Strong Laser Pulses Research Articles

Enhanced chiral-sensitivity of Coulomb-focused electrons in strong field ionization

Strong-field light–matter interactions initiate a wide range of phenomena in which the quantum paths of electronic wavepackets can be manipulated by tailoring the laser field. Among the electrons released by a strong laser pulse from atomic and molecular targets, some are subsequently driven back to the vicinity of the ionic core by the oscillating laser field. The trajectories of these returning electrons are bent toward the core by the ionic potential, an effect known as Coulomb focusing. This process, studied over the past two decades, has been associated with the long range influence of the Coulomb potential. Here we explore the structural properties of the Coulomb focusing phenomenon. Specifically, we numerically study the sensitivity of the returning electron dynamics to the anisotropy of the ionic potential. We employ orthogonally polarized two-color strong fields and chiral molecules, whose asymmetric features lead to unambiguous fingerprints of the potential on the freed electrons. The Coulomb-focused electrons show an enhanced sensitivity to chirality, related to an asymmetric attoclock-like angular streaking stemming from field-assisted scattering of the electrons onto the chiral ionic potential. Anisotropic features of the ionic potential thus monitor the motion of Coulomb-focused electrons throughout their returning paths, shedding light on the structural properties of the interaction.

Optimal control for suppressing wave-packet spreading with strong nonresonant laser pulses

We use nonlinear optimal control simulation to systematically examine how to suppress wave-packet spreading with strong nonresonant near-infrared (NIR) laser pulses through a case study of a vibrational wave packet in the $B$ state of ${\mathrm{I}}_{2}$. As the degree of spreading of the vibrational wave packet is controllable by adjusting the pump pulse shape, the vibrational wave packet may be regarded as a prototype system to study spreading suppression by using nonresonant laser pulses. We quantitatively define spreading suppression in terms of the probability of finding the wave packet in the analytically defined, initially excited state at every vibrational period. From optimal control simulation and simulation by using model Gaussian pulse trains that mimic optimal pulses, we conclude that a simple periodic NIR pulse train without highly tuned temporal widths, amplitudes, or irradiation timings can almost perfectly stop the wave-packet spreading over a long control period, provided that the degree of spreading is not too large.

Ultrafast quantum imaging in the dissociation of H2+ via the induced conical intersection of two lowest adiabatic states by strong field laser pulses

• A non-Born-Oppenheimer time-dependent Schrödinger equation is numerically solved to study the dissociative-ionization of H 2 + molecule subjected to strong field six-cycle laser pulses ( I = 4 × 10 14 W / cm 2 , λ = 800 nm ) . • Analysis of the time evolution of the H 2 + nuclear wave packet is performed using Floquet formalism in the adiabatic representation to gain newly insightful images of the nonlinear processes such as vibrational trapping, above-threshold dissociation, tunneling, and bond-softening of the laser-driven nuclear wave packet on two lowest field-dressed adiabatic potential energy curves. • Starting from an initial state-selected vibrational level of H 2 , the images enable ones to fully comprehend the dissociative-ionization dynamics of H 2 + , notably in the vicinity of a conical intersection up to eventually entering the two exit channels with the main one producing fragments H + H + via the H 2 + ( 2 ∑ u + ) exit channel. • The direct treatment of the electronic-nuclear wave packet of H 2 enables one to image the one-electron nuclear localization of H 2 + on the right or left of field-distorted double-well Coulomb potential to reveal the influence of the field on the charge-resonance enhanced ionization coupled with the other nonlinear processes. A non-Born–Oppenheimer time-dependent Schrödinger equation is numerically solved to study the dissociative-ionization of H 2 + subjected to strong field six-cycle laser pulses ( I = 4 × 10 14 W / cm 2 , λ = 800 nm ) . Analysis of the evolution of H 2 + nuclear wave packet is performed using Floquet formalism in the adiabatic representation to gain newly insightful images of the nonlinear processes in the vicinity of a conical intersection of two lowest field-dressed adiabatic states. The coherent superposition of the states leads to the right or left localization of H 2 + on the field-distorted double-well Coulomb potential.

Radiation induced acceleration of ions in a laser irradiated transparent foil

Radiation friction can have a substantial impact on electron dynamics in a transparent target exposed to a strong laser pulse. In particular, by modifying quiver electron motion, it can strongly enhance the longitudinal charge separation field, thus inducing ion acceleration. We present a model and simulation results for such a radiation induced ion acceleration regime and study the scalings of the maximal attainable and average ion energies with respect to the laser and target parameters. We also compare the performance of this mechanism to the conventional ones.

Optical Stark shift to control the dark exciton occupation of a quantum dot in a tilted magnetic field

When a detuned and strong laser pulse acts on an optical transition, a Stark shift of the corresponding energies occurs. We analyze how this optical Stark effect can be used to prepare and control the dark exciton occupation in a semiconductor quantum dot. The coupling between the bright and dark exciton states is facilitated by an external magnetic field. Using sequences of laser pulses, we show how the dark exciton and different superposition states can be prepared. We give simple analytic formulas, which yield a good estimate for optimal preparation parameters. The preparation scheme is quite robust against the influence of acoustic phonons. We further discuss the experimental feasibility of the used Stark pulses. Giving a clear physical picture our results will stimulate the usage of dark excitons in schemes to generate photons from quantum dots.

Controlling quantum wave packet of electronic motion on field‐dressed Coulomb potential of by carrier‐envelope phase‐dependent strong field laser pulses

AbstractSolving numerically a non‐Born‐Oppenheimer time‐dependent Schrödinger equation to study the dynamics of H2 subjected to strong field six‐cycle laser pulses ( W/cm2, nm) leads to the newly ultrafast electron imaging in the dissociative‐ionization of . This includes the electron distribution in oscillates symmetrically with laser cycle with periodicity where the distribution concentrates between two protons for about 8 fs, being trapped in a Coulomb potential well. Nonetheless, the most important finding reveals that the electron symmetrical distribution begins to break up in the field‐free region after 24 fs when the internuclear distance stretches larger than 9 a.u. It is a result of the distortion of Coulomb potential where the ejected electron preferentially localizes in one of the double‐well potential separated by the inner Coulomb potential barrier, leading to the new images of charge resonance enhanced ionization. Controlling laser carrier‐envelope phase enables one to quantify such phenomena with the highest total asymmetries of 0.75 and −0.75 occur at and , respectively, associated with the electron preferential directionality being ionized to the right and the left paths along the molecular axis.

Experimentally validated modeling of the optical energy deposition in highly ionized ambient air by strong femtosecond laser pulses

The deposition of femtosecond laser optical energy in gases leads to the emission of secondary electromagnetic and acoustic radiation. These optoacoustic components have a complex nonlinear dependency on the laser beam characteristics, such as the pulse energy, duration, wavelength and the focusing conditions, as well as on the optical and elastic characteristics of the gaseous medium. The initial interaction times are governed by the high electronic excitation and ionization. These phenomena result in a self-modulation of the laser pulse, significantly affecting the optical energy deposition on the medium. Such complex nonlinear phenomena are very difficult to be studied via analytical equations. To address this, a multiphysics Particle-In-Cell model is applied for the evaluation of the optical energy deposition and plasma generation from tightly focused femtosecond pulses in ambient air. The computational domain of the model is built to describe optical energy deposition in its full spatiotemporal scale. The model is validated by experimental results of the absorbed energy. The agreement between the computational and experimental results provides the basis for the future development of an advanced microstructural Finite Element Method model, which, combined with the Particle-In-Cell model, will have the ability of delivering detailed insights for all the sub-domains and timescales varying from nano- to femto-seconds of the laser-induced breakdown phenomenon.Graphic

Coherent control at gold needle tips approaching the strong-field regime

Abstract We demonstrate coherent control in photoemission from a gold needle tip using an ω − 2ω field composed of strong few-cycle laser pulses with a nearfield intensity of ∼4 TW/cm2. We obtain the nearfield intensity from electron energy spectra, showing the tell-tale plateau of field-driven electron rescattering at the metal surface induced by the fundamental field. Changing the relative phase between the fundamental field centered at 1560 nm and its second harmonic modulates the total emitted photocurrent with visibilities of up to 80% despite the strong and broadband excitation of the plasmonic material. Our work combines a two-color coherent control scheme and strong-field physics enabled by a nanoplasmonic emitter.

Microstructure and performance evolution of Ti-6Al-4 V alloy coating by laser cladding and laser shocking composite remanufacture

Regarding occurrences of excessive tensile stress, low basket organization refinement and unoptimized mechanical properties of Ti-6Al-4 V layer during laser remanufacture, a new composite remanufacture process of laser cladding and laser shocking was proposed. Optimized pulse laser remanufacture process of low laser power and high duty cycle was adopted to prepare a homogeneous alloy layer on the Ti-6Al-4 V matrix. Water confinement and strong pulse laser was used to shock the cladding surface, as a strategy to tailor metallographic structure, mechanical properties and residual stress of the cladding layer. The experiment results showed that, the originally cladding layer was mainly composed α + β two-phase structure with some acicular martensite transformation, along with metallographic structure interlaced in basket organization. Microhardness of the layer was distributed between 345 ~ 445HV0.1, and residual stress distribution was −190 ~ 180 MPa.In addition, friction and wear coefficient was found in a range of 0.19 ~ 0.58.After shocking optimization, the α phase redissolution of the layer was aggravated, the α and β phase grains were refined. It was also noted some α phase particles were dispersed among the grains. The microhardness of the layer was increased to 365 ~ 478HV0.1, the residual stress is mainly distributed in −70 ~ -652Mpa.The tensile stress was all transformed into compressive stress, along with the friction and wear coefficient ranged in 0.04 ~ 0.15.These results confirmed the possibility of the laser remanufacture outcome improvement via adoption of the composite process of laser cladding and laser shocking.

Laser streaking of dissociating D2+ in a strong midinfrared laser pulse

Laser streaking of photoelectrons has been a well-established technique to extract ultrafast information. In analogy to the streaking of photoelectrons, in this paper, we study the laser streaking of nuclei by numerically simulating the time-dependent Schr\"odinger equations of ${\mathrm{D}}_{2}^{+}$ in strong laser fields. We first demonstrate the nuclear streaking, where the dissociating wave packet excited by an isolated attosecond pulse and propagating on molecular potential surfaces is streaked by a strong midinfrared laser pulse. Subsequently, we use an attosecond pulse train composing two successive attosecond pulses to initiate the dissociation of ${\mathrm{D}}_{2}^{+}$. Depending on the time delay between the attosecond pulse train and the midinfrared laser pulse, the streaked nuclear wave packets show a novel delay-dependent interference pattern. By tracing the wave-packet evolution in the adiabatic presentation, the complicated dissociation pathways are revealed. Based on the streaked nuclear pattern, one may conceive a strategy to probe the real-time nuclear streaking process and extract instantaneous dynamics information experimentally.

Strong-field triple ionisation of atoms with p 3 valence shell

The interaction of strong pulsed femtosecond laser field with atoms having three equivalent electrons in the outer shell (p 3 configuration, e.g. nitrogen) is studied via numerical integration of a time-dependent Schrödinger equation on a spatial grid. Single ionisation, double ionisation (DI) and triple ionisation (TI) yields originating from a completely antisymmetric wave function are calculated and extracted using a restricted-geometry model with the soft-core potential and three active electrons. The observed suppression of the ionisation yields for the non-sequential processes, in both DI and TI cases, is attributed to the action of the Pauli principle. Compared against earlier results investigating the s 2 p 1 configuration, we propose that the differences found here might in fact be accessible through electron’s momentum distribution.

Quantum-Optical Spectrometry in Relativistic Laser–Plasma Interactions Using the High-Harmonic Generation Process: A Proposal

Quantum-optical spectrometry is a recently developed shot-to-shot photon correlation-based method, namely using a quantum spectrometer (QS), that has been used to reveal the quantum optical nature of intense laser–matter interactions and connect the research domains of quantum optics (QO) and strong laser-field physics (SLFP). The method provides the probability of absorbing photons from a driving laser field towards the generation of a strong laser–field interaction product, such as high-order harmonics. In this case, the harmonic spectrum is reflected in the photon number distribution of the infrared (IR) driving field after its interaction with the high harmonic generation medium. The method was implemented in non-relativistic interactions using high harmonics produced by the interaction of strong laser pulses with atoms and semiconductors. Very recently, it was used for the generation of non-classical light states in intense laser–atom interaction, building the basis for studies of quantum electrodynamics in strong laser-field physics and the development of a new class of non-classical light sources for applications in quantum technology. Here, after a brief introduction of the QS method, we will discuss how the QS can be applied in relativistic laser–plasma interactions and become the driving factor for initiating investigations on relativistic quantum electrodynamics.

Population transfer in a nonlinear three-level Λ-system by Stark - chirped rapid adiabatic passage

We propose the use of the Stark-chirped rapid adiabatic passage (SCRAP) method to induce a complete population transfer in a nonlinear three-level Λ-type system (nl-SCRAP). We also use the nl-SCRAP method for creating stable diatomic ground molecular Bose–Einstein condensates (BECs) from atomic BECs. In this three-laser technique the pump and Stokes pulses are slightly detuned from transition frequencies, and a third strong hyperbolic-tangent laser pulse induces dynamic Stark shifts of the relevant transitions and compensates for third-order nonlinearities. If the timing of the three pulses is appropriately chosen, the nonlinear quantum system is prepared to almost complete population inversion between the two lower states in the Λ-like scheme. The paper shows that the efficiency of the nl-SCRAP is higher than the nonlinear stimulated Raman adiabatic passage (nl-STIRAP) technique, and this method can be used in one-photon as well as multi-photon transitions. The transfer process is robust concerning fluctuations of experimental parameters, such as peak Rabi frequencies, the time delay between pulses, and static detunings.

Laser-nucleus interactions in the sudden regime

The interaction between medium-weight nuclei and a strong zeptosecond laser pulse of MeV photons is investigated theoretically. Multiple absorption of photons competes with nuclear equilibration. We investigate the sudden regime. Here the rate of photon absorption is so strong that there is no time for the nucleus to fully equilibrate after each photon absorption process. We follow the temporal evolution of the system in terms of a set of rate equations. These account for dipole absorption and induced dipole emission, equilibration (modeled in terms of particle-hole states coupled by the residual nuclear interaction), and neutron decay (populating a chain of proton-rich nuclei). Our results are compared with earlier work addressing the adiabatic regime where equilibration is instantaneous. We predict the degree of excitation and the range of nuclei reached by neutron evaporation. These findings are relevant for planning future experiments.

Hydrodynamical model of QED cascade expansion in an extremely strong laser pulse

The development of a self-sustained quantum electrodynamical (QED) cascade in a single strong laser pulse is studied analytically and numerically. A hydrodynamical approach is used to construct an analytical model of cascade evolution, which includes the key features of the cascade observed in 3D QED particle-in-cell (QED-PIC) simulations, such as the magnetic field dominance in the cascade plasma and laser energy absorption. The equations of the model are derived in closed form and solved numerically. Direct comparison between the solutions of the model equations and 3D QED-PIC simulations shows that our model is able to describe the complex nonlinear process of cascade development qualitatively well. Various regimes of the interaction based on the intensity of the laser pulse are revealed in both the solutions of the model equations and the results of the QED-PIC simulations.

Non uniqueness of non runaway solutions of Abraham—Lorentz—Dirac equation in an external laser pulse

In the paper (Carati et al 1995 Non Linearity 8 65-79) it was shown that, for motions on a line under the action of a potential barrier, the third-order Abraham–Lorentz–Dirac equation presents the phenomenon of non uniqueness of non runaway solutions. Namely, at least for a sufficiently steep barrier, the physical solutions of the equation are not determined by the ‘mechanical state’ of position and velocity, and knowledge of the initial acceleration too is required. Due to recent experiments, both in course and planned, on the interactions between strong laser pulses and ultra relativistic electrons, it becomes interesting to establish whether such a non uniqueness phenomenon extends to the latter case, and for which ranges of the parameters. In the present work we will consider just the simplest model, i.e., the case of an electromagnetic plane wave, and moreover the Abraham–Lorentz–Dirac equation will be dealt with in the non relativistic approximation. The result we found is that the non uniqueness phenomenon occurs if, at a given frequency of the incoming wave, the field intensity is sufficiently large. An analytic estimate of such a threshold is also given. At the moment it is unclear whether such a phenomenon applies also in the full relativistic case, which is the one of physical interest.

Theory of femtosecond strong field ion excitation and subsequent lasing in

Delayed cavity-free forward lasing at the wavelengths of 391 and 428 nm was observed in recent experiments in air or pure nitrogen pumped with an intense femtosecond laser pulse at wavelength of 800 nm. The mechanism responsible for the lasing is highly controversial. In this article we propose a model explaining the delayed lasing, which contains two parts: (i) ionization of neutral nitrogen molecules and subsequent excitation of nitrogen ions in a strong pump laser pulse, and (ii) coherent emission of excited ions due to the presence of long-lived polarizations maintained by a weak laser post-pulse and coupling simultaneously ground state to states A2Π u and of singly ionized nitrogen molecules . Two regimes of signal amplification are identified: a signal of a few picosecond duration at low gas pressures and a short (sub-picosecond) signal at high gas pressures. The theoretical model compares favorably with results obtained by different experimental groups.

HeTDSE: A GPU based program to solve the full-dimensional time-dependent Schrödinger equation for two-electron helium subjected to strong laser fields* *Project supported the National Natural Science Foundation of China (Grant Nos. 11904192, 11604119, 11627807, and 11604131), the Natural Science Basic Research Plan of Shaanxi Province of China (Grant No. 2016JM1012), the Natural Science Foundation of the Education Committee of Shaanxi

Electron–electron correlation plays an important role in the underlying dynamics in physics and chemistry. Helium is the simplest and most fundamental two-electron system. The dynamic process of helium in a strong laser field is still a challenging issue because of the large calculation cost. In this study, a graphic processing unit (GPU) openACC based ab initio numerical simulations package HeTDSE is developed to solve the full-dimensional time-dependent Schrödinger equation of helium subjected to a strong laser pulse. HeTDSE uses B-spline basis sets expansion method to construct the radial part of the wavefunction, and the spherical harmonic functions is used to express for the angular part. Adams algorithm is employed for the time propagation. Our example shows that HeTDSE running on an NVIDIA Kepler K20 GPU can outperform the one on an Intel E5-2640 single CPU core by a factor of 147. HeTDSE code package can be obtained from the author or from the author’s personal website (doi: 10.13140/RG.2.2.15334.45128) directly under the GPL license, so HeTDSE can be downloaded, used and modified freely.

Laser-induced ultrafast insulator-metal transition in BaBiO3

We investigate ultra-fast coherent quantum dynamics of undoped $\text{BaBiO}_{3}$ driven by a strong laser pulse. Our calculations demonstrate that in a wide range of radiation frequencies and intensities the system undergoes a transient change from the insulating to the metallic state, where the charge density wave and the corresponding energy spectrum gap vanish. The transition takes place on the ultra-fast time scale of tens femtoseconds, comparable to the period of the corresponding lattice vibrations. The dynamics are determined by a complex interplay of the particle-hole excitation over the gap and of the tunnelling through it, giving rise to the highly non-trivial time evolution which comprises high harmonics and reveals periodic reappearance of the gap. The time evolution is obtained by solving the dynamical mean-field theory equations with the realistic parameters for the system and radiation. Results are summarized in the phase diagram, helpful for a possible experimental setup to achieve a dynamical control over the conduction state of this and other materials with the similarly strong electron-phonon interaction.

Bayesian optimization for inverse problems in time-dependent quantum dynamics.

We demonstrate an efficient algorithm for inverse problems in time-dependent quantum dynamics based on feedback loops between Hamiltonian parameters and the solutions of the Schrödinger equation. Our approach formulates the inverse problem as a target vector estimation problem and uses Bayesian surrogate models of the Schrödinger equation solutions to direct the optimization of feedback loops. For the surrogate models, we use Gaussian processes with vector outputs and composite kernels built by an iterative algorithm with the Bayesian information criterion (BIC) as a kernel selection metric. The outputs of the Gaussian processes are designed to model an observable simultaneously at different time instances. We show that the use of Gaussian processes with vector outputs and the BIC-directed kernel construction reduces the number of iterations in the feedback loops by, at least, a factor of 3. We also demonstrate an application of Bayesian optimization for inverse problems with noisy data. To demonstrate the algorithm, we consider the orientation and alignment of polyatomic molecules SO2 and propylene oxide (PPO) induced by strong laser pulses. We use simulated time evolutions of the orientation or alignment signals to determine the relevant components of the molecular polarizability tensors. We show that, for the five independent components of the polarizability tensor of PPO, this can be achieved with as few as 30 quantum dynamics calculations.