Generation of High-Power High-Intensity Short X-Ray Free-Electron-Laser Pulses

Stimulated X-ray emission spectroscopy

We describe an emerging hard X-ray spectroscopy technique, stimulated X-ray emission spectroscopy (S-XES). S-XES has the potential to characterize the electronic structure of 3d transition metal complexes with spectral information currently not reachable and might lead to the development of new ultrafast X-ray sources with properties beyond the state of the art. S-XES has become possible with the emergence of X-ray free-electron lasers (XFELs) that provide intense femtosecond X-ray pulses that can be employed to generate a population inversion of core-hole excited states resulting in stimulated X-ray emission. We describe the instrumentation, the various types of S-XES, the potential applications, the experimental challenges, and the feasibility of applying S-XES to characterize dilute systems, including the Mn4Ca cluster in the oxygen evolving complex of photosystem II.

Femtosecond-Terawatt Hard X-Ray Pulse Generation with Chirped Pulse Amplification on a Free Electron Laser

Advances of high intensity lasers have opened up the field of strong field physics and led to a broad range of technological applications. Recent x-ray laser sources and optics development makes it possible to obtain extremely high intensity and brightness at x-ray wavelengths. In this Letter, we present a system design that implements chirped pulse amplification for hard x-ray free electron lasers. Numerical modeling with realistic experimental parameters shows that near-transform-limit single-femtosecond hard x-ray laser pulses with peak power exceeding 1 TW and brightness exceeding 4×10^{35}  s^{-1} mm^{-2} mrad^{-2}0.1%  bandwdith^{-1} can be consistently generated. Realization of such beam qualities is essential for establishing systematic and quantitative understanding of strong field x-ray physics and nonlinear x-ray optics phenomena.

Online single-shot characterization of ultrafast pulses from high-gain free-electron lasers

X-ray free-electron lasers (FELs) provide cutting-edge tools for fundamental researches to study nature down to the atomic level at a time-scale that fits this resolution. A precise knowledge of temporal information of FEL pulses is the central issue for its applications. Here we proposed and demonstrated a novel method to determine the FEL temporal profiles online. This robust method, designed for ultrafast FELs, allows researchers to acquire real-time longitudinal profiles of FEL pulses as well as their arrive times with respect to the external optical laser with a resolution better than 6 fs. Based on this method, we can also directly measure various properties of FEL pulses and correlations between them online. This helps us to further understand the FEL lasing processes and realize the generation of stable, nearly fully coherent soft X-ray laser pulses at the Shanghai Soft X-ray FEL facility. This method will enhance the experimental opportunities for ultrafast science in various areas.

Tunable x-ray free electron laser multi-pulses with nanosecond separation

X-ray Free Electron Lasers provide femtosecond x-ray pulses with narrow bandwidth and unprecedented peak brightness. Special modes of operation have been developed to deliver double pulses for x-ray pump, x-ray probe experiments. However, the longest delay between the two pulses achieved with existing single bucket methods is less than 1 picosecond, thus preventing the exploration of longer time-scale dynamics. We present a novel two-bucket scheme covering delays from 350 picoseconds to hundreds of nanoseconds in discrete steps of 350 picoseconds. Performance for each pulse can be similar to the one in a single pulse operation. The method has been experimentally tested with the Linac Coherent Light Source (LCLS-I) and the copper linac with LCLS-II hard x-ray undulators.

Intensity-dependent angular distribution of low-energy electrons generated by intense high-frequency laser pulse

By solving the three-dimensional time-dependent Schrödinger equation, we investigate the angular distributions of the low-energy electrons when an intense high-frequency laser pulse is applied to the hydrogen atom. Our numerical results show that the angular distributions of the low-energy electrons which generated by the nonadiabatic transitions sensitively depend on the laser intensity. The angular distributions evolve from a two-lobe to a four-lobe structure as the laser intensity increases. By analyzing nonadiabatic process in the Kramers-Henneberger frame, we illustrate that this phenomenon is attributed to the intensity-dependent adiabatic evolution of the ground state wavefunction. When the laser intensity further increases, the pathway of nonadiabatic transition from the ground state to the excited state and then to the continuum states is non-negligible, which results in the ring-like structure in the photoelectron momentum distribution. The angular distributions of the low-energy electrons provide a way to monitor the evolution of the electron wavefunction in the intense high frequency laser fields.

Experimental demonstration of novel beam characterization using a polarizable X-band transverse deflection structure

The PolariX TDS (Polarizable X-Band Transverse Deflection Structure) is an innovative TDS-design operating in the X-band frequency-range. The design gives full control of the streaking plane, which can be tuned in order to characterize the projections of the beam distribution onto arbitrary transverse axes. This novel feature opens up new opportunities for detailed characterization of the electron beam. In this paper we present first measurements of the Polarix TDS at the FLASHForward beamline at DESY, including three-dimensional reconstruction of the charge-density distribution of the bunch and slice emittance measurements in both transverse directions. The experimental results open the path toward novel and more extensive beam characterization in the direction of multi-dimensional-beam-phase-space reconstruction.

Observation of Seeded Mn Kβ Stimulated X-Ray Emission Using Two-Color X-Ray Free-Electron Laser Pulses

Kβ x-ray emission spectroscopy is a powerful probe for electronic structure analysis of 3d transition metal systems and their ultrafast dynamics. Selectively enhancing specific spectral regions would increase this sensitivity and provide fundamentally new insights. Recently we reported the observation and analysis of Kα amplified spontaneous x-ray emission from Mn solutions using an x-ray free-electron laser to create the 1s core-hole population inversion [Kroll et al., Phys. Rev. Lett. 120, 133203 (2018)PRLTAO0031-900710.1103/PhysRevLett.120.133203]. To apply this new approach to the chemically more sensitive but much weaker Kβ x-ray emission lines requires a mechanism to outcompete the dominant amplification of the Kα emission. Here we report the observation of seeded amplified Kβ x-ray emission from a NaMnO_{4} solution using two colors of x-ray free-electron laser pulses, one to create the 1s core-hole population inversion and the other to seed the amplified Kβ emission. Comparing the observed seeded amplified Kβ emission signal with that from conventional Kβ emission into the same solid angle, we obtain a signal enhancement of more than 10^{5}. Our findings are the first important step of enhancing and controlling the emission of selected final states of the Kβ spectrum with applications in chemical and materials science.

Two-Electron Interference in Strong-Field Ionization of He by a Short Intense Extreme Ultraviolet Laser Pulse

Double ionization of helium by a single intense (above 10^{18}  W/cm^{2}) linearly polarized extreme ultraviolet laser pulse is studied by numerically solving the full-dimensional time-dependent Schrödinger equation. For the laser intensities well beyond the perturbative limit, novel gridlike interference fringes are found in the correlated energy spectrum of the two photoelectrons. The interference can be traced to the multitude of two-electron wave packets emitted at different ionization times. A semianalytical model for the dressed two-photon double ionization is shown to qualitatively account for the interference patterns in the joint energy spectrum. Similar signatures of interferences between transient induced time-delayed ionization bursts are expected for other atomic and molecular multielectron systems.

Shortening the pulse duration in seeded free-electron lasers by chirped microbunching

In externally seeded free-electron lasers (FELs) that rely on a frequency upconversion scheme to generate intense short-wavelength light pulses, the slippage effect in the radiator imposes a lower limit on the FEL pulse duration, which is typically on the order of a few tens of femtoseconds. Recently it was proposed that a combination of a chirped microbunch and a tapered undulator can be used to break this limit. Although the method has the potential to reduce the FEL pulse duration down to a level that cannot be achieved by current state-of-the-art technology, it requires a very short seed pulse (∼ one optical cycle or less), making it challenging to put this concept into practical use. Here, we propose an alternative technique to relax the requirement on the seed pulse length. We show that the modified scheme allows generation of FEL pulses with durations much shorter than that determined by the seed pulse and the slippage effect. The performance of the method, which can easily be implemented at existing seeded FEL user facilities, is evaluated through a campaign of analytical calculations and simulations. For our set of typical seeded FEL parameters, we expect the generation of 1.6 fs long pulses at 26 nm with a peak power of 10 GW using a 20 fs long chirped seed pulse operating at 260 nm.

Is radiation damage the limiting factor in high-resolution single particle imaging with X-ray free-electron lasers?

The prospect of single particle imaging with atomic resolution is one of the scientific drivers for the development of X-ray free-electron lasers. The assumption since the beginning has been that damage to the sample caused by intense X-ray pulses is one of the limiting factors for achieving subnanometer X-ray imaging of single particles and that X-ray pulses need to be as short as possible. Based on the molecular dynamics simulations of proteins in X-ray fields of various durations (5 fs, 25 fs, and 50 fs), we show that the noise in the diffracted signal caused by radiation damage is less than what can be expected from other sources, such as sample inhomogeneity and X-ray shot-to-shot variations. These findings show a different aspect of the feasibility of high-resolution single particle imaging using free-electron lasers, where employing X-ray pulses of longer durations could still provide a useful diffraction signal above the noise due to the Coulomb explosion.

Imaging electron-density fluctuations by multidimensional X-ray photon-coincidence diffraction

The ultrafast spontaneous electron-density fluctuation dynamics in molecules is studied theoretically by off-resonant multiple X-ray diffraction events. The time- and wavevector-resolved photon-coincidence signals give an image of electron-density fluctuations expressed through the four-point correlation function of the charge density in momentum space. A Fourier transform of the signal provides a real-space image of the multipoint charge-density correlation functions, which reveal snapshots of the evolving electron density in between the diffraction events. The proposed technique is illustrated by ab initio simulations of the momentum- and real-space inelastic scattering signals from a linear cyanotetracetylene molecule.

Dynamic interference as signature of atomic stabilization

We study the ionization of atoms by very intense linearly polarized pulse with moderately high frequency by numerically solving the time-dependent Schrödinger equation (TDSE). In this regime, the photon energy exceeds the ionization potential allowing for one-photon ionization which is, however, strongly influenced by strong nonlinear photon-atom interactions. We find that the onset of atomic stabilization can be monitored by the appearance of a dynamic interference pattern in the photoelectron spectrum.

High-Power Femtosecond Soft X Rays from Fresh-Slice Multistage Free-Electron Lasers

We demonstrate a novel multistage amplification scheme for self-amplified spontaneous-emission free electron lasers for the production of few femtosecond pulses with very high power in the soft x-ray regime. The scheme uses the fresh-slice technique to produce an x-ray pulse on the bunch tail, subsequently amplified in downstream undulator sections by fresh electrons. With three-stages amplification, x-ray pulses with an energy of hundreds of microjoules are produced in few femtoseconds. For single-spike spectra x-ray pulses the pulse power is increased more than an order of magnitude compared to other techniques in the same wavelength range.

Dispersion-Based Fresh-Slice Scheme for Free-Electron Lasers

The fresh-slice technique improved the performance of several self-amplified spontaneous emission free-electron laser schemes by granting selective control on the temporal lasing slice without spoiling the other electron bunch slices. So far, the implementation has required a special insertion device to create the beam yaw, called a dechirper. We demonstrate a novel scheme to enable fresh-slice operation based on electron energy chirp and orbit dispersion that can be implemented at any free-electron laser facility without additional hardware.

Stimulated X-Ray Emission Spectroscopy in Transition Metal Complexes

We report the observation and analysis of the gain curve of amplified Kα x-ray emission from solutions of Mn(II) and Mn(VII) complexes using an x-ray free electron laser to create the 1s core-hole population inversion. We find spectra at amplification levels extending over 4 orders of magnitude until saturation. We observe bandwidths below the Mn 1s core-hole lifetime broadening in the onset of the stimulated emission. In the exponential amplification regime the resolution corrected spectral width of ∼1.7  eV FWHM is constant over 3 orders of magnitude, pointing to the buildup of transform limited pulses of ∼1  fs duration. Driving the amplification into saturation leads to broadening and a shift of the line. Importantly, the chemical sensitivity of the stimulated x-ray emission to the Mn oxidation state is preserved at power densities of ∼10^{20}  W/cm^{2} for the incoming x-ray pulses. Differences in signal sensitivity and spectral information compared to conventional (spontaneous) x-ray emission spectroscopy are discussed. Our findings build a baseline for nonlinear x-ray spectroscopy for a wide range of transition metal complexes in inorganic chemistry, catalysis, and materials science.