Generation Of Attosecond X-ray Pulses Research Articles

Generation of single attosecond relativistic electron bunch from intense laser interaction with a nanosphere

Ultrahigh-intensity laser-plasma physics provides unique light and particle beams as well as novel physical phenomena. A recently available regime is based on the interaction between a relativistic intensity few-cycle laser pulse and a sub-wavelength-sized mass-limited plasma target. Here, we investigate the generation of electron bunches under these extreme conditions by means of particle-in-cell simulations. In a first step, up to all electrons are expelled from the nanodroplet and gain relativistic energy from time-dependent local field enhancement at the surface. After this ejection, the electrons are further accelerated as they copropagate with the laser pulse. As a result, a few, or under specific conditions isolated, pC-class relativistic attosecond electron bunches are generated with laser pulse parameters feasible at state-of-the-art laser facilities. This is particularly interesting for some applications, such as generation of attosecond x-ray pulses via Thomson backscattering.

Efficient generation of relativistic near-single-cycle mid-infrared pulses in plasmas

Ultrashort intense optical pulses in the mid-infrared (mid-IR) region are very important for broad applications ranging from super-resolution spectroscopy to attosecond X-ray pulse generation and particle acceleration. However, currently, it is still difficult to produce few-cycle mid-IR pulses of relativistic intensities using standard optical techniques. Here, we propose and numerically demonstrate a novel scheme to produce these mid-IR pulses based on laser-driven plasma optical modulation. In this scheme, a plasma wake is first excited by an intense drive laser pulse in an underdense plasma, and a signal laser pulse initially at the same wavelength (1 micron) as that of the drive laser is subsequently injected into the plasma wake. The signal pulse is converted to a relativistic multi-millijoule near-single-cycle mid-IR pulse with a central wavelength of ~5 microns via frequency-downshifting, where the energy conversion efficiency is as high as approximately 30% when the drive and signal laser pulses are both at a few tens of millijoules at the beginning. Our scheme can be realized with terawatt-class kHz laser systems, which may bring new opportunities in high-field physics and ultrafast science.

High-intensity isolated attosecond X-ray pulse generation by using low-intensity ultraviolet–mid-infrared laser beam

An efficient approach to obtain the high-intensity isolated attosecond X-ray pulse has been proposed and studied by using the low-intensity ultraviolet–mid-infrared (UV–MIR) laser beam. It is found that with the superposition of the UV–MIR beam, not only the harmonic efficiency can be enhanced, but also the harmonic cutoff can be extended to the X-ray region. In detail, the results can be separated into two parts. Firstly, when the fundamental field is chosen to be the linearly polarization MIR field, the enhancement of the harmonic efficiency is sensitive to the pulse duration and the delay time of the UV pulse. Moreover, the enhancement of the harmonic spectrum is coming from the multiple harmonic emission peaks (HEPs). Secondly, when the fundamental field is chosen to be the polarization gating two circularly polarization MIR fields, the enhancement of the harmonic efficiency is independent on the pulse duration and the delay time of the UV pulse, which is much better for experimental realization. Moreover, the enhancement of the harmonic spectrum is nearly coming from the single HEP. Further, with the introduction of the unipolar pulse, the harmonic cutoff can be further extended and some sub-40 as single X-ray pulses with the intensity enhancement of 450 dB can be obtained.

Relativistic single-cycle tunable infrared pulses generated from a tailored plasma density structure

The availability of intense, ultrashort coherent radiation sources in the infrared region of the spectrum is enabling the generation of attosecond X-ray pulses via high harmonic generation, pump-probe experiments in the "molecular fingerprint" region and opening up the area of relativistic-infrared nonlinear optics of plasmas. These applications would benefit from multi-millijoule single-cycle pulses in the mid to long wavelength infrared (LW-IR) region. Here we present a new scheme capable of producing tunable relativistically intense, single-cycle infrared pulses from 5-14$\mu$m with a 1.7% conversion efficiency based on a photon frequency downshifting scheme that uses a tailored plasma density structure. The carrier-envelope phase (CEP) of the LW-IR pulse is locked to that of the drive laser to within a few percent. Such a versatile tunable IR source may meet the demands of many cutting-edge applications in strong-field physics and greatly promote their development.

Sub-cycle light transients for attosecond, X-ray, four-dimensional imaging

This paper reviews the revolutionary development of ultra-short, multi-TW laser pulse generation made possible by current laser technology. The design of the unified laser architecture discussed in this paper, based on the synthesis of ultrabroadband optical parametric chirped-pulse amplifiers, promises to provide powerful light transients with electromagnetic forces engineerable on the electron time scale. By coherent combination of multiple amplifiers operating in different wavelength ranges, pulses with wavelength spectra extending from less than 1 m to more than 10 m, with sub-cycle duration at unprecedented peak and average power levels can be generated. It is shown theoretically that these light transients enable the efficient generation of attosecond X-ray pulses with photon flux sufficient to image, for the first time, picometre-attosecond trajectories of electrons, by means of X-ray diffraction and record the electron dynamics by attosecond spectroscopy. The proposed system leads to a tool with sub-atomic spatio-temporal resolution for studying different processes deep inside matter.

Improved polarization gating scheme on attosecond source generation

ABSTRACTAn improved polarization gating scheme on attosecond X-ray pulse generation from high-order harmonic generation has been investigated. It is found that by properly adding a linearly polarized pulse to the circularly polarized pulse, the harmonic spectrum is remarkably enhanced and controlled, leading to an efficient generation of the ultra-broadband supercontinuum covered by the spectrum range from ultraviolet to X-ray. Classical and the quantum time–frequency analyses have been shown to explain the harmonic emission process. Moreover, from analyzing the harmonic phases, we see that the phases of the generated harmonics are almost linear, which is beneficial to the attosecond pulse generation. As a result, a series of phase-steady and wavelength-tunable sub-50 as X-ray pulses can be obtained.

Generation of bright attosecond x-ray pulse trains via Thomson scattering from laser-plasma accelerators.

Generation of attosecond x-ray pulse attracts more and more attention within the advanced light source user community due to its potentially wide applications. Here we propose an all-optical scheme to generate bright, attosecond hard x-ray pulse trains by Thomson backscattering of similarly structured electron beams produced in a vacuum channel by a tightly focused laser pulse. Design parameters for a proof-of-concept experiment are presented and demonstrated by using a particle-in-cell code and a four-dimensional laser-Compton scattering simulation code to model both the laser-based electron acceleration and Thomson scattering processes. Trains of 200 attosecond duration hard x-ray pulses holding stable longitudinal spacing with photon energies approaching 50 keV and maximum achievable peak brightness up to 1020 photons/s/mm2/mrad2/0.1%BW for each micro-bunch are observed. The suggested physical scheme for attosecond x-ray pulse trains generation may directly access the fastest time scales relevant to electron dynamics in atoms, molecules and materials.

Asymmetric molecular harmonic assisted generation of the attosecond X-ray pulse

In this paper, we theoretically investigate the asymmetric molecular high-order harmonic emission and the attosecond X-ray pulse generation when the asymmetric molecular HeH2+ ion is exposed to an intense chirped pulse. It is found that the molecular harmonics are strongly dependent on the internuclear distance R as well as the chirp parameter, showing a smooth and intense harmonic plateau at R = 8.0 a.u. and β = 0.4. Further, by adding a subharmonic controlling pulse with the optimised conditions, a supercontinuum with a bandwidth of 602 eV can be obtained. Finally, by properly superposing the selected harmonics in the supercontinuum region, a series of isolated X-ray pulses with durations of sub-80 as are directly generated.

Attosecond Hard X-ray Free Electron Laser

In this paper, several schemes of soft X-ray and hard X-ray free electron lasers (XFEL) and their progress are reviewed. Self-amplified spontaneous emission (SASE) schemes, the high gain harmonic generation (HGHG) scheme and various enhancement schemes through seeding and beam manipulations are discussed, especially in view of the generation of attosecond X-ray pulses. Our recent work on the generation of attosecond hard X-ray pulses is also discussed. In our study, the enhanced SASE scheme is utilized, using electron beam parameters of an XFEL under construction at Pohang Accelerator Laboratory (PAL). Laser, chicane and electron beam parameters are optimized to generate an isolated attosecond hard X-ray pulse at 0.1 nm (12.4 keV). The simulations show that the manipulation of electron energy beam profile may lead to the generation of an isolated attosecond hard X-ray of 150 attosecond pulse at 0.1 nm.

Generation of attosecond X-ray pulse of wavelength below 0.4 nm from the interaction of ultra-relativistic intense lasers with thin foil targets

By one-dimensional particle-in-cell simulations, the relativistic electron sheets generated by interaction between the ultra-relativistic intense laser pulse with intensity above 1022 W/cm2 and the thin foil target, as well as the attosecond X-ray pulses induced by Thomson backscattering from electron bunch are studied in this paper. The results indicate that increasing the intensity of the driving laser, reducing the density and thickness of foil target corresponding make the longitudinal momentum of the electrons enhanced and the wavelength of X-ray radiation reduced. Attosecond X-ray pulse with wavelength 1.168 nm can be obtained through optimizing correlated parameters. Especially, using probing laser pulse with doubling frequency and optimizing parameters of the drive light and thin film target can make the wavelength of coherent attosecond X-ray radiation reduced obviously, even below 0.4 nm, and the energy of the scattered photons can achieve more than 2 keV.

Generating stable attosecond x-ray pulse trains with a mode-locked seeded free-electron laser

Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. We propose a novel scheme for the generation of coherent stable attosecond x-ray pulse trains in a seeded FEL, via a process of mode-locked amplification. Three modulators and two chicanes are used for generating separated attosecond scale microstructures in the electron beam using the beam echo effect. Such electron beam will produce high harmonic radiation with a comb of longitudinal modes at the very beginning of the radiator. By using a series of spatiotemporal shifts between the copropagating radiation and electron beam in the radiator, all these modes can be preserved and amplified to saturation. Using a representative realistic set of parameters, three-dimensional simulation results show that trains of 200 attosecond soft x-ray pulses with stable peak powers at gigawatt level can be generated directly from ultraviolet seed lasers. The even spacing between the attosecond pulses can be easily altered from subfemtosecond to tens of femtoseconds by slightly changing the wavelength of one seed laser.

Generation of attosecond soft x-ray pulses in a longitudinal space charge amplifier

A longitudinal space charge amplifier (LSCA), operating in soft x-ray regime, was recently proposed. Such an amplifier consists of a few amplification cascades (focusing channel and chicane) and a short radiator undulator in the end. The broadband nature of LSCA supports generation of few-cycle pulses as well as wavelength compression. In this paper we consider an application of these properties of LSCA for generation of attosecond x-ray pulses. It is shown that a compact and cheap addition to the soft x-ray free-electron laser facility FLASH would allow one to generate 60 attosecond (FWHM) long x-ray pulses with the peak power at the 100 MW level and a contrast above 98%.

Generation of attosecond X-ray pulse in the interaction between the pulses and the relativistic electrons

The attosecond X-ray pulse which is produced by the interaction between the laser pulse and the relativistic electrons is studied in this paper. The attosecond X-ray pulse is generated by Thomson backscattering from the relativistic electrons. It also discusses the effect of the plasma parameters on the attosecond X-ray. The wavelength of attosecond X-ray pulse becomes shorter when the frequency of the laser or the velocity of the relativistic electrons increases. We obtained the "water window" X-ray by selecting the appropriate laser and plasma parameters. This paper also discusses the effect of relativistic electrons density and density grad on the translation efficiency.

Generation of attosecond x-ray pulses with a multicycle two-color enhanced self-amplified spontaneous emission scheme

Generation of attosecond x-ray pulses is attracting much attention within the x-ray free-electron laser (FEL) user community. Several schemes using extremely short laser pulses to manipulate the electron bunches have been proposed. In this paper, we extend the attosecond two-color ESASE scheme proposed by Zholents et al. to the long optical cycle regime using a second detuned laser and a tapered undulator. Both lasers can be about ten-optical-cycles long, with the second laser frequency detuned from the first to optimize the contrast between the central and side current spikes. A tapered undulator mitigates the degradation effect of the longitudinal space charge (LSC) force in the undulator and suppresses the FEL gain of all side current peaks. Simulations using the LCLS parameters show a single attosecond x-ray spike of {approx} 110 attoseconds can be produced. The second laser can also be detuned to coherently control the number of the side x-ray spikes and the length of the radiation pulse.

Self-amplified spontaneous emission FEL with energy-chirped electron beam and its application for generation of attosecond x-ray pulses

Influence of a linear energy chirp in the electron beam on a SASE FEL operation is studied analytically and numerically using 1-D model. Explicit expressions for Green's functions and for output power of a SASE FEL are obtained for high-gain linear regime in the limits of small and large energy chirp parameter. Saturation length and power versus energy chirp parameter are calculated numerically. It is shown that the effect of linear energy chirp on FEL gain is equivalent to the linear undulator tapering (or linear energy variation along the undulator). A consequence of this fact is a possibility to perfectly compensate FEL gain degradation, caused by the energy chirp, by means of the undulator tapering independently of the value of the energy chirp parameter. An application of this effect for generation of attosecond pulses from a hard X-ray FEL is proposed. Strong energy modulation within a short slice of an electron bunch is produced by few-cycle optical laser pulse in a short undulator, placed in front of the main undulator. Gain degradation within this slice is compensated by an appropriate undulator taper while the rest of the bunch suffers from this taper and does not lase. Three-dimensional simulations predict that short (200 attoseconds) high-power (up to 100 GW) pulses can be produced in Angstroem wavelength range with a high degree of contrast. A possibility to reduce pulse duration to sub-100 attosecond scale is discussed.

Attosecond x-Ray Pulse Generation by Linear ThomsonScattering of Intense Laser Beam with Relativistic Electron

Linear Thomson scattering of a short pulse laser byrelativistic electron has been investigated using computer simulations. Itis shown that scattering of an intense laser pulse of ∼33 fs fullwidth at half maximum, with an electron of γ0 = 10 initialenergy, generates an ultrashort, pulsed radiation of 76 attoseconds with aphoton wavelength of 2.5 nm in the backward direction. The scatteredradiation generated by a highly relativistic electron has superior qualityin terms of its pulse width and angular distribution in comparison to theone generated by lower relativistic energy electron.

Possibilities for controlling attosecond x-ray pulse generation during molecular ionization by femtosecond laser radiation

We discuss the role of different factors (molecular sizes and configuration, orientation of the molecular axis with respect to the electric field of a laser pulse, the type of the molecular orbital, etc.), which characterize molecules and their state, in the formation of the nonlinear response of a molecule ionized by a high-power femtosecond laser pulse. In numerical experiments within the framework of a two-dimensional model for the H + molecular ion, we study possibilities for controlling the process of nonlinear frequency conversion of femtosecond optical radiation into X-ray radiation of attosecond duration by means of preliminary vibrational or electronic excitation of molecules. We demonstrate the possibilities of using the attosecond pulse generation as a diagnostic tool for probing vibration-rotational dynamics of molecules.

Controlling the Phase Evolution of Few-Cycle Light Pulses

Using a coherent nonlinear optical technique, slipping of the carrier through the envelope of 6-fs light wave packets emitted from a mode-locked-oscillator/pulse-compressor system has been measured, permitting the generation of intense, few-cycle light with precisely reproducible electric and magnetic fields. These pulses open the way to controlling the evolution of strong-field interactions on the time scale of the light oscillation cycle and are indispensable to reproducible attosecond x-ray pulse generation.