Polarized Laser Pulses Research Articles

Interplay of pulse duration, peak intensity, and particle size in laser-driven electron emission from silica nanospheres.

We present the results of a systematic study of photoelectron emission from isolated dielectric nanoparticles (SiO2) irradiated by intense 25 fs, 780 nm linearly polarized laser pulses as a function of particle size (20 nm to 750 nm in diameter) and laser intensity. We also introduce an experimental technique to reduce the effects of focal volume averaging. The highest photoelectron energies show a strong size dependence, increasing by a factor of six over the range of particles sizes studied at a fixed intensity. For smaller particle sizes (up to 200 nm), our findings agree well with earlier results obtained with few-cycle, ∼4 fs pulses. For large nanoparticles, which exhibit stronger near-field localization due to field-propagation effects, we observe the emission of much more energetic electrons, reaching energies up to ∼200 times the ponderomotive energy. This strong deviation in maximum photoelectron energy is attributed to the increase in ionization and charge interaction for many-cycle pulses at similar intensities.

Investigating nonsequential double ionization mechanisms with attosecond two-electron angular streaking

The emission direction of ionized electrons from atoms and molecules is determined by the strong transient laser field at the moment of ionization. Thus, we can use circularly polarized laser pulses to record the emission direction of two electrons in strong-field double ionization. Mapping the relative emission angle between the two electrons onto their ionization time delay, we show relative emission time delay changes in nonsequential double ionization of a model benzene molecule with a classical ensemble model, within the range from 150 to 400 attoseconds, as the laser intensity decreases. Such emission time delay changes reflect the double ionization mechanism changes from dominant direct impact double ionization to dominant recollision-induced excitation with a subsequent field-ionization mechanism.

Ultrafast control of spin interactions in honeycomb antiferromagnetic insulators

Light enables ultrafast, direct and nonthermal control of the exchange and Dzyaloshinskii-Moriya interactions. We consider two-dimensional honeycomb lattices described by the Kane-Mele-Hubbard model at half filling and in the strongly correlated limit, i.e., the Mott insulator phase of a canted antiferromagnet. Based on Floquet theory, we demonstrate that by changing the amplitude and frequency of polarized laser pulses, one can tune the amplitudes and signs of and even the ratio between the exchange and Dzyaloshinskii-Moriya spin interactions. Furthermore, the renormalizations of the spin interactions are independent of helicity. Our results pave the way for ultrafast optical spin manipulation in recently discovered two-dimensional magnetic materials.

Multipass-cell-based post-compression of radially and azimuthally polarized pulses to the sub-two-cycle regime

Nonlinear spectral broadening of radially and azimuthally polarized pulses by using the multipass cell technique is proposed and investigated with numerical simulations. The scheme features post-compression of optical pulses from 30 fs down to sub-5 fs centered around 800 nm wavelength with negligible beam distortion and high spectral broadening homogeneity. The performance of the multipass cell is studied with solid-state and gaseous media at μJ and mJ energy levels, respectively. The simulations show that dispersion management in the cell is crucial to obtain sub-two-cycle pulses, while dispersive coatings are considered. It is also shown that multipass cells with silver mirrors provide an alternative option that eases the requirement for dispersion control with an acceptable sacrifice on the net transmission. The sub-two-cycle radially/azimuthally polarized laser pulses are of great interest for generation of radially/azimuthally polarized isolated attosecond extreme ultraviolet pulses and for novel schemes of electron acceleration.

3D particle simulation of harmonic radiation excited by double circularly-polarized laser pulses irradiating over-dense plasma surface

In this paper, we propose a robust and novel scheme to generate ultra-bright circularly polarized harmonics using non-collinear double-circularly polarized laser pulses irradiating over-dense plasma surface. Numerous advantages are demonstrated by three-dimensional particle-in-cell simulations including high efficiency of harmonic generation, the separation of the harmonics from the pump beams, the production of both left and right-circularly polarized harmonics at the same wavelength and the capability of separating the harmonics without a spectrometer. Relativistically oscillating mirror model (ROM) is adopted to explain the harmonic generation mechanism and a set of novel selection rules for our HHG technique is derived, which is consistent with the simulation results.

Multi-stage scheme for nonlinear Breit–Wheeler pair-production utilising ultra-intense laser-solid interactions

Multi-petawatt (PW) lasers enable intensities exceeding 1023 W cm−2, at which point quantum electrodynamics (QED) processes, such as electron–positron pair-production via the nonlinear Breit–Wheeler process, will play a significant role in laser-plasma interactions. Using 2D QED-particle-in-cell simulations, we present a two-stage scheme in which nonlinear pair-production is induced via an ultra-intense laser-solid interaction. The first stage is the generation of a γ-ray beam, through the interaction of an ultra-intense laser pulse with a thick target, whose features are found to be strongly dependent on collective plasma effects. This compact, high energy γ-ray beam (characterised by a divergence half-angle ∼10° and average photon energy ∼10 MeV) then interacts with two counter-propagating laser pulses. By varying the laser polarisation and angle of incidence, we show that in the case of two circularly polarised laser pulses propagating at an angle equal to the divergence half-angle of the γ-ray beam, the produced positron distribution is highly anisotropic compared to the case of a standard head-on collision.

Frustrated tunneling ionization in the elliptically polarized strong laser fields.

We theoretically investigated frustrated tunneling ionization (FTI) in the interaction of atoms with elliptically polarized laser pulses by a semiclassical ensemble model. Our results show that the yield of frustrated tunneling ionization events exhibits an anomalous behavior which maximizes at the nonzero ellipticity. By tracing back the initial tunneling coordinates, we show that this anomalous behavior is due to the fact that the initial transverse velocity at tunneling of the FTI events is nonzero in the linear laser pulses and it moves across zero as the ellipticity increases. The FTI yield maximizes at the ellipticity when the initial transverse momentum for being trapped is zero. Moreover, the angular momentum distribution of the FTI events and its ellipticity dependence are also explored. The anomalous behavior revealed in our work is very similar to the previously observed ellipticity dependence of the near- and below-threshold harmonics, and thus our work may uncover the mechanism of the below-threshold harmonics which is still a controversial issue.

Collisionless electrostatic shock acceleration of proton using high intensity laser

An experiment on ion acceleration in near-critical density plasma is conducted using a high-intensity 1053 nm linearly polarized laser pulse with a laser intensity of 3.5 × 1018W/cm2. In this experiment a thin 0.7 µm thick CH foil is irradiated with a low-intensity, nanosecond pulse to create a near-critical density plasma. Then the high-intensity pulse is focused on the target at different time delays in order to verify what density profile is optimal for collisionless shock generation. Ion acceleration and electron spectra are observed by Radio Chromic Film and magnetic electron spectrometer. Proton dominant acceleration is confirmed at some time delays by CR-39. It is suggested that the accelerated ions are produced by Collisionless Electrostatic Shock comparing the experimental results with numerical/computational results.

Numerical attoclock on atomic and molecular hydrogen

Numerical attoclock is a theoretical model of attosecond angular streaking driven by a very short, nearly a single oscillation, circularly polarized laser pulse. The reading of such an attoclock is readily obtained from a numerical solution of the time-dependent Schr\"odinger equation as well as a semi-classical trajectory simulation. By making comparison of the two approaches, we highlight the essential physics behind the attoclock measurements. In addition, we analyze the predictions of the Keldysh-Rutherford model of the attoclock [Phys. Rev. Lett. 121, 123201 (2018)]. In molecular hydrogen, we highlight a strong dependence of the width of the attoclock angular peak on the molecular orientation and attribute it to the two-center electron interference. This effect is further exemplified in the weakly bound neon dimer.

High-flux x-ray photon emission by a superluminal hybrid electromagnetic mode of intense laser in a plasma waveguide

The feasibility of several novel ultrafast x-ray sources has been demonstrated through the interaction between laser and a micro-structured target. However, the resulting photon flux is still deficient for some applications. Here, we proposed a new method to yield high-flux synchrotron radiation by adopting compact hollow plasma waveguide. We verified the method theoretically and numerically. Linearly polarized laser pulses propagating in the waveguide are mainly converted into the electromagnetic mode HE11 with a superluminal phase speed of 1.02c when the pulse is coupled with the waveguide. The mode fields lead to a strong oscillating force and short oscillation period for energetic electrons, which are accelerated via the longitudinal acceleration mechanism. Then, the short x-ray beam is generated with high yield of 1011–1012. The corresponding photon flux reaches 3.5 × 1021 photons s−1 · 0.1% bandwidth. The high-flux source can be even used for single shot ultrafast imaging or the ultrafast x-ray diffraction and absorption studies.

Simultaneous polarization transformation and amplification of multi-petawatt laser pulses in magnetized plasmas.

With increasing laser peak power, the generation and manipulation of high-power laser pulses become a growing challenge for conventional solid-state optics due to their limited damage threshold. As a result, plasma-based optical components that can sustain extremely high fields are attracting increasing interest. Here, we propose a type of plasma waveplate based on magneto-optical birefringence under a transverse magnetic field, which can work under extremely high laser power. Importantly, this waveplate can simultaneously alter the polarization state and boost the peak laser power. It is demonstrated numerically that an initially linearly polarized laser pulse with 5 petawatt peak power can be converted into a circularly polarized pulse with a peak power higher than 10 petawatts by such a waveplate with a centimeter-scale diameter. The energy conversion efficiency of the polarization transformation is about 98%. The necessary waveplate thickness is shown to scale inversely with plasma electron density ne and the square of magnetic field B0, and it is about 1 cm for ne = 3 × 1020 cm-3 and B0 = 100 T. The proposed plasma waveplate and other plasma-based optical components can play a critical role for the effective utilization of multi-petawatt laser systems.

All-optical spin switching under different spin configurations

All-optical spin switching represents a new frontier in femtomagnetism. However, its underlying principles are quite different from traditional thermal activated spin switching. Here, we employ an atomic spin model and present a systematic investigation from a single spin to a large system of over a million spins. We find that for a single spin without an external perturbation, the conservation of total angular momentum requires that the spin change, if any, exactly matches the orbital momentum change, but a laser pulse significantly alters this relation, where the spin change does not necessarily follow the orbital change. This is reflected in the strong dependence of switching on laser polarization. To have an efficient spin switching, the electron initial momentum direction must closely follow the spin’s orientation, so the orbital angular momentum is transverse to the spin and consequently the spin–orbit torque lies in the same direction as the spin. The module of the spin–orbit torque is , where is the angle between spin and position (momentum ) and is the angle between and . These findings are manifested in a much larger system. We find that the spin response depends on underlying spin structures. A linearly polarized laser pulse creates a dip in a uniform inplane-magnetized thin film, but has little effects on Néel and Bloch walls. Both right- and left- circularly polarized light ( and ) have stronger but different effects in both uniform spin domains and Néel walls. While light creates a basin of spins pointing down, light creates a mound of spins pointing up. In the vicinity of the structure spins are reversed, similar to the experimental observation. light has a dramatic effect, disrupting spins in Bloch walls. By contrast, light has a small effect on Bloch walls because only switches down spins up and once the spins already point up, there is no major effect. These findings are expected to have important implications in the future.

Deformation of an inner valence molecular orbital in ethanol by an intense laser field.

Valence molecular orbitals play a crucial role in chemical reactions. Here, we reveal that an intense laser field deforms an inner valence orbital (10a') in the ethanol molecule. We measure the recoil-frame photoelectron angular distribution (RFPAD), which corresponds to the orientation dependence of the ionization probability of the orbital, using photoelectron-photoion coincidence momentum imaging with a circularly polarized laser pulse. Ab initio simulations show that the orbital deformation depends strongly on the laser field direction and that the measured RFPAD cannot be reproduced without taking the orbital deformation into account. Our findings suggest that the laser-induced orbital deformation occurs before electron emission on a suboptical cycle time scale.

Dependence of photoelectron-momentum distribution of molecule on orientation angle and laser ellipticity**Project supported by the National Natural Science Foundation of China (Grant Nos. 11271158, 11574117, and 61575077) and the Natural Science Foundation of Jilin Province of China (Grants No. 20180101225JC).

By numerically solving the two-dimensional time-dependent Schrödinger equation under the frozen-nuclei approximation, we are able to study the molecular photoelectron-momentum distribution (MPMD) of with different orientation angles driven by elliptically polarized laser pulse with varying ellipticities. Our numerical results show that the MPMD is sensitive to the orientation angle and the laser ellipticity, which can be explained by the attosecond perturbation ionization theory and the exactly solvable photoionization model. When the ellipticity ε=0, the final MPMD of x-oriented shows a distinct six-lobe pattern that is different from that with ε=0.5 and ε=1. The evolutions of electron wave packet (EWP) and MPMD with x-oriented are presented to interpret this distinct pattern.

Ultrarelativistic Electron-Beam Polarization in Single-Shot Interaction with an Ultraintense Laser Pulse.

Spin polarization of an ultrarelativistic electron beam head-on colliding with an ultraintense laser pulse is investigated in the quantum radiation-dominated regime. We develop a MonteCarlo method to model electron radiative spin effects in arbitrary electromagnetic fields by employing spin-resolved radiation probabilities in the local constant field approximation. Because of spin-dependent radiation reaction, the applied elliptically polarized laser pulse polarizes the initially unpolarized electron beam and splits it along the propagation direction into two oppositely transversely polarized parts with a splitting angle of about tens of milliradians. Thus, a dense electron beam with above 70% polarization can be generated in tensof femtoseconds with realistic laser pulses. The proposed method demonstrates a way for relativistic electron beam polarization with currently achievable laser facilities.

Alignment of the CS2 dimer embedded in helium droplets induced by a circularly polarized laser pulse

Dimers of carbon disulfide (CS$_2$) molecules embedded in helium nanodroplets are aligned using a moderately intense, 160ps, non-resonant, circularly polarized laser pulse. It is shown that the intermolecular carbon-carbon (C-C) axis aligns along the axis perpendicular to the polarization plane of the alignment laser pulse. The degree of alignment, quantified by $\langle \cos^2(\theta_\text{2D}) \rangle$, is determined from the emission directions of recoiling CS$_2$$^+$ fragment ions, created when an intense 40fs probe laser pulse doubly ionizes the dimers. Here, $\theta_\text{2D}$ is the projection of the angle between the C-C axis on the 2D ion detector and the normal to the polarization plane. $\langle \cos^2(\theta_\text{2D}) \rangle$ is measured as a function of the alignment laser intensity and the results agree well with $\langle \cos^2(\theta_\text{2D}) \rangle$ calculated for gas-phase CS$_2$ dimers with a rotational temperature of 0.4K.

Coherent Optical Control of a Quantum-Dot Spin-Qubit in a Waveguide-Based Spin-Photon Interface

Waveguide-based spin-photon interfaces on the GaAs platform have emerged as a promising system for a variety of quantum information applications directly integrated into planar photonic circuits. The coherent control of spin states in a quantum dot can be achieved by applying circularly polarized laser pulses that may be coupled into the planar waveguide vertically through radiation modes. However, proper control of the laser polarization is challenging since the polarization is modified through the transformation from the far field to the exact position of the quantum dot in the nanostructure. Here we demonstrate polarization-controlled excitation of a quantum-dot electron spin and use that to perform coherent control in a Ramsey interferometry experiment. The Ramsey interference reveals a pure dephasing time of $ 2.2\pm0.1 $ ns, which is comparable to the values so far only obtained in bulk media. We analyze the experimental limitations in spin initialization fidelity and Ramsey contrast and identify the underlying mechanisms.

Pure even and odd harmonics produced in modeled hydrogen molecular ions in single-color strong laser fields

The even and odd high-order harmonic generation in a modeled hydrogen molecular ion in strong laser fields is studied by numerically simulating the time-dependent Schrodinger equation. By applying a linearly polarized laser pulse whose polarization axis is perpendicular to the molecular axis, the pure odd harmonics polarized along the laser polarization direction are produced. Meanwhile, the pure even harmonics polarized along the molecular axis may be produced simultaneously either by adding an extra weak direct-current electric field along the molecular axis, or by artificially setting asymmetric charges for the two nuclei. The Bohmian trajectories reveal that the pure even harmonics are contributed by the dipole formed by the asymmetric expansion of the ionized wave packet along the molecular axis, instead of the asymmetric Coulomb attraction for the rescattering electron as expected. Moreover, the relationship between the symmetry of molecular orbital and the parity of high-order harmonics is also explored.

Ultrafast structural dynamics of photo-reactions observed by time-resolved x-ray cross-correlation analysis.

We applied angular X-ray Cross-Correlation analysis (XCCA) to scattering images from a femtosecond resolution X-ray free-electron laser pump-probe experiment with solvated PtPOP {[Pt2(P2O5H2)4]4–} metal complex molecules. The molecules were pumped with linear polarized laser pulses creating an excited state population with a preferred orientational (alignment) direction. Two time scales of 1.9 ± 1.5 ps and 46 ± 10 ps were revealed by angular XCCA associated with structural changes and rotational dephasing of the solvent molecules, respectively. These results illustrate the potential of XCCA to reveal hidden structural information in the analysis of time-resolved x-ray scattering data from molecules in solution.

Direct probing of tunneling time in strong-field ionization processes by time-dependent wave packets.

We propose a straightforward approach to directly probe the tunneling time by observing the transition of photoelectron wave packets in strong-field ionization processes, where Coulomb potentials do not affect the results. A circularly polarized laser pulse is used to avoid the impact of scattering electrons on the direct ionization electrons, and a pure transmission photoelectron wave packet can be obtained. Then, a positive tunneling time is extracted. The results demonstrate that the tunneling time is dominated mainly by the laser frequency in some laser intensity range. At the same time, we also investigate the tunneling time by analyzing the instantaneous ionization rate, and the consentaneous results are obtained.