Fluence scan: an unexplored property of a laser beam.

We present an extended theoretical background of so-called fluence scan (f-scan or F-scan) method, which is frequently being used for offline characterization of focused short-wavelength (EUV, soft X-ray, and hard X-ray) laser beams [J. Chalupský et al., Opt.

Double core-hole creation by sequential attosecond photoionization.

X-ray fluorescence spectroscopy demonstrates that a single core-hole krypton with a 170-as lifetime can be photoionized again to a double core-hole state by an intense x-ray pulse. The observation indicates that unconventional interaction between intense x rays and atoms is no more negligible in applications with x-ray free-electron lasers. Quantitative analysis of the double core-hole creation including effects of a pulsed and spiky temporal structure enables estimation of the x-ray pulse duration in the sub-10-fs range.

Soft x-ray free-electron laser imaging by LiF crystal and film detectors over a wide range of fluences.

LiF crystal and film detectors were used to measure the far-field fluence profile of a self-amplified spontaneous-emission free-electron laser beam and diffraction imaging with high spatial resolution. In these measurements the photoluminescence (PL) response of LiF crystal and film was compared over a wide range of soft x-ray fluences. It was found that the soft x-ray fluence dependences of LiF crystal and film differ.

Assessment of radiation damage in single-shot coherent diffraction of DNA molecules by an extreme-ultraviolet free-electron laser.

We have investigated the progress of structural distortions in DNA molecules by single-shot coherent diffraction using extreme-ultraviolet radiation from a free-electron laser. A speckle pattern of DNA molecules was successfully acquired using photons in a single pulse with a 100 fs pulse width. The radiation damage was assessed by a cross correlation, revealing that the first exposure has significantly deformed most of the original structures.

Demonstration of up-conversion fluorescence from Ar clusters in intense free-electron-laser fields.

Extreme ultraviolet (EUV) fluorescence emitted from Ar clusters irradiated by intense EUV free electron laser (FEL) pulses has been investigated. The EUV fluorescence spectra display rich structure at wavelengths shorter than the incident FEL wavelength of 51 nm. The results suggest that multiply-charged ions are produced following the ion-electron recombination processes which occur in the nanoplasma created by multi-photon excitation during the intense EUV-FEL pulses.

Investigating the interaction of x-ray free electron laser radiation with grating structure.

The interaction of free electron laser pulses with grating structure is investigated using 4.6±0.1 nm radiation at the FLASH facility in Hamburg.

Optical features of a LiF crystal soft X-ray imaging detector irradiated by free electron laser pulses.

Optical features of point defects photoluminescence in LiF crystals, irradiated by soft X-ray pulses of the Free Electron Laser with wavelengths of 17.2 - 61.5 nm, were measured.

Enhanced nonlinear double excitation of He in intense extreme ultraviolet laser fields.

Nonlinear, three-photon double excitation of He in intense extreme ultraviolet free-electron laser fields (∼24.1  eV, ∼5  TW/cm2) is presented. Resonances to the doubly excited states converging to the He+ N=3 level are revealed by the shot-by-shot photoelectron spectroscopy and identified by theoretical calculations based on the time-dependent Schrödinger equation for the two-electron atom under a laser field.

Observation of free-electron-laser-induced collective spontaneous emission (superfluorescence).

We have observed and characterized 501.6 nm collective spontaneous emission (superfluorescence) following 1s(2) → 1s3p excitation of helium atoms by 53.7 nm free-electron laser radiation.

Second-order autocorrelation of XUV FEL pulses via time resolved two-photon single ionization of He.

Second-order autocorrelation spectra of XUV free-electron laser pulses from the Spring-8 Compact SASE Source (SCSS) have been recorded by time and momentum resolved detection of two-photon single ionization of He at 20.45 eV using a split-mirror delay-stage in combination with high-resolution recoil-ion momentum spectroscopy (COLTRIMS). From the autocorrelation trace we extract a coherence time of 8 ± 2 fs and a mean pulse duration of 28 ± 5 fs, much shorter than estimations based on electron bunch-length measurements.

Wavefront analysis of nonlinear self-amplified spontaneous-emission free-electron laser harmonics in the single-shot regime.

The single-shot spatial characteristics of the vacuum ultraviolet self-amplified spontaneous emission of a free electron laser (FEL) is measured at different stages of amplification up to saturation with a Hartmann wavefront sensor. We show that the fundamental radiation at 61.5 nm tends towards a single-mode behavior as getting closer to saturation.

Extreme ultraviolet free electron laser seeded with high-order harmonic of Ti:sapphire laser.

The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.

Multiphoton double ionization of Ar in intense extreme ultraviolet laser fields studied by shot-by-shot photoelectron spectroscopy.

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field (hν ∼ 21  eV, ∼ 5  TW/cm²), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of Ar(+): Ar+hν→Ar (+) + e⁻ and Ar²(+) + 2hν→Ar(+) + e⁻. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from Ar(+) is significantly enhanced by intermediate resonances in Ar(+).

Ion-ion coincidence studies on multiple ionizations of N(2) and O(2) molecules irradiated by extreme ultraviolet free-electron laser pulses.

We have investigated multiple ionization of N(2) and O(2) molecules by 52 nm extreme-ultraviolet light pulses at the free-electron laser facility SCSS in Japan. Coulomb break-up of parent ions with charge states up to 5+ is found by the ion-ion coincidence technique. The charge-state dependence of kinetic energy release distributions suggests that the electrons are emitted sequentially in competition with the elongation of the bond length.

The extreme ultraviolet split and femtosecond delay unit at the plane grating monochromator beamline PG2 at FLASH.

An extreme ultraviolet split and femtosecond delay unit based on grazing incidence Mach-Zehnder geometry has been designed and implemented on the plane grating monochromator beamline PG2 at FLASH, the Free Electron Laser at DESY. This device splits the FLASH radiation into two beams, which can independently be steered, filtered and temporally delayed between -5.1 and +5.

Note: Measurement of saturable absorption by intense vacuum ultraviolet free electron laser using fluorescent material.

Advances in free electron lasers (FELs) which generate high energy photons are expected to open novel nonlinear optics in the x-ray and vacuum ultraviolet (VUV) regions. In this paper, we report a new method for performing VUV-FEL focusing experiments. A VUV-FEL was focused with Kirkpatrick-Baez optics on a multilayer target, which contains fused silica as a fluorescent material.

Response-time improved hydrothermal-method-grown ZnO scintillator for soft x-ray free-electron laser timing-observation.

For pump and probe experiments in x-ray free-electron laser (XFEL) facilities, accurate timing synchronization between short-wavelength femtosecond pulses from XFELs and short optical pulses from other light sources is required. For this purpose, the response time of a hydrothermal-method-grown ZnO is improved by over one order of magnitude via intentional iron ion doping. The fluorescence rise- and decay-time constants are measured to be less than 10 and 100 ps, respectively.

Dynamics of electron-phonon scattering: crystal- and angular-momentum transfer probed by resonant inelastic x-ray scattering.

Experimentally, we observe angular-momentum transfer in electron-phonon scattering, although it is commonly agreed that phonons transfer mostly linear momentum. Therefore, the incorporation of angular momentum to describe phonons is necessary already for simple semiconductors and bears significant implications for the formation of new quasiparticles in correlated functional materials. Separation of linear and angular-momentum transfer in electron-phonon scattering is achieved by highly selective excitations on the femtosecond time scale of resonant inelastic x-ray scattering.

Longitudinal coherence measurements of an extreme-ultraviolet free-electron laser.

We have measured the average single-pulse longitudinal coherence characteristics of FLASH, a self amplified spontaneous emission free electron laser, at extreme UV wavelengths. Electric field autocorrelation measurements in the time domain were enabled by a wavefront division beam splitter applied to a tunable delay Mach-Zehnder interferometer. These data agree with the spectral bandwidth measurements made in the frequency domain.

Ultra-fast switching of light by absorption saturation in vacuum ultra-violet region.

Advances in free electron lasers producing high energy photons [Nat. Photonics 2(9), 555-559 (2008)] are expected to open up a new science of nonlinear optics of high energy photons. Specifically, lasers of photon energy higher than the plasma frequency of a metal can show new interaction features because they can penetrate deeply into metals without strong reflection.

Spin-orbit mediated interference in the radiative and nonradiative channels of the La 4d core resonances.

Symmetrical fluorescence yield profiles and asymmetrical electron yield profiles of the preresonances at the La N_{IV,V} x-ray absorption edge are experimentally observed in LaPO_{4} nanoparticles. Theoretical studies show that they are caused by interference effects. The spin-orbit interaction and the giant resonance produce symmetry entangled intermediate states that activate coherent scattering and alter the spectral distribution of the oscillator strength.

Vacuum-compatible pulse selector for free-electron laser.

We developed a vacuum-compatible pulse selector for a free-electron laser. A rotating cylinder with eight apertures in a vacuum is driven by a closed-loop stepping motor system through a magnetically coupled rotary feedthrough. A field programmable gate array is used to synchronize the cylinder rotation with a trigger signal of the accelerator at a maximum repetition rate of 60 Hz.

Cold-target recoil-ion momentum spectroscopy for diagnostics of high harmonics of the extreme-ultraviolet free-electron laser light source at SPring-8.

We have developed a cold-target recoil-ion momentum spectroscopy apparatus dedicated to the experiments using the extreme-ultraviolet light pulses at the free-electron laser facility, SPring-8 Compact SASE Source test accelerator, in Japan and used it to measure spatial distributions of fundamental, second, and third harmonics at the end station.

Geometric and electronic structure of lanthanide orthophosphate nanoparticles determined with X-rays.

The evolution of the geometric and electronic structures within the entire series of lanthanide orthophosphate nanoparticles ( approximately 2- approximately 5 nm) has been determined experimentally with X-ray diffraction and near edge X-ray absorption fine structure spectroscopy. In particular, the interplay between electronic structure, crystal morphology, and crystal phase has been systematically studied. A missing local order in the crystal structure accompanied by multiple ion sites in the nanoparticles was revealed to be due to the small crystal size and large surface contribution.

Probing the electron delocalization in liquid water and ice at attosecond time scales.

We determine electron delocalization rates in liquid water and ice using core-hole decay spectroscopy. The hydrogen-bonded network delocalizes the electrons in less than 500 as. Broken or weak hydrogen bonds--in the liquid or at the surface of ice--provide states where the electron remains localized longer than 20 fs.