Circularly Polarized Laser Pulse Research Articles

Photoelectron momentum distributions of triatomic CO2 molecules by circularly polarized attosecond pulses

Abstract Molecular-frame photoelectron momentum distributions (MF-PMDs) have been studied for imaging molecular structures. We investigate the MF-PMDs of CO2 molecules exposed to circularly polarized (CP) attosecond laser pulses by solving the time-dependent Schrödinger equations based on the single-active-electron approximation frames. Results show that high-frequency photons lead to photoelectron diffraction patterns, indicating molecular orbitals, these diffraction patterns can be illustrated by the ultrafast photoionization models. However, for the driving pulses with 30 nm, a deviation between MF-PMDs and theoretically predicted results of the ultrafast photoionization models is produced because the Coulomb effect strongly influences the molecular photoionization. Meanwhile, the MF-PMDs rotates in the same direction as the helicity of driving laser pulses. Our results also demonstrate that the MF-PMDs in a CP laser pulse are the superposition of those in the parallel and perpendicular linearly polarized cases. The simulations efficiently visualize molecular orbital geometries and structures by ultrafast photoelectron imaging. Furthermore, we determine the contribution of HOMO and HOMO-1 orbitals to ionization by varying the relative phase and the ratio of these two orbitals.

Electron vortices in the amplitude of the atomic ionization by a few-cycle circularly polarized laser pulse

Electron vortices in the amplitude of the atomic ionization by a few-cycle circularly polarized laser pulse

Effect of Ion Motion on Generation and Lifetime of Magnetic Fields in a Cluster Plasma Irradiated with Intense Circularly Polarized Laser Pulses

Effect of Ion Motion on Generation and Lifetime of Magnetic Fields in a Cluster Plasma Irradiated with Intense Circularly Polarized Laser Pulses

Ultrafast Chiral Precession of Spin and Orbital Angular Momentum Induced by Circularly Polarized Laser Pulse in Elementary Ferromagnets.

Despite spin (SAM) and orbital (OAM) angular momentum dynamics being well-studied in demagnetization processes, their components receive less focus. Here, we utilize real-time time-dependent density functional theory (rt-TDDFT) to unveil significant x and y components of SAM and OAM induced by circularly left (σ+) and right (σ-) polarized laser pulses in ferromagnetic Fe, Co, and Ni. Our results show that the magnitude of the OAM is an order of magnitude larger than that of the SAM, highlighting a stronger optical response from the orbital degrees of freedom of electrons. Intriguingly, σ+ and σ- pulses induce chirality in the precession of SAM and OAM, respectively, with clear associations with laser frequency and duration. Finally, we demonstrate the time scale of the OAM and SAM precession occurs even earlier than that of the demagnetization process and the OISTR effect. Our results provide detailed insight into the dynamics of SAM and OAM during and shortly after a polarized laser pulse.

From chiral laser pulses to femto- and attosecond electronic chirality flips in achiral molecules

Chirality is an important topic in biology, chemistry and physics. Here we show that ultrashort circularly polarized laser pulses, which are chiral, can be fired on achiral oriented molecules to induce chirality in their electronic densities, with chirality flips within femtoseconds or even attoseconds. Our results, obtained by quantum dynamics simulations, use the fact that laser pulses can break electronic symmetry while conserving nuclear symmetry. Here two laser pulses generate a superposition of three electronic eigenstates. This breaks all symmetry elements of the electronic density, making it chiral except at the periodic rare events of the chirality flips. As possible applications, we propose the combination of the electronic chirality flips with Chiral Induced Spin Selectivity.

Modulation of High-Order Harmonic Generation from a Monolayer ZnO by Co-rotating Two-Color Circularly Polarized Laser Fields

By numerically solving the two-dimensional semiconductor Bloch equation, we study the high-order harmonic emission of a monolayer ZnO under the driving of co-rotating two-color circularly polarized laser pulses. By changing the relative phase between the fundamental frequency field and the second one, it is found that the harmonic intensity in the platform region can be significantly modulated. In the higher order, the harmonic intensity can be increased by about one order of magnitude. Through time-frequency analysis, it is demonstrated that the emission trajectory of monolayer ZnO can be controlled by the relative phase, and the harmonic enhancement is caused by the second quantum trajectory with the higher emission probability. In addition, near-circularly polarized harmonics can be generated in the co-rotating two-color circularly polarized fields. With the change of the relative phase, the harmonics in the platform region can be altered from left-handed near-circularly polarization to right-handed one. Our results can obtain high-intensity harmonic radiation with an adjustable ellipticity, which provides an opportunity for syntheses of circularly polarized attosecond pulses.

Tunable Tesla-Scale Magnetic Attosecond Pulses through Ring-Current Gating.

Coherent control over electron dynamics in atoms and molecules using high-intensity circularly polarized laser pulses gives rise to current loops, resulting in the emission of magnetic fields. We propose, and demonstrate with ab initio calculations, "current-gating" schemes to generate direct or alternating-current magnetic pulses in the infrared spectral region, with highly tunable waveform and frequency, and showing femtosecond-to-attosecond pulse duration. In optimal conditions, the magnetic pulse can be highly isolated from the driving laser and exhibits a high flux density (∼1 T at a few hundred nanometers from the source, with a pulse duration of 787 attoseconds) for application in forefront experiments of ultrafast spectroscopy. Our work paves the way toward the generation of attosecond magnetic fields to probe ultrafast magnetization, chiral responses, and spin dynamics.

Dichroism Effects in the Ionization of Polarized Atoms by Short Circularly Polarized Laser Pulses

The dichroism effects, i.e., asymmetries of the ionization probability with respect to the inversion of either the atomic orientation (magnetic dichroism, MD) or the circular polarization degree of the photon beam (circular dichroism, CD), are investigated using the time-dependent perturbation theory (PT). It is shown that the magnitude of these effects depends not only on the polarization states of the atom and laser pulse but also on the intensity and duration of the latter. We find that the CD can also be observed in the ionization of oriented initial bound states, which is impossible in long-pulse ionization. Predictions of our PT analysis are supported by the results obtained by numerical solutions of the time-dependent Schrödinger equation (TDSE) describing the ionization of the excited 2P-states of the hydrogen atom.

Generation of Ultrafast Attosecond Magnetic Field from Ne Dimer in Circularly Polarized Laser Pulses

By numerically solving time-dependent Schrödinger equations, we investigate the generation of electron currents, ultrafast magnetic fields and photoelectron momentum distributions (PMD) when circularly polarized laser pulses interact with a Ne dimer in the charge migration (CM) process. By adjusting the laser wavelength, we consider two cases: (i) coherent resonance excitation (λ = 76 nm) and (ii) direct ionization (λ = 38 nm). The results show that the current and magnetic field generated by the Ne dimer under resonance excitation are stronger than under direct ionization. This phenomenon is due to the quantum interference between the initial state 2pσ g and the excited state 3sσ g under resonance excitation, so the CM efficiency of the dimer can be improved and the strength of the PMD under different ionization conditions is opposite to the strength of the electron current and induced magnetic field. In addition, we also find that both 2pπ g and 2pπ u have coherent resonance excitation with 3sσ g state and generate periodic oscillating currents for the Ne dimer. The study of the dynamics of the Ne dimer under different ionization conditions lays a foundation for research of ultrafast magnetism in complex molecular systems.

Internal collision double ionization of molecules driven by co-rotating two-color circularly polarized laser pulses

The double ionization process of molecules driven by co-rotating two-color circularly polarized fields is investigated with a three-dimensional classical ensemble model. Numerical results indicate that a considerable part of the sequential double ionization (DI) events of molecules occur through internal collision double ionization (ICD), and the ICD recollision mechanism is significantly different from that in non-sequential double ionization (NSDI). By analyzing the results of internuclear distances R = 5 a.u. and 2 a.u., these two recollision mechanisms are studied in depth. It is found that the dynamic behaviors of the recollision mechanisms of NSDI and ICD are similar. For NSDI, the motion range of electrons after the ionization is relatively large, and the electrons will return to the core after a period of time. In the ICD process, electrons will rotate around the parent ion before ionization, and the distance of the electron motion is relatively small. After a period of time, the electrons will come back to the core and collide with another electron. Furthermore, the molecular internuclear distance has a significant effect on the electron dynamic behavior of the two ionization mechanisms. This study will help to understand the multi-electron ionization process of complex systems.

Collision off-axis position dependence of relativistic nonlinear Thomson inverse scattering of an excited electron in a tightly focused circular polarized laser pulse

This paper presents a novel view of the impact of electron collision off-axis positions on the dynamic properties and relativistic nonlinear Thomson inverse scattering of excited electrons within tightly focused, circularly polarized laser pulses of varying intensities. We examine the effects of the transverse ponderomotive force, specifically how the deviation angle and speed of electron motion are affected by the initial off-axis position of the electron and the peak amplitude of the laser pulse. When the laser pulse intensity is low, an increase in the electron’s initial off-axis distance results in reduced spatial radiation power, improved collimation, super-continuum phenomena generation, red-shifting of the spectrum’s harmonic peak, and significant symmetry in the radiation radial direction. However, in contradiction to conventional understandings, when the laser pulse intensity is relatively high, the properties of the relativistic nonlinear Thomson inverse scattering of the electron deviate from the central axis, changing direction in opposition to the aforementioned effects. After reaching a peak, these properties then shift again, aligning with the previous direction. The complex interplay of these effects suggests a greater nuance and intricacy in the relationship between laser pulse intensity, electron position, and scattering properties than previously thought.

Circularly polarized attosecond pulses generation from laser interaction with magnetized sub-critical plasmas

We propose a method to generate circularly polarized (CP) attosecond pulses by the interactions of a relativistic-intensity right-hand CP laser pulse and magnetized sub-critical plasma. It is theoretically and numerically demonstrated that when an external magnetic field with an appropriate strength is applied to a sub-critical plasma along the laser propagation, the ponderomotive force of a right-hand CP laser at the vacuum-plasma boundary is significantly enhanced. The electrons are then steadily pushed forward until the timely-increasing charge separation field becomes strong enough to pull them back, forming a dense and counter-moving electron sheet. The relativistic-velocity electron sheet works as a flying mirror to compress the tail of the driving laser and efficiently generate a single CP attosecond pulse. The present scheme shows a stable efficiency on different scale lengths of preplasma and thus may provide a robust way to generate bright and CP attosecond pulses.

Production of Multioriented Polarization for Relativistic Electron Beams via a Mutable Filter for Nonlinear Compton Scattering

We propose a feasible scenario to directly polarize a relativistic electron beam and obtain overall polarization in various directions through a filter mechanism for single-shot collision between an ultrarelativistic unpolarized electron beam and an ultraintense circularly polarized laser pulse. The electrons are scattered to a large angular range of several degrees and the polarization states of the electrons are connected with their spatial position after the collision. Therefore, we can employ a filter to filter out a part of the scattered electrons based on their position and obtain high-degree overall polarization for the filtered beam. Through spin-considered Monte-Carlo simulations, polarization with a degree up to 62% in arbitrary transverse directions and longitudinal polarization up to 10% are obtained for the filtered beams at currently achievable laser intensity. We theoretically analyze the distribution formation of the scattered electrons and investigate the influence of different initial parameters through simulations to demonstrate the robustness of our scheme. This scenario provides a simple and flexible way to produce relativistic polarized electron beams for various polarization directions.

Timescales and contribution of heating and helicity effect in helicity-dependent all-optical switching

The heating and helicity effects induced by circularly polarized laser excitation are entangled in the helicity-dependent all-optical switching (HD-AOS), which hinders understanding the magnetization dynamics involved. Here, applying a dual-pump laser excitation, first with a linearly polarized (LP) laser pulse followed by a circularly polarized (CP) laser pulse, the timescales and contribution from heating and helicity effects in HD-AOS were identified with a Pt/Co/Pt triple-layer. When the LP laser pulses preheat the sample to a nearly fully demagnetized state, the CP laser pulses with a power reduced by 80% switch the sample’s magnetization. By varying the time delay between the two pump pulses, the results show that the helicity effect, which gives rise to the deterministic helicity-induced switching, arises almost instantly within 200 fs close to the pulse width upon laser excitation. The results reveal that the transient magnetization state upon which CP laser pulses impinge is the key factor for achieving HD-AOS, and importantly, the tunability between heating and helicity effects with the unique dual-pump laser excitation approach will enable HD-AOS in a wide range of magnetic material systems having wide-ranging implications for potential ultrafast spintronics applications.Graphical abstract

Generation of isolated and polarized γ-ray pulse by few-cycle laser irradiating a nanofoil

An isolated ultra-short γ-ray pulse is a unique tool for measuring ultrafast-physics processes, such as imaging intra-nuclear dynamics and inner-shell electron dynamics. Here, we propose an all-optical efficient scheme for generating isolated ultra-short γ-ray pulse from a laser-driven nanofoil. When a few-cycle circularly polarized laser pulse with an intensity of 1022 W cm−2 irradiates a nanofoil, the electrons in the nanofoil are pushed forwards collectively, forming a single relativistic electron sheet (RES) with a charge of nC. The electrons are substantially accelerated to high energies by the super-ponderomotive force of the laser. Then, a counter-propagating laser pulse with a peak intensity of 1021 W cm−2 collides with the RES, resulting in the generation of an isolated sub-femtosecond γ-ray pulse via nonlinear Compton scattering. The effect of laser polarization on the polarization degree of γ-rays is investigated by using a proof-of-principle calculation. It is shown that a highly polarized isolated γ-ray pulse with a cut-off energy of 100 MeV can eventually be generated in a head-on collision configuration when the scattering laser is linearly polarized. Such an isolated ultra-short polarized γ-ray source would provide critical applications in high-energy physics, laboratory astrophysics and nuclear physics.

Formation of periodic nanostructures induced by circularly-polarized femtosecond laser

We investigated the formation of periodic nanostructures on GaN induced by circularly-polarized femtosecond laser pulses. The structure shape changed from spiral to dots structures with increasing the pulse number. The structure change explained the previous inconsistent results, and we suggest a hypothesis for the formation dynamics. The period of the dots structures was approximately 150 nm which is almost 1/7 of the laser wavelength, and it kept crystalline comparable to the original substrate. The laser-induced periodic surface structures are expected to apply as a new fine processing technology.

Ultra-fast polarization of a thin electron layer in the rotational standing-wave field driven by double ultra-intense laser pulses

We explore radiative polarization of electrons in a standing-wave formed by two circularly-polarized laser pulses irradiating a thin layer. Here the electron radiative spin dynamics in external electromagnetic fields is described by the generalized Sokolov–Ternov model implemented in the particle-in-cell simulations. We find that significant polarization is established in roughly one laser period from the circular motion in the standing wave. However, such motion is unstable at the magnetic nodes such that electrons migrate to different phases. The beam polarization is then transferred to transverse directions following the T-BMT precession and splits into two groups with opposite signs. The induced polarization distribution allows for filtering out electron population of high polarization purity via certain emitting angles and energies, approaching maximum of 78% polarization at light intensities of the order ∼1023 W cm−2.

Theoretical Study of the Efficient Ion Acceleration Driven by Petawatt-Class Lasers via Stable Radiation Pressure Acceleration

Laser-driven radiation pressure acceleration (RPA) is one of the most promising candidates to achieve quasi-monoenergetic ion beams. In particular, many petawatt systems are under construction or in the planning phase. Here, a stable radiation pressure acceleration (SRPA) scheme is investigated, in which a circularly-polarized (CP) laser pulse illuminates a CH2 thin foil followed by a large-scale near-critical-density (NCD) plasma. In the laser-foil interaction, a longitudinal charge-separated electric field is excited to accelerate ions together with the heating of electrons. The heating can be alleviated by the continuous replenishment of cold electrons of the NCD plasma as the laser pulse and the pre-accelerated ions enter into the NCD plasma. With the relativistically transparent propagation of the pulse in the NCD plasma, the accelerating field with large amplitude is persistent, and its propagating speed becomes relatively low, which further accelerates the pre-accelerated ions. Our particle-in-cell (PIC) simulation shows that the SRPA scheme works efficiently with the laser intensity ranging from 6.85×1021 W cm−2 to 4.38×1023 W cm−2, e.g., a well-collimated quasi-monoenergetic proton beam with peak energy ∼1.2 GeV can be generated by a 2.74 × 1022 W cm−2 pulse, and the energy conversion efficiency from the laser pulse to the proton beam is about 16%. The QED effects have slight influence on this SRPA scheme.

The High-Order Harmonic Generation from Atom Driven by Co-Rotating Laser Pulses Composed of Fundamental Frequency and High Frequency

By numerically solving the time-dependent Schrödinger equation (TDSE), the harmonic generation process of atoms irradiated by corotating laser pulses composed of a fundamental-frequency and high-frequency field is systematically studied. Compared with the harmonic generated from atoms irradiated by counter-rotating two-color circularly polarized laser pulses, the harmonic efficiency of atoms irradiated by co-rotating two-color circularly polarized (CRTCCP) laser pulses with the same laser parameters is higher. The harmonics are generated by the multiphoton radiation transition after the bound electrons undergo a multiphoton absorption transition to a higher energy level. In addition, the variation of the harmonic efficiency with the field strength of different frequency components in the driving laser pulse is also studied. The circularly polarized harmonics with higher intensity can be obtained by optimizing the field strength of the driving laser field.

The nonsequential double ionization of Ar atoms with different initial angular momenta irradiated by a circularly polarized laser pulse

The non-sequential double ionization process of Ar atoms with different initial angular momenta in the 500 nm circularly polarized field is investigated with a classical ensemble model. The numerical results show that, under the circularly polarized laser pulse, the double ionization yield as a function of the laser intensity for Ar atoms with different initial angular momenta exhibit an obvious Knee structure. The ionization yield in the counter-rotating case where the electronic initial rotation direction is opposite to the vector potential of the laser field, significantly higher than that in the co-rotating case where the electronic initial rotation direction is consistent with the vector potential of the laser field. Based on the classical trajectory analysis, it is found that this ionization difference comes from the angle between the direction of the electronic momentum and the force of the laser field at the single ionization instant. The angle in the counter-rotating case is greater than that in the co-rotating case, and the counter-rotating electrons are more likely to return to the parent ion for recollision. Besides, the recollision intants of Ar atoms with the different initial angular momenta are mapped to the electron momentum spectrum. This may provide a method to obtain the electron’s recollision instant of Ar atoms with different initial angular momenta.