Multi-cycle Laser Pulses Research Articles

Polarization control of terahertz waves generated by a femtosecond three-color pulse scheme

Polarization characteristics of terahertz waves generated from a short air plasma excited by femtosecond three-color pulses with a frequency ratio of 1:2:3 have been theoretically investigated, and the results show that flexible and effective control of terahertz polarization can be achieved by means of changing the polarization combination and relative phase of three-color pulses, which is related to the electric field spatiotemporal distribution of the synthetic pulse formed via three-color pulse superposition. The complicated spatiotemporal distribution can be made clear by analyzing the projection component of the electric field in the three-dimensional Cartesian coordinate system. For terahertz waves generated from a short air plasma filament, the proposed method of terahertz polarization control on the basis of a three-color pulse scheme can be realized by ordinary multi-cycle laser pulses and overcome the disadvantage of few-cycle laser pulses utilized to obtain nearly circularly polarized intense terahertz waves or elliptically polarized intense terahertz waves with large ellipticity in the two-color pulse scheme.

Laser wakefield acceleration of 10-MeV-scale electrons driven by 1-TW multi-cycle laser pulses in a sub-millimeter nitrogen gas cell

By focusing conventional 1-TW 40-fs laser pulses into a dense 450-μm-long nitrogen gas cell, we demonstrate the feasibility of routinely generating electron beams from laser wakefield acceleration (LWFA) with primary energies scaling up to 10 MeV and a high charge in excess of 50 pC. When electron beams are generated with a charge of ≈30 pC and a beam divergence of ≈40 mrad from the nitrogen cell having a peak atom density of 7.6×1018 cm−3, increasing the density inside the cell by 25%—controlled by tuning the backing pressure of fed nitrogen gas—can induce defocusing of the pump pulse that leads to a twofold increase in the output charge but with a trade-off in beam divergence. Therefore, this LWFA scheme has two preferred regimes for acquiring electron beams with either lower divergence or higher beam charge depending on a slight variation of the gas/plasma density inside the cell. Our results identify the high potential for implementing sub-millimeter nitrogen gas cells in the future development of high-repetition-rate LWFA driven by sub-TW or few-TW laser pulses.

Isolated ultra-bright attosecond pulses via non-collinear gating

When light with relativistic intensity is incident on a solid target, bright attosecond pulses of extreme ultraviolet and x-ray radiation can be generated in the reflected beam. Unfortunately, the use of multi-cycle laser pulses results in trains of these attosecond pulses. Here we investigate a non-collinear gating scheme applied to surface high-harmonic generation to allow for the extraction of a single intense attosecond pulse from this train. Using 3D and 2D particle in cell (PIC) simulations we demonstrate that it is possible to angularly isolate a single attosecond pulse from the main driving laser pulse using this interaction geometry with intensities I > 1020 W cm−2. This result opens the door to generating bright attosecond pulses from relativistic plasmas without the need to spectrally filter the driving laser pulse.

Controlling electron recollision with combined linear and circular polarization

We theoretically investigate the non-sequential double ionization of Ar atoms in the combined fields of linearly polarized laser and circularly polarized laser through 3D semiclassical simulations. By partially overlapping the two time-delayed multicycle laser pulses, we construct an optical waveform whose polarization ellipticity increase slowly for consecutive optical cycles. This composite laser pulses with the time-dependent ellipticity can tunnel-ionize atoms and steer the first tunneling electron to recollision with the second bound electron through different trajectories, in which the recollision occurs with different return times of the first ionized electron. Through tuning delay time between the two laser pulses, the double ionization yields and recollision trajectories with different return times can be controlled. The time-dependent ellipticity with different delay time can enhance or suppress the probability of different return times. This work provides a scheme exploring electron dynamics in few optical cycle or even subcycle time scale in a multicycle laser field without having to be limited to near-single-cycle laser pulses.

Determination of transition dipole moments of solids with high-order harmonics driven by multicycle ultrashort pulses

We propose an all-optical method to reconstruct the transition dipole moment of solid materials. Our method is realized by high-order harmonic generation driving by a multicycle laser pulse. The relations between the transition dipole moment and the harmonic yield are established. It is found that the transition dipole phase is related to the intensity ratio of the neighboring odd and even harmonics, and the modulus of the transition dipole moment is related to the odd harmonics. We investigate the high-order harmonic generation in symmetric crystals and asymmetric crystals by solving the semiconductor Bloch equations and use these relations to successfully reconstruct their relative transition dipole moments.

Low-Energy Protons in Strong-Field Dissociation of H 2 + via Dipole-Transitions at Large Bond Lengths

More than ten years ago, the observation of the low-energy structure in the photoelectron energy spectrum, regarded as an “ionization surprise,” has overthrown our understanding of strong-field physics. However, the similar low-energy nuclear fragment generation from dissociating molecules upon the photon energy absorption, one of the well-observed phenomena in light-molecule interaction, still lacks an unambiguous mechanism and remains mysterious. Here, we introduce a time-energy-resolved manner using a multicycle near-infrared femtosecond laser pulse to identify the physical origin of the light-induced ultrafast dynamics of molecules. By simultaneously measuring the bond-stretching times and photon numbers involved in the dissociation of H 2 + driven by a polarization-skewed laser pulse, we reveal that the low-energy protons (below 0.7 eV) are produced via dipole-transitions at large bond lengths. The observed low-energy protons originate from strong-field dissociation of high vibrational states rather than the low ones of H 2 + cation, which is distinct from the well-accepted bond-softening picture. Further numerical simulation of the time-dependent Schrödinger equation unveils that the electronic states are periodically distorted by the strong laser field, and the energy gap between the field-dressed transient electronic states may favor the one- or three-photon transitions at the internuclear distance larger than 5 a.u. The time-dependent scenario and our time-energy-resolved approach presented here can be extended to other molecules to understand the complex ultrafast dynamics.

Strong-field ionization of water. II. Electronic and nuclear dynamics en route to double ionization

We investigate the role of nuclear motion and strong-field-induced electronic couplings during the double ionization of deuterated water using momentum-resolved coincidence spectroscopy. By examining the three-body dicationic dissociation channel, D$^{+}$/D$^{+}$/O, for both few- and multi-cycle laser pulses, strong evidence for intra-pulse dynamics is observed. The extracted angle- and energy-resolved double ionization yields are compared to classical trajectory simulations of the dissociation dynamics occurring from different electronic states of the dication. In contrast with measurements of single photon double ionization, pronounced departure from the expectations for vertical ionization is observed, even for pulses as short as 10~fs in duration. We outline numerous mechanisms by which the strong laser field can modify the nuclear wavefunction en-route to final states of the dication where molecular fragmentation occurs. Specifically, we consider the possibility of a coordinate-dependence to the strong-field ionization rate, intermediate nuclear motion in monocation states prior to double ionization, and near-resonant laser-induced dipole couplings in the ion. These results highlight the fact that, for small and light molecules such as D$_2$O, a vertical-transition treatment of the ionization dynamics is not sufficient to reproduce the features seen experimentally in the strong field coincidence double-ionization data.

GV m−1 on-chip particle accelerator driven by few-cycle femtosecond laser pulse

Particle accelerator on chip with high acceleration gradient has been an unremitting goal of researchers. Dielectric laser accelerator (DLA) is a possible candidate to achieve this goal. However, due to the limitation of dielectric breakdown, it is difficult for the available DLAs to reach an acceleration gradient as high as 1 GV m−1 since a long-duration multi-cycle laser pulse with high fluence have to be used. Here we propose to use a few-cycle laser pulse to drive a DLA based on the inverse Cherekov radiation effect. It significantly reduces the required pulse duration and the laser fluence, remarkably increasing the achievable acceleration gradient. Moreover, by using a cascade acceleration scheme, we realize a high energy-gain acceleration for low-energy electrons in a microscale device by simulation, which paves the way for the development of a fully on-chip particle accelerator.

Femtosecond Single Cycle Pulses Enhanced the Efficiency of High Order Harmonic Generation.

High-order harmonic generation is a nonlinear process that converts the gained energy during light-matter interaction into high-frequency radiation, thus resulting in the generation of coherent attosecond pulses in the XUV and soft x-ray regions. Here, we propose a control scheme for enhancing the efficiency of HHG process induced by an intense near-infrared (NIR) multi-cycle laser pulse. The scheme is based on introducing an infrared (IR) single-cycle pulse and exploiting its characteristic feature that manifests by a non-zero displacement effect to generate high-photon energy. The proposed scenario is numerically implemented on the basis of the time-dependent Schrödinger equation. In particular, we show that the combined pulses allow one to produce high-energy plateaus and that the harmonic cutoff is extended by a factor of 3 compared to the case with the NIR pulse alone. The emerged high-energy plateaus is understood as a result of a vast momentum transfer from the single-cycle field to the ionized electrons while travelling in the NIR field, thus leading to high-momentum electron recollisions. We also identify the role of the IR single-cycle field for controlling the directionality of the emitted electrons via the IR-field induced electron displacement effect. We further show that the emerged plateaus can be controlled by varying the relative carrier-envelope phase between the two pulses as well as the wavelengths. Our findings pave the way for an efficient control of light-matter interaction with the use of assisting femtosecond single-cycle fields.

Clocking Dissociative Above-Threshold Double Ionization of H_{2} in a Multicycle Laser Pulse.

The dissociative above-threshold double ionization (ATDI) of H_{2} in strong laser fields involves the sequential releasing of two electrons at specific instants with the stretching of the molecular bond. By mapping the releasing instants of two electrons to their emission directions in a multicycle polarization-skewed femtosecond laser pulse, we experimentally clock the dissociative ATDI of H_{2} via distinct photon-number-resolved pathways, which are distinguished in the kinetic energy release spectrum of two protons measured in coincidence. The timings of the experimentally resolved dissociative ATDI pathways are in good accordance with the classical predictions. Our results verify the multiphoton scenario of the dissociative ATDI of H_{2} in both time and energy fashion, strengthening the understanding of the strong-field phenomenon and providing a robust tool with a subcycle time resolution to clock abundant ultrafast dynamics of molecules.

Probing spectral and temporal resolved dynamics in high-order harmonic generation of atom driven by multicycle near-infrared laser pulses

We present an ab initio investigation of the time-frequency characteristics of high-order harmonic generation (HHG) calculated by solving accurately and efficiently the time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We extend a new synchrosqueezing transform (SST) to obtain the time-frequency spectra of the HHG and the dynamical phase associated with the dipole-emission time profile. The SST time-frequency analysis allows us to explore in depth the dynamics and contributions of the quantum trajectories in HHG. Particularly it uncovers the subtle details of the spectral and temporal fine structures of the HHG and provides novel insights regarding the dynamical origin of the HHG in below-threshold harmonic regimes.

Suppression of individual peaks in two-colour high harmonic generation

This work investigates the suppression of individual harmonics, simultaneously affecting specific even and odd orders in the high-harmonic spectra generated by strongly tailored, two-colour, multi-cycle laser pulses in neon. The resulting spectra are systematically studied as a function of the electric-field shape in a symmetry-broken (ω–2ω) and symmetry-preserved (ω–3ω) configuration. The peak suppression is reproduced by macroscopic strong-field approximation calculations and is found to be unique to symmetry-broken fields (ω–2ω). Additionally, semi-classical calculations further corroborate the observation and reveal their underlying mechanism, where a nontrivial spectral interference between subsequent asymmetric half-cycles is found to be responsible for the suppression.

Coulomb exploded directional double ionization of N2O molecules in multicycle femtosecond laser pulses**Project supported by the National Key R&D Program of China (Grant No. 2018YFA0306303), the National Natural Science Fundation of China (Grant Nos. 11425416, 11834004, and 11761141004), and the 111 Project of China (Grant No. B12024).

We experimentally investigate Coulomb exploded directional double ionization of N2O molecules in elliptically polarized femtosecond laser pulses. The denitrogenation and deoxygenation channels are accessed via various pathways. It leads to distinct asymmetries in directional breaking of the doubly ionized N2O molecules versus the instantaneous laser field vector, which is revealed by tracing the sum-momentum spectra of the ionic fragments as a recoil of the ejected electrons. Our results demonstrate that the accessibility of the Coulomb exploded double ionization channels of N2O molecules are ruled by the detailed potential energy curves, and the directional emission of the fragments are governed by the joint effects of the electron localization-assisted enhanced ionization of the stretched molecules and the profiles of the molecular orbitals.

Carrier-envelope phase-dependent molecular high-order harmonic generation from H2+ in a multi-cycle regime.

Carrier-envelope phase (CEP) dependence of high-order harmonic generation (HHG) from H2+ in a multi-cycle laser pulse is investigated by solving the non-Born-Oppenheimer time-dependent Schrödinger equation (TDSE). It is found that high harmonics in the plateau exhibit counterintuitive frequency modulation (FM) as the CEP of the multi-cycle laser varies. Based on the classical electron trajectories and time-frequency analysis, this multi-cycle CEP-dependent FM is demonstrated to result from the interference of half-cycle HHG radiations, which is modulated by laser-driven nuclear motion. The mechanism of the CEP-dependent FM is further confirmed by simulations based on a simple algorithm in the time domain, which satisfactorily reproduces the TDSE results. The CEP-dependent FM encodes rich information on the correlated electron and nuclear dynamics, which paves the way for probing nuclear motion with attosecond resolution.

Single-Shot Carrier-Envelope Phase Determination of Long Superintense Laser Pulses.

The impact of the carrier-envelope phase (CEP) of an intense multicycle laser pulse on the radiation of an electron beam during nonlinear Compton scattering is investigated. We have identified a CEP effect specific to the ultrarelativistic regime. When the electron beam counterpropagates with the laser pulse, pronounced high-energy x-ray double peaks emerge near the backward direction relative to the initial electron motion. This is achieved in the relativistic interaction domain, where both the electron energy is required to be lower than for the electron reflection condition at the laser peak and the stochasticity effects in the photon emission need to be weak. The asymmetry parameter of the double peaks in the angular radiation distribution is shown to serve as a sensitive measure for the CEP of up to 10-cycle long laser pulses and can be applied for the characterization of extremely strong laser pulses in present and near future laser facilities.

Terawatt-scale optical half-cycle attosecond pulses

Extreme-ultravoilet (XUV) attosecond pulses with durations of a few tens of attosecond have been successfully applied for exploring ultrafast electron dynamics at the atomic scale. But their weak intensities limit the further application in demonstrating nonlinear responses of inner-shell electrons. Optical attosecond pulses will provide sufficient photon flux to initiate strong-field processes. Here we proposed a novel method to generate an ultra-intense isolated optical attosecond pulse through relativistic multi-cycle laser pulse interacting with a designed gas-foil target. The underdense gas target sharpens the multi-cycle laser pulse, producing a dense layer of relativistic electrons with a thickness of a few hundred nanometers. When the dense electron layer passes through an oblique foil, it emits single ultra-intense half-cycle attosecond pulse in the visible and ultraviolet spectral range. The emitted pulse has a peak intensity exceeding 1018 W/cm2 and full-width-half-maximum duration of 200 as. The peak power of this attosecond light source reaches 2 terawatt. The proposed method relaxes the single-cycle requirement on the driving pulse for isolated attosecond pulse generation and significantly boosts the peak power, thus it may open up the route to new experiments tracking the nonlinear response of inner-shell electrons as well as nonlinear attosecond phenomena investigation.

Wavepacket dynamics of a Rydberg atom monitored by a pair of time-delayed laser pulses

We have investigated the Rydberg state population of an argon atom by an intense laser pulse and its wavepacket dynamics monitored by another successive laser pulse in the tunneling regime. A wavepacket comprising a superposition of close high-lying Rydberg states is irradiated by a multicycle laser pulse, where the sub-wave components in the wavepacket have fixed relative phases. A time-delayed second laser pulse is employed to apply on the excited Rydberg atom. If the time is properly chosen, one of the sub-wave components will be guided towards the ionization area while the rest remains intact. By means of this pump–probe technique, we could control and monitor the Rydberg wavepacket dynamics and reveal some interesting phenomenon such as the survival rate of individual Rydberg states related to the classical orbital period of electron.

Subcycle characterization of photoelectron emission with multicycle laser pulses

By partially overlapping two time-delayed orthogonally polarized multicycle laser pulses of the same carrier frequency, we construct an optical waveform whose polarization axis rotates slowly for consecutive optical cycles. This unique laser field tunnel-ionizes atoms and accelerates the freed electrons to different directions at different instants. Classical trajectory Monto Carlo simulations support the resulting angle-resolved photoelectron momentum distribution. Taking advantage of this optical field, we investigate characteristics of electrons triggered by different quarters of an optical cycle and explore the Coulomb focusing effect on electrons traveling along different trajectories. Our experiment using a polarization-skewed near-infrared laser pulse verifies the numerical simulations and demonstrates the feasibility of retrieving subcycle dynamics with multicycle laser pulses.

Frequency gating to isolate single attosecond pulses with overdense plasmas using particle-in-cell simulations.

We present the isolation of single attosecond pulses for multi-cycle and few-cycle laser pulses from high harmonic generation in overdense plasmas, calculated with particle-in-cell simulations. By the combination of two laser pulses of equal amplitude and a small frequency shift between them, we demonstrate that it is possible to shorten the region in which the laser pulse is most intense, therefore restricting the generation of high harmonic orders in the form of attosecond pulses to a narrower time window. The creation of this window is achieved due to the combination of the laser pulse envelope and the slow oscillating wave obtained from the coherent sum of the two pulses. A parametric scan, performed with particle-in-cell simulations, reveals how the pulse isolation behaves for different input laser pulse lengths and which are the optimal frequency shifts between the two laser pulses in each case, giving the conditions for having a good isolation of an attosecond pulse when working with laser-plasma interaction in overdense targets.

Isolated broadband attosecond pulse generation with near- and mid-infrared driver pulses via time-gated phase matching.

We present a theoretical analysis of the time-gated phase matching (ionization gating) mechanism in high-order harmonic generation for the isolation of attosecond pulses at near-infrared and mid-infrared driver wavelengths, for both few-cycle and multi-cycle driving laser pulses. Results of our high harmonic generation and three-dimensional propagation simulations show that broadband isolated pulses spanning from the extreme-ultraviolet well into the soft X-ray region of the spectrum can be generated for both few-cycle and multi-cycle laser pulses. We demonstrate the key role of absorption and group velocity matching for generating bright, isolated, attosecond pulses using long wavelength multi-cycle pulses. Finally, we show that this technique is robust against carrier-envelope phase and peak intensity variations.