured HHG spectra quantitatively under a variety of macroscopic conditions.

High-order harmonic generation in H2+ via multicolor beam superposition: barrier suppression ionization regime

Generation of high-order harmonics (HOHs) and attosecond pulses in the barrier suppression ionization regime for a target of hydrogen molecular ion, $${\text{H}}_{2}^{+}$$ , with large inter nuclear distance is introduced in this work. Some broadband femtosecon laser pulses are designed based on multicolor beam superposition method in order to interact with $${\text{H}}_{2}^{+}$$ . Behavioral study of harmonics cutoff, attosecond pulse widths and intensities, along with the maximum ionization intensity with respect to the spectral width of driving pulses was carried out. Molecular orbitals in this study are estimated via the linear combination of atomic orbital approximation, and temporal variations of these orbitals are studied in the various ionization regimes. It is seen in this work that although the cutoff harmonic is increased by the driving pulse intensity, but it is insensitive to the spectral driving pulsewidth. This study showed that intensities of the generated attosecond pulses gradually decreased by increasing the bandwidth of the driving laser pulse, but the duration of them is shown to be insensitive in this respect.

Probing time delays and coherent imaging of multiphoton resonant ionization

We measure the laser-intensity-dependent photoelectron momentum and energy spectra and identify the emerged resonant $(4s,3d$, and $4f)$ states of multiphoton ionization of Ne at 400 nm. We employ parallelly polarized two-color fields involving a strong ionizing 400-nm field and a weak 800-nm field to probe the relative time delay of resonant states. By analyzing phase-resolved three-dimensional photoelectron momentum distributions, we find the time delay of resonant states has an anisotropic angle dependence. Using an intuitive model, we show that the angle-dependent time delay carries the information of subcycle evolution of quantum states and reflects the temporal coherent properties of multiphoton resonant states during absorption and emission of photons. We find that the relative time delays among the resonant ionization via $4s,3d$, and $4f$ states along the polarization depend on the number of photons absorbed. This study has implications for coherent imaging of electron wave packets in the multiphoton ionization regime.

Magnetic quantum number dependence of hydrogen photoelectron spectra under circularly polarized pulse in barrier suppression ionization regime

Photoelectron spectra and transition mechanisms of a hydrogen atom in the n = 2 state under circularly polarized pulses in the barrier suppression ionization (BSI) regime, found by solving the time-dependent Schrödinger equation, are presented in detail. The photoelectron peak emerges from low-energy, in contrast to being centered at higher energy in the case of ionization from the ground state. We show that the magnetic quantum number dependence of photoelectron angular distributions on initial states yields 3- and 2-band structures for 2p− and 2p0 in zenith angle, respectively, and 1-band structures for both 2p+ and 2s. The intermediate transition pathways from these initial states to the corresponding final states are also revealed.

Above-threshold ionization of hydrogen atom in chirped laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11664035, 11465016, 11764038, 11364038, and 11564033).

Above-threshold ionization (ATI) of a hydrogen atom exposed to chirped laser fields is investigated theoretically by solving the time-dependent Schrödinger equation. By comparing the energy spectra, the two-dimensional momentum spectra, and the angular distributions of photoelectron for the laser pulses with different chirp rates, we show a very clear chirp dependence both in the multiphoton and tunneling ionization processes but no chirp dependence in the single-photon ionization. We find that the chirp dependence in the multiphoton ionization based ATI can be attributed to the excited bound states. In the single-photon and tunneling ionization regimes, the electron can be removed directly from the ground state and thus the excited states may not be very important. It indicates that the chirp dependence in the tunneling ionization based ATI processes is mainly due to the laser pulses with different chirp rates.

The influence of the ionization regime on femtosecond laser beam machining mono-crystalline diamond

Diamond is widely used in a series of industrial and scientific applications, including the micro-electro-mechanical systems, optical elements, ultra-precision machining tools, etc. Femtosecond laser beam machining is promising for high precision and high accuracy machining diamond material for its famous non-thermal machining regime. Initially, the laser ablation threshold was determined for the low and high laser fluence stage, respectively. Then, the ionization regime of femtosecond pulsed laser beam machining mono-crystalline diamond with different laser power was discussed. The influence of the ionization regime on laser machining also was analyzed basing on Raman analysis and laser machined surface topography. The laser ablation threshold was 0.29 J/cm2 for the low laser fluence stage and 2.35 J/cm2 for the high laser fluence stage, respectively. The calculation of the Keldysh parameter indicated that the multi-photon ionization prevailed in the low laser fluence stage while the tunneling ionization became dominant in the high laser fluence stage. The transition of the ionization regime from the multi-photon ionization to the tunneling ionization occurred around the laser fluence 3.2 J/cm2. Graphitization of diamond material was discovered on laser machined diamond surface by means of Raman analysis. Graphitization would be enhanced by the tunneling ionization regime in the high laser fluence stage. A strong surface cracking tendency was also observed in laser machining in the high fluence stage.

Impact of Coulomb potential on peak structures arising in momentum and low-energy photoelectron spectra produced in strong-field ionization of laser-irradiated atoms

The phenomenon of pronounced peak structure(s) of longitudinal momentum distributions as well as a spike-like structure of low-energy spectra of photoelectrons emitted from laser-irradiated Ar and Ne atoms in a single ionization process is theoretically studied in the tunneling and multiphoton regimes of ionization. The problem is addressed assuming only the direct above-threshold ionization (ATI) as a physical mechanism underlying the phenomenon under consideration (viz. solely contributing to observed photoelectron momentum distributions (PMD)) and using the Coulomb–Volkov (CV) ansatz within the frame of conventional strong-field approximation (SFA) applied in the length-gauge formulation. The developed CV-SFA approach also incorporates the density functional theory essentially exploited for numerical composition of initial (laser-free) atomic state(s) constructed from atomic orbitals of Gaussian type. Our presented CV-SFA based (and laser focal-volume averaged) calculation results proved to be well reproducing both the pronounced double-peak and/or ATI-like multi-peak structure(s) experimentally observed in longitudinal PMD under conditions of tunneling and/or multiphoton regime, respectively. In addition, our CV-SFA results presented for tunneling regime also suggest and remarkably reproduce a pronounced structure observed in relevant experiments as a ‘spike-like’ enhanced maximum arising in low-energy region (around the value of about 1 eV) of photoelectron spectra. The latter consistency allows to identify and interpret these results as the so-called low-energy structure (LES) since the phenomenon proved to appear as the most prominent if the influence of Coulomb potential on photoelectron continuum states is maximally taken into account under calculations (viz. if the parameter Z in CV’s functions is put equal to 1). Moreover, the calculated LES proved to correspond (viz., established as closely related) to the mentioned double-peak structure arising in the low-momentum region ( a.u.) of longitudinal PMDs calculated under condition of the tunneling regime. Thus, the phenomena under consideration can be well understood and adequately interpreted beyond the terms and/or concepts of various different alternative strong-field approaches and models (such as e.g., extensively invoked and exploited nowadays though, more sophisticated SFA-based ‘rescattering’ mechanism) compared to which, the currently applied CV-SFA model (through the same underlying physical mechanism of solely direct ATI suggested) is additionally able to provide and reveal an intimate and transparent interrelation between the phenomena of LES and double-peak structure arising in PMDs observed in the tunneling regime.

Dynamics of tunneling ionization using Bohmian mechanics

Recent attoclock experiments and theoretical studies regarding the strong-field ionization of atoms by few-cycle infrared pulses revealed new features that have attracted much attention. Here we investigate tunneling ionization and the dynamics of the electron probability using Bohmian Mechanics. We consider a one-dimensional problem to illustrate the underlying mechanisms of the ionization process. It is revealed that in the major part of the below-the-barrier ionization regime, in an intense and short infrared pulse, the electron does not tunnel \through" the entire barrier, but rather already starts from the classically forbidden region. Moreover, we highlight the correspondence between the probability of locating the electron at a particular initial position and its asymptotic momentum. Bohmian Mechanics also provides a natural definition of mean tunneling time and exit position, taking account of the time dependence of the barrier. Finally, we find that the electron can exit the barrier with significant kinetic energy, thereby corroborating the results of a recent study [Camus et al., Phys. Rev. Lett. 119 (2017) 023201].

Symmetry Breaking and Strong Persistent Plasma Currents via Resonant Destabilization of Atoms.

The ionization rate of an atom in a strong optical field can be resonantly enhanced by the presence of long-living atomic levels (so-called Freeman resonances). This process is most prominent in the multiphoton ionization regime, meaning that the ionization event takes many optical cycles. Nevertheless, here, we show that these resonances can lead to rapid subcycle-scale plasma buildup at the resonant values of the intensity in the pump pulse. The fast buildup can break the cycle-to-cycle symmetry of the ionization process, resulting in the generation of persistent macroscopic plasma currents which remain after the end of the pulse. This, in turn, gives rise to a broadband radiation of unusual spectral structure, forming a comb from terahertz to visible. This radiation contains fingerprints of the attosecond electron dynamics in Rydberg states during ionization.

Insights on the Body Charging and Noise Generation by Impact Ionization in Fully Depleted SOI MOSFETs

Thin and ultrathin body-contacted Silicon-on-Insulator MOS-transistors have been used for the direct experimental measurement of the stationary body potential and impact ionization current generated at moderate and high electric field regimes. The large influence of the channel length on the evolution of the body potential as well as the severe loss of electrostatic control of the body by the gate terminal due to the hole injection have been successfully monitored through the body-contact. In our short devices, the impact ionization current tends to saturate as the gate bias increases for high values of the drain bias, whereas the impact ionization ratio has been found to be larger at low inversion charge. At moderate impact ionization regime, an update set of experimentally extracted ionization coefficients have been proposed revealing significant differences with the ones used in bulk technologies. Finally, low-frequency noise characterization has shown the impact ionization contribution to the fluctuation of the drain current. The power spectral density of the noise at the body-contact of the transistor has revealed the signature of the parasitic bipolar triggering at high impact ionization regime.

Adiabatic theory of strong-field photoelectron momentum distributions near a backward rescattering caustic

We present a comprehensive treatise on the derivation of the factorization formula describing strong-field photoelectron momentum distributions near the outermost backward rescattering caustic within the adiabatic theory and its validation by calculations. The formula derived holds for ionization by linearly polarized laser pulses of sufficiently low frequency and becomes exact as the frequency tends to zero for a fixed pulse amplitude. The convergence of the results obtained from the formula to accurate photoelectron momentum distributions obtained by solving the time-dependent Schr\odinger equation is demonstrated. The formula is shown to work quantitatively in both tunneling and over-the-barrier regimes of ionization for finite-range potentials as well as potentials with a Coulomb tail. This paves the way for future applications of the present theory in strong-field physics. In particular, the explicit analytical form of the returning photoelectron wave packet given here enables one to extract differential cross sections for elastic scattering of a photoelectron on the parent ion from experimental photoelectron momentum distributions.

Stark resonance parameters for the 3a1 orbital of the water molecule

The Stark resonance parameters for the 3a1 molecular orbital of H2O are computed by solving a system of partial differential equations in spherical polar coordinates. The starting point of the calculation is the non-spherical quantum potential derived for this orbital from a single-center expanded Hartree–Fock orbital. The resonance positions and widths are obtained after applying an exterior complex scaling technique to describe the ionization regime for external fields applied along the two distinct directions associated with the symmetry axis. Comparison is made with previous results for the 1b1 and 1b2 orbitals obtained from a spherical potential approximation.

Quantum entanglement in strong-field ionization

We investigate the time-evolution of quantum entanglement between an electron, liberated by a strong few-cycle laser pulse, and its parent ion-core. Since the standard procedure is numerically prohibitive in this case, we propose a novel way to quantify the quantum correlation in such a system: we use the reduced density matrices of the directional subspaces along the polarization of the laser pulse and along the transverse directions as building blocks for an approximate entanglement entropy. We present our results, based on accurate numerical simulations, in terms of several of these entropies, for selected values of the peak electric field strength and the carrier-envelope phase difference of the laser pulse. The time evolution of the mutual entropy of the electron and the ion-core motion along the direction of the laser polarization is similar to our earlier results based on a simple one-dimensional model. However, taking into account also the dynamics perpendicular to the laser polarization reveals a surprisingly different entanglement dynamics above the laser intensity range corresponding to pure tunneling: the quantum entanglement decreases with time in the over-the-barrier ionization regime.

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.