Strong Laser Pulses Research Articles

Pair creation by collision of an intense laser pulse with a high-frequency photon beam

We consider pair creation by collision of a couple of counterpropagating electromagnetic pulses with arbitrary frequency ratio, mostly in the context of setup with collision of optical and coherent hard X- (or gamma-) ray pulses. This problem is non-perturbative and in general does not admit exact analytical solutions. We discuss several known approaches according to the ratio of the parameters. Certain regions of the parameter space are not covered by the existing approximations or models. We present a new simplified exactly solvable model with one of the pulses being a delta-pulse. This model partially fills the yet unexplored gap in the space of parameters. The shape of the momentum distribution of the created particles in such a model is discussed. Even though the model may not have immediate implications for the forthcoming experiments, it can still provide some hints for better understanding of the fully non-perturbative regime and vacuum instability in QED interactions of extremely strong and short laser pulses.

Domain of validity of the perturbative approach to femtosecond optical spectroscopy

We have performed numerical nonperturbative simulations of transient absorption pump-probe responses for a series of molecular model systems. The resulting signals as a function of the laser field strength and the pump-probe delay time are compared with those obtained in the perturbative response function formalism. The simulations and their theoretical analysis indicate that the perturbative description remains valid up to moderately strong laser pulses, corresponding to a rather substantial depopulation (population) of the initial (final) electronic states.

Simulation of femtosecond “double-slit” experiments for a chromophore in a dissipative environment

We performed simulations of the prototypical femtosecond "double-slit" experiment with strong pulsed laser fields for a chromophore in solution. The chromophore is modeled as a system with two electronic levels and a single Franck-Condon active underdamped vibrational mode. All other (intra- and inter-molecular) vibrational modes are accounted for as a thermal bath. The system-bath coupling is treated in a computationally accurate manner using the hierarchy equations of motion approach. The double-slit signal is evaluated numerically exactly without invoking perturbation theory in the matter-field interaction. We show that the strong-pulse double-slit signal consists of a superposition of N-wave-mixing (N = 2, 4, 6...) responses and can be split into population and coherence contributions. The former reveals the dynamics of vibrational wave packets in the ground state and the excited electronic state of the chromophore, while the latter contains information on the dephasing of electronic coherences of the chromophore density matrix. We studied the influence of heat baths with different coupling strengths and memories on the double-slit signal. Our results show that the double-slit experiment performed with strong (nonperturbative) pulses yields substantially more information on the photoinduced dynamics of the chromophore than the weak-pulse experiment, in particular, if the bath-induced dephasings are fast.

The ionization rate inversion of H induced by the single and double UV photon(s)

The ionization of H in the strong UV laser pulse is studied by numerically solving the time-dependent Schrödinger equation. In analogy to Young’s double-slit interference, the ionized electron originating from two nuclei will constructively, or destructively interfere, depending on the UV frequencies. The fluctuation of the ionization rate as a function of the laser frequency is observed. The destructive interference suppresses the single-photon ionization rate, so that the double-photon ionization rate can be larger than the single-photon ionization rate. When such an ionization-rate inversion happens, the electron momentum spectra splits into several peaks.

Excitation of rotons in parahydrogen crystals: The laser-induced-molecular-alignment mechanism

Solid parahydrogen (p-H2) is known to support long-lived coherences, of the order of 100 ps, which enables high-resolution spectroscopy in the time domain. Rotational Raman-type excitations to sublevels of J = 2 are delocalized due to electric-quadrupole–quadrupole coupling in p-H2 crystals, and the resulting states can be characterized as rotons. Wave packets of rotons exhibit molecular alignment with respect to laboratory coordinates. Here the concept of field-free molecular alignment, induced by strong ultrashort laser pulses, is extended into a molecular solid case. We derive a solid-state analog for the gas-phase alignment measure and illustrate the time-dependent alignment degree in p-H2 crystals by numerical simulations. To underscore the Raman gain effect of the solid, general properties of the field-free alignment are revisited by comparing gaseous p-H2 with N2. The interplay between the polarization direction of the excitation pulses and the axis directionality of the crystal is shown to affect the alignment dynamics via the spatial (M = 0,±1,±2) composition of the roton wave packets. We simulate experimental traces by incorporating the induced alignment degree in the calculation of heterodyne-detected realization of femtosecond pump-probe optical Kerr effect spectroscopy. With the help of dispersed, two-dimensional resolved images of the calculated signal we reproduce the experiment as a whole. To that end, the effects of probe chirp, shape, and power must be explored in detail. We find good agreement with previous experiments and unravel the ambiguity of tracing back the wave-packet composition from the signal; in particular, we find that the effect of quantum phase factors of all the components should be taken into account when explaining the signal properties.

Ac Stark effect in the electronic continuum and its impact on the photoionization of atoms by coherent intense short high-frequency laser pulses

We investigate the photoionization of an atom by a high-frequency strong coherent laser pulse, where the energy of a single photon is sufficient to ionize the system. Ab initio theory exemplified by numerical calculations of the photoionization of the hydrogen atom illustrates that the laser pulse induces a time-dependent ac (or dynamic) Stark shift of the binding energy of the system due to interaction with the electronic continuum. The energy shift follows the pulse envelope. Because of this shift, the energy of the emitted photoelectron also follows the intensity envelope of a short laser pulse. This results in a strong dynamic interference of the photoelectrons of the same kinetic energy emitted at different times, when the pulse arrives and expires, respectively. The dynamic interference leads to interesting modifications of the photoionization process and results in a multiple peak structure in the photoelectron spectrum. The present work provides a detailed theoretical and numerical study of the results reported in Demekhin and Cederbaum [Phys. Rev. Lett. 108, 253001 (2012)]. The working equations are derived explicitly and an extended analysis of the theory is provided. Additional results are presented. In particular, we demonstrate that the dynamic interference can be controlled by the intensity, duration, and shape of the pulse envelope.

ClassSTRONG: Classical simulations of strong field processes

A set of Mathematica functions is presented to model classically two of the most important processes in strong field physics, namely high-order harmonic generation (HHG) and above-threshold ionization (ATI). Our approach is based on the numerical solution of the Newton-Lorentz equation of an electron moving on an electric field and takes advantage of the symbolic languages features and graphical power of Mathematica. Similarly as in the Strong Field Approximation (SFA), the effects of atomic potential on the motion of electron in the laser field are neglected. The SFA has proven to be an essential tool in strong field physics in the sense that it is able to predict with great precision the harmonic (in the HHG) and energy (in the ATI) limits. We have extended substantially the conventional classical simulations, where the electric field is only dependent on time, including spatial nonhomogeneous fields and spatial and temporal synthesized fields. Spatial nonhomogeneous fields appear when metal nanosystems interact with strong and short laser pulses and temporal synthesized fields are routinely generated in attosecond laboratories around the world. Temporal and spatial synthesized fields have received special attention nowadays because they would allow to exceed considerably the conventional harmonic and electron energy frontiers. Classical simulations are an invaluable tool to explore exhaustively the parameters domain at a cheap computational cost, before massive quantum mechanical calculations, absolutely indispensable for detailed analysis, are performed.

Reactive scattering dynamics of rotational wavepackets: A case study using the model H+H2 and F+H2 reactions with aligned and anti-aligned H2

We propose a method to steer the outcome of reactive atom-diatom scattering, using rotational wavepackets excited by strong non-resonant laser pulses. Full close-coupled quantum mechanical scattering calculations of the D+H2 and F+H2 reactions are presented, where the H2 molecule exists as a coherent superposition of rotational states. The nuclear spin selective control over the molecular bond axis alignment afforded by the creation of rotational wavepackets is applied to reactive scattering systems, enabling a nuclear spin selective influence to be exerted over the reactive dynamics. The extension of the conventional eigenstate-to-eigenstate scattering problem to the case in which the initial state is composed of a coherent superposition of rotational states is detailed, and a selection of example calculations are discussed, along with their mechanistic implications. The feasibility of the corresponding experiments is considered, and a suitable simple two pulse laser scheme is shown to strongly differentiate the reactivities of o-H2 and p-H2.

Orbital alignment in atoms generated by photodetachment in a strong laser field

A pump-probe laser scheme is employed to investigate orbital alignment and its dynamics in the ground state of laser-generated neutral atoms. The alignment is initiated by electron photodetachment of an atomic negative ion in a strong laser pulse. The electron density distribution in the ground state of the residual atom is probed by means of strong-field ionization in a second laser pulse at a delayed time. The principle of the probe method relies on the fact that the portion of the electron density distribution oriented along the laser polarization axis constitutes the ionization yield in the high-energy jets of emitted electrons. A systematic study is carried out on C, Si, and Ge atoms, which possess two electrons in an open $p$ shell. A pronounced temporal modulation in the yield of high-energy electrons is observed for C and Si, revealing a periodic spatial rearrangement of the electron density distribution in these atoms. Its period is defined by the beat between the $J=1$ and $J=2$ spin-orbit components of the ground state.

Photoionization of hydrogen atoms by coherent intense high-frequency short laser pulses: Direct propagation of electron wave packets on large spatial grids

The time-dependent Schr\"{o}dinger equation for the hydrogen atom and its interaction with coherent intense high-frequency short laser pulses is solved numerically exactly by propagating the single-electron wave packets. Thereby, the wavefunction is followed in space and time for times longer than the pulse duration. Results are explicitly shown for 3 and 10 fs pulses. Particular attention is paid to identifying the effect of dynamic interference of photoelectrons emitted with the same kinetic energy at different times during the rising and falling sides of the pulse predicted in [\emph{Ph.V. Demekhin and L.S. Cederbaum}, Phys. Rev. Lett. \textbf{108}, 253001 (2012)]. In order to be able to see the dynamic interference pattern in the computed electron spectra, the photoelectron wave packet has to be propagated over long distances. Clearly, complex absorption potentials often employed to compute spectra of emitted particles cannot be used to detect dynamic interference. For the considered high-frequency pulses of 3 and 10 fs durations, this requires enormously large spatial grids. The presently computed photoionization and above-threshold ionization spectra are found to exhibit pronounced dynamic interference patterns. Where available, the patterns are in very good agreement with previously published results on the photoionization spectra which have been computed using a completely different method, thus supporting the previously made assumption that the above-threshold ionization processes are very weak for the considered pulse intensities and high carrier frequency. The quiver motion in space and time of a free electron in strong laser pulses is also investigated numerically. Finally, a discussion is presented of how fast the atom is ionized by an intense pulse.

Manipulation of ferromagnets via the spin-selective optical Stark effect

We investigate the non-resonant all-optical switching of magnetization. We treat the inverse Faraday effect (IFE) theoretically in terms of the spin-selective optical Stark effect for linearly or circularly polarized light. In the dilute magnetic semiconductors (Ga,Mn)As, strong laser pulses below the band gap induce effective magnetic fields of several teslas in a direction which depends on the magnetization and light wave vectors. Our theory demonstrates that the polarized light catalyzes the angular momentum transfer between lattice and the magnetization.

Resonant Auger decay of core-excited CO molecules in intense x-ray laser pulses: the O(1s → π*) excitation

The dynamics of the resonant Auger (RA) decay of the core-excited CO*(1s−1π*) molecule in intense x-ray laser pulses is studied theoretically. The present approach includes the impact of the analogue of conical intersections of the complex potential energy surfaces of the ground and ‘dressed’ resonant states induced by intense x-ray pulses. It also takes into account the decay of the resonance and the direct photoionization of the ground state, both populating the same final ionic states coherently, as well as the direct photoionization of the resonance state itself. The individual impacts of these physical processes on the total ion yield, the CO+(A 2Π) electron spectrum and the ro-vibrational distributions of the neutral molecules remaining in the ground electronic state after the laser pulse has expired are analysed and compared to those reported previously for the C*O resonance. It is also demonstrated that the RA effect of molecules by strong laser pulses of resonant carrier frequency is an efficient process to produce two-site double-core-hole–one-particle states of CO*.

Enhanced Electron-Positron Pair Production of a Vacuum in a Strong Laser Pulse Field by Frequency Variation

Effects of frequency variation on vacuum electron-positron pair production in strong pulsed laser fields are investigated in the framework of a quantum kinetic method. It is found that the slowly varying frequency can influence the pair production to some extent, while rapid varying frequency can increase the pair creation rate a few orders if the time lies within a narrow time window, in which frequency jump occurs. The possible physical mechanism behind the pair enhancement phenomenon is discussed briefly.

Imprints of molecular orbitals using photoelectron angular distribution by strong laser pulses of circular polarization

We theoretically investigate the strong-field ionization of H+2 molecules in four different electronic states by calculating photoelectron angular distributions in circularly polarized fields. We find that the structure of photoelectron angular distribution depends on the molecular orbital as well as the energy of the photoelectron. The location of main lobes changes with the symmetric property of the molecular orbital. Generally, for molecules with bonding electronic states, the photoelectron's angular distribution shows a rotation of π/2 with respect to the molecular axis, while for molecules with antibonding electronic states, no rotation occurs. We use an interference scenario to interpret these phenomena. We also find that, due to the interference effect, a new pair of jets appears in the waist of the main lobes, and the main lobes or jets of the photoelectron's angular distribution are split into two parts if the photoelectron energy is sufficiently high.

Time-dependent restricted-active-space self-consistent-field theory for laser-driven many-electron dynamics

We present the time-dependent restricted-active-space self-consistent field (TD-RASSCF) theory as a new framework for the time-dependent many-electron problem. The theory generalizes the multiconfigurational time-dependent Hartree-Fock (MCTDHF) theory by incorporating the restricted-active-space scheme well known in time-independent quantum chemistry. Optimization of the orbitals as well as the expansion coefficients at each time step makes it possible to construct the wave function accurately while using only a relatively small number of electronic configurations. In numerical calculations of high-order harmonic generation spectra of a one-dimensional model of atomic beryllium interacting with a strong laser pulse, the TD-RASSCF method is reasonably accurate while largely reducing the computational complexity. The TD-RASSCF method has the potential to treat large atoms and molecules beyond the capability of the MCTDHF method.

Modifying molecular scattering from rough solid surfaces using ultrashort laser pulses

We consider solid surface scattering of molecules that were subject to strong non-resonant ultrashort laser pulses just before hitting the surface. The pulses modify the rotational state of the molecules, causing their field-free alignment, or a rotation with a preferred sense. We show that field-free laser-induced molecular alignment leads to correlation between the scattering angle and the sense of rotation of the scattered molecules. Moreover, by controlling the sense of laser-induced unidirectional molecular rotation, one may affect the scattering angle of the molecules. This provides a new means for separation of mixtures of molecules (such as isotopes and nuclear-spin isomers) by laser-controlled surface scattering.

Demonstration of cascaded optical inverse free-electron laser accelerator.

We report on a proof-of-principle demonstration of a two-stage cascaded optical inverse free-electron laser (IFEL) accelerator in which an electron beam is accelerated by a strong laser pulse after being packed into optical microbunches by a weaker initial laser pulse. We show experimentally that injection of precisely prepared optical microbunches into an IFEL allows net acceleration or deceleration of the beam, depending on the relative phase of the two laser pulses. The experimental results are in excellent agreement with simulation. The demonstrated technique holds great promise to significantly improve the beam quality of IFELs and may have a strong impact on emerging laser accelerators driven by high-power optical lasers.

Temporal mode selectivity by frequency conversion in second-order nonlinear optical waveguides

We explore theoretically the feasibility of using frequency conversion by sum- or difference-frequency generation, enabled by three-wave-mixing, for selectively multiplexing orthogonal input waveforms that overlap in time and frequency. Such a process would enable a drop device for use in a transparent optical network using temporally orthogonal waveforms to encode different channels. We model the process using coupled-mode equations appropriate for wave mixing in a uniform second-order nonlinear optical medium pumped by a strong laser pulse. We find Green functions describing the process, and employ Schmidt (singular-value) decompositions thereof to quantify its viability in functioning as a coherent waveform discriminator. We define a selectivity figure of merit in terms of the Schmidt coefficients, and use it to compare and contrast various parameter regimes via extensive numerical computations. We identify the most favorable regime (at least in the case of no pump chirp) and derive the complete analytical solution for the same. We bound the maximum achievable selectivity in this parameter space. We show that including a frequency chirp in the pump does not improve selectivity in this optimal regime. We also find an operating regime in which high-efficiency frequency conversion without temporal-shape selectivity can be achieved while preserving the shapes of a wide class of input pulses. The results are applicable to both classical and quantum frequency conversion.

Scheme for a spin-based quantum computer employing induction detection and imaging

A theoretical spin-based scheme for performing a variety of quantum computations is presented. It makes use of an array of multiple identical “computer” vectors of phosphorus-doped silicon where the nuclei serve as logical qubits and the electrons as working qubits. The spins are addressed by a combination of electron spin resonance and nuclear magnetic resonance techniques operating at a field of $$\sim $$ 3.3 T and cryogenic temperatures with an ultra-sensitive surface microresonator. Spin initialization is invoked by a combination of strong pre-polarization fields and laser pulses, which shortens the electrons’ $$T_{1}$$ . The set of universal quantum gates for this system includes an arbitrary rotation of single qubits and c-NOT operation in two qubits. The efficient parallel readout of all the spins in the system is performed by high sensitivity induction detection of the electron spin resonance signals with one-dimensional imaging. Details of the suggested scheme are provided, which show that it is scalable to a few hundreds of qubits.

Manipulating the singlet–triplet transition in ion strings by nonresonant dynamic Stark effect

Using strong laser pulses, we show that it is possible to control the spin state in a model system based on a two-electron extension with spin couplings of the Shin–Metiu Hamiltonian, truncated to account for the lowest electronic energy states. We consider two different models depending on the number of electronic states included in the calculation. The initial electronic state determines when the spin state is stable or not in the absence of an external field. In the latter case, by nonresonant dynamic Stark effect, we show that it is possible to avoid spin transitions with strong fields, using different pulse frequencies. This effective spin locking requires minimizing absorption to excited singlets as well as decoupling the singlet and triplet electronic states. In the first case, we show that it is possible to force the spin transition by a combination of two pulses, a chirped pulse and a transform limited pulse, where the time-delay must be chosen to maximize spin switching on a different electronic state. Our results show that forcing the spin switching is a more difficult goal than avoiding it and that this goal becomes highly restricted when many electronic pathways or multi-photon processes are available.