Low-frequency Laser Field Research Articles

On the adiabatic approximation for the laser-dressed atom

The adiabatic approximation is analyzed for non-perturbative interaction of an atomic system with an intense low-frequency laser field. We find that the well-known adiabatic ansatz, which consists in using the DC-based Green’s and wave functions for an atom in the instantaneous AC field to calculate transition matrix elements, exceeds the level of accuracy of the adiabatic approximation and requires further theoretical revision. It is shown that the non-analyticity in the field strength of the DC-based Green’s and wave functions leads to violation of the adiabaticity constrains and the appearance of rapidly oscillating components in the transition matrix elements. We suggest an improved adiabatic approach utilizing the analytical parts in the DC-based Green’s and wave functions. We illustrate our revised adiabatic approach for analysis of atomic polarizability in two-component laser field with an intense infrared and perturbative extreme ultraviolet components within the zero-range potential model.

Role of nuclear dynamics in high harmonic generation redshift of H2 +

The detailed spectral and temporal structures of high-order harmonic generation (HHG) have provided us with a wealth of information about the highly nonlinear response of atoms, molecules, and electronic structures to intense low-frequency laser fields. In this study, we aim to investigate the underlying physics behind the molecular high harmonic spectra redshift beyond the Born–Oppenheimer approximation for and its isotopes with numerical simulations of the time-dependent Schrödinger equation. One widely recognized phenomenon is the occurrence of a spectral redshift at the trailing edge of a laser pulse. However, our findings reveal that even when driven by a sinusoidal laser pulse without a falling edge, the HHG redshift appears and highlights that the observed spectral redshift is not solely due to the declining intensity of the laser pulse. We show that it is a direct consequence of the decrease in ionization energy during the transition from recombination time to ionization time within wave packets at short internuclear distances. Interestingly, our results also demonstrate how a sinusoidal laser pulse without a rising part can disrupt the standard picture of redshift in literatures in the past. This finding suggests that there are additional factors influencing the redshift mechanism beyond just the trailing edge of the laser pulse. These phenomena are elucidated through the analysis of the dynamics of the nuclear wave packet. Our study provides valuable insights into the complex dynamics of HHG processes and sheds light on how different laser pulse characteristics can influence spectral redshifts and harmonic generation in molecular systems.

Highly non-resonant lasing without inversion gain in a ladder scheme

We show that it is possible to get gain without inversion at high frequency in a three-level ladder type scheme driven by a single driving laser. The transition from the ground state to first excited state is driven by a low-frequency resonant laser field. The next excited state lies far above the first one; therefore its resonant frequency can be much higher than that of the lower transition, and hence one should go beyond the most commonly used rotating wave approximation consisting of retaining only resonant terms (slowly oscillating) in the Hamiltonian. We solve the master equation, both analytically (under proper approximations) and numerically, with the full Hamiltonian keeping counter-rotating highly non-resonant terms. We show a dressed state picture explaining spectral features of gain-absorption profiles. Also we present a kind of phase diagram in the plane γ–Λ, where γ and Λ are spontaneous emission rate and incoherent pumping rate, respectively. The whole γ–Λ plane is divided into four areas: gain-inversion, absorption-no inversion, gain without inversion and absorption with inversion. The first two are usual regimes of incoherently pumped lasers, while the last two stem from the quantum interference phenomena. We analyse the parameter areas where such quantum regimes are possible. The exact numerical solution of the time-dependent master equation shows that keeping counter-rotating terms (no rotating wave approximation) gives rise to new spectral features in the form of additional peaks at combination frequencies. Actually the considered system works as a coherent frequency up-converter.

Spontaneous decay processes in a classical strong low-frequency laser field

The spontaneous emission of an excited two-level emitter driven by a strong classical coherent low-frequency electromagnetic field is investigated. We find that for relatively strong laser driving, multi-photon processes are induced, thereby opening additional decay channels for the atom. We analyze the interplay between the strong low-frequency driving and the interfering multiphoton decay channels, and discuss its implications for the spontaneous emission dynamics.

Substantially enhanced deuteron-triton fusion probabilities in intense low-frequency laser fields

Deuteron-triton (DT) fusion is the primary fusion reaction used in controlled fusion research, mainly for its relatively high reaction cross sections compared to other fusion options. Even so, to attain appreciable reaction probabilities very high temperatures (on the order of 10-100 million kelvins) are required, which are extremely challenging to achieve and maintain. We show that intense low-frequency laser fields, such as those in the near-infrared regime for the majority of intense laser facilities around the world, are highly effective in transferring energy to the DT system and enhancing the DT fusion probabilities. The fusion probabilities are shown to be enhanced by at least an order of magnitude in 800-nm laser fields with intensities on the order of 10$^{21}$ W/cm$^2$. The demanding temperature requirement of controlled nuclear fusion may be relaxed if intense low-frequency lasers are exploited.

Direct numerical observation of real-space recollision in high-order harmonic generation from solids

Real-space picture of electron recollision with the parent ion guides our understanding of the highly nonlinear response of atoms and molecules to intense low-frequency laser fields. It is also among several leading contestants for the dominant mechanism of high harmonic generation (HHG) in solids, where it is typically viewed in the momentum space, as the recombination of the conduction band electron with the valence band hole, competing with another HHG mechanism, the strong-field driven Bloch oscillations. In this work, we use numerical simulations to directly test and confirm the real-space recollision picture as the key mechanism of HHG in solids. Our tests take advantage of the well-known characteristic features in the molecular harmonic spectra, associated with the real-space structure of the molecular ion. We show the emergence of analogous spectral features when similar real-space structures are present in the periodic potential of the solid-state lattice. This work demonstrates the capability of HHG imaging of spatial structures of a unit cell in solids.

Topological strong-field physics on sub-laser-cycle timescale

Sub-laser cycle time scale of electronic response to strong laser fields enables attosecond dynamical imaging in atoms, molecules and solids. Optical tunneling and high harmonic generation are the hallmarks of attosecond imaging in optical domain, including imaging of phase transitions in solids. Topological phase transition yields a state of matter intimately linked with electron dynamics, as manifested via the chiral edge currents in topological insulators. Does topological state of matter leave its mark on optical tunneling and sub-cycle electronic response? We identify distinct topological effects on the directionality and the attosecond timing of currents arising during electron injection into conduction bands. We show that electrons tunnel across the band gap differently in trivial and topological phases, for the same band structure, and identify the key role of the Berry curvature in this process. These effects map onto topologically-dependent attosecond delays in high harmonic emission and the helicities of the emitted harmonics, which can record the phase diagram of the system and its topological invariants. Thus, the topological state of the system controls its attosecond, highly non-equilibrium electronic response to strong low-frequency laser fields, in bulk. Our findings create new roadmaps in studies of topological systems, building on ubiquitous properties of sub-laser cycle strong field response - a unique mark of attosecond science.

A cold electron emission spectrum in a low-frequency laser field

We consider the process of cold emission of electrons from a metal in a strong low-frequency laser field. An expression is obtained for the electron spectrum with allowance for their rescattering at the metal surface as a result of an extension of the Fowler-Nordheim theory and modification of the Simplemen model. The numerically simulated features of the electron spectrum are similar to the ATI spectra for atoms and have the same form as the experimental surface emission dependencies for a metal.

Manipulation by Photoelectron Currents for the Generation of Terahertz Light Pulses

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Coulomb Corrections in Photoelectron Spectra in the Adiabatic Limit

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Adiabatic-limit Coulomb factors for photoelectron and high-order-harmonic spectra

A momentum-dependent Coulomb factor in the probability for nonlinear ionization of atoms by a strong low-frequency laser field is calculated analytically in the adiabatic approximation. Expressions for this Coulomb factor, valid for an arbitrary laser pulse waveform, are obtained and analyzed in detail for the cases of linear and circular polarizations. The dependence of the Coulomb factor on the photoelectron momentum is shown to be significant in both cases. Using a similar technique, the Coulomb factor for emission of high-order harmonics by an atom in a bichromatic laser field is also calculated. In contrast to the case of a single-frequency field, for bichromatic fields the Coulomb factor depends significantly on the harmonic energy.

Dynamic-Stark-effect-induced coherent mixture of virtual paths in laser-dressed helium: energetic electron impact excitation

We theoretically investigate quantum virtual path interference caused by the dynamic Stark effect in bound–bound electronic transitions. The effect is studied in an intermediate resonant region and in connection with the energetic electron impact excitation of a helium atom embedded in a weak low-frequency laser field. The process under investigation is dealt with via a Born–Floquet approach. Numerical calculations show a resonant feature in laser-assisted cross sections. The latter is found to be sensitive to the intensity of the laser field dressing. We show that this feature is a signature of quantum beats which result from the coherent mixture of different quantum virtual pathways, and that excitation may follow in order to end up with a common final channel. This mixture arises from the dynamic Stark effect, which produces a set of avoided crossings in laser-dressed states. The effect allows one to coherently control quantum virtual path interference by varying the intensity of the laser field dressing. Our findings suggest that the combination of an energetic electron and a weak laser field is a useful tool for the coherent control of nonadiabatic transitions in an intermediate resonant region.

Behaviour of tunnelling transition rate of argon atom exposed to strong low-frequency elliptical laser field

We considered the tunnelling ionization of an electron under the influence of a monochromatic laser beam with the elliptical polarization. Arbitrary values of ellipticity were observed. The influence of ponderomotive potential and Stark shift on the ionization rate was discussed. A brief description of the dependence of the ponderomotive potential and the Keldysh parameter on the field intensity and ellipticity is given.

Laser Field Ionization Rates in the Barrier-Suppression Regime

We analyze the influence of ponderomotive and Stark shifts on the barrier suppression transition rate for noble and alkali atoms for a low-frequency linearly-polarized laser field. We consider three approaches to the barrier suppression ionization (BSI) rate: through a critical field, an ion charge, and the Airy function. We show that ponderomotive and Stark shifts affect the rate differently in these three approaches. Generally, the rate curve of K is more strongly lowered than the one of Ar.

Low-frequency dipole response of a diatomic heteronuclear molecule on an intense ultrashort laser pulse

In the present work we analyze the dipole response of a diatomic heteronuclear molecule on the nonperturbative action of the external low-frequency laser field. Main results are obtained by numerical solution of the time-dependent Schrodinger equation for the nuclear subsystem of the molecule. We study the spectrum of polarization response and the influence of the coherent repopulation of the ro-vibrational states on it. Generally, we consider two cases: repopulation due to Raman-type transitions leading to the interference stabilization of the molecule and repopulation arising as a result of extremely short pulse action. We also study the transformation of a weak probe pulse propagating through the highly excited medium of the studied molecules.

Rescattering effects in laser-assisted electron–atom bremsstrahlung

Rescattering effects in non-resonant spontaneous laser-assisted electron–atom bremsstrahlung (LABrS) are analyzed within the framework of time-dependent effective-range (TDER) theory. It is shown that high energy LABrS spectra exhibit rescattering plateau structures that are similar to those that are well-known in strong field laser-induced processes as well as those that have been predicted theoretically in laser-assisted collision processes. In the limit of a low-frequency laser field, an analytic description of LABrS is obtained from a rigorous quantum analysis of the exact TDER results for the LABrS amplitude. This amplitude is represented as a sum of factorized terms involving three factors, each having a clear physical meaning. The first two factors are the exact field-free amplitudes for electron–atom bremsstrahlung and for electron–atom scattering, and the third factor describes free electron motion in the laser field along a closed trajectory between the first (scattering) and second (rescattering) collision events. Finally, an extension of these TDER results to the case of LABrS in a Coulomb field is discussed.

Nonperturbative Bethe–Heitler pair creation in combined high- and low-frequency laser fields

The nonperturbative regime of electron–positron pair creation by a relativistic proton beam colliding with a highly intense bichromatic laser field is studied. The laser wave is composed of a strong low-frequency and a weak high-frequency mode, with mutually orthogonal polarization vectors. We show that the presence of the high-frequency field component can strongly enhance the pair-creation rate. Besides, a characteristic influence of the high-frequency mode on the angular and energy distributions of the created particles is demonstrated, both in the nuclear rest frame and the laboratory frame.

Strongly enhanced pair production in combined high- and low-frequency laser fields

Production of particle-antiparticle pairs by a high-energy probe photon propagating through a high-intensity laser field is considered in the nonperturbative interaction regime. The laser field consists of a strong low-frequency and a weak high-frequency component. While for each component alone photoproduction of pairs is strongly suppressed, we show that their combination can largely amplify the pair production probability by facilitating to bridge the energetic barrier of the process. Our predictions can be tested by utilizing presently available high-intensity laser devices.

Effect of nuclear motion on tunneling ionization rates of molecules

We show that the observable rate of tunneling ionization of a molecule in an intense low-frequency laser field is affected by nuclear motion and can essentially differ from a bare electronic characteristic calculated for fixed nuclei. Both the absolute value of the rate and the shape of its orientation dependence are affected. The effect is significant for $I\ensuremath{\sim}{10}^{14}$ W/cm${}^{2}$ and becomes more pronounced at lower intensities. An isotope effect in tunneling ionization of H${}_{2}$ and D${}_{2}$ is predicted. The results are compared with available experiments.

Laser-assisted electron-argon scattering at small angles

Electron-argon scattering in the presence of a linearly polarized, low frequency laser field is studied theoretically. The scattering geometries of interest are small angles where momentum transfer is nearly perpendicular to the field, which is where the Kroll-Watson approximation has the potential to break down. The Floquet R matrix method solves the velocity gauge Schr\"odinger equation, using a larger reaction volume than previous treatments in order to carefully assess the importance of the long range polarization potential to the cross section. A comparison of the cross sections calculated with the target potential fully included inside 20 and 100 a.u. shows no appreciable differences, which demonstrates that the long range interaction can not account for the high cross sections measured in experiments.