Free-electron Laser Pulses Research Articles

Observation of Free Induction Signals of Radicals Excited by Terahertz Free-Electron Laser Pulses

Observation of Free Induction Signals of Radicals Excited by Terahertz Free-Electron Laser Pulses

Observation of Free Induction Decay Signals of OH Radicals Excited by Terahertz Free-Electron Laser Pulses

The first experimental observation of free induction decay signals of free radicals is reported. The signals were observed on the line of the rotational transition of OH radicals in the terahertz region. OH radicals were generated by the chemical reaction of excited oxygen atoms with water molecules. Radicals were excited by free-electron laser radiation pulses. Free induction decay radiation was detected in real time using ultra fast terahertz radiation detectors. The possibility of using free induction decay signals of radicals as a new method for detection of free radicals in combustion processes is discussed.

Nonlinear plasmonic response of doped nanowires observed by infrared nanospectroscopy

We report a strong shift of the plasma resonance in highly-doped GaAs/InGaAs core/shell nanowires (NWs) for intense infrared excitation observed by scattering-type scanning near-field infrared microscopy. The studied NWs show a sharp plasma resonance at a photon energy of about 125 meV in the case of continuous wave excitation by a CO2 laser. Probing the same NWs with the pulsed free-electron laser with peak electric field strengths up to several 10 kV cm−1 reveals a power-dependent redshift to about 95 meV and broadening of the plasmonic resonance. We assign this effect to a substantial heating of the electrons in the conduction band and subsequent increase of the effective mass in the nonparabolic Γ-valley.

Superfluorescence, Free-Induction Decay, and Four-Wave Mixing: Propagation of Free-Electron Laser Pulses through a Dense Sample of Helium Ions.

We report an experimental and numerical study of the propagation of free-electron laser pulses (wavelength 24.3nm) through helium gas. Ionization and excitation populates the He^{+} 4p state. Strong, directional emission was observed at wavelengths of 469, 164, 30.4, and 25.6nm. We interpret the emissions at 469 and 164nm as 4p-3s-2p cascade superfluorescence, that at 30.4nm as yoked superfluorescence on the 2p-1s transition, and that at 25.6nm as free-induction decay of the 3p state.

Generating attosecond x-ray pulses through an angular dispersion enhanced self-amplified spontaneous emission free electron laser

We propose a scheme that can generate attosecond x-ray pulses through an angular dispersion enhanced self-amplified spontaneous emission (ESASE) free electron laser (FEL). In this scheme, an angular dispersion section is implemented into a conventional ESASE set-up, together with a few cycle laser pulse to manipulate the electron beam effectively. During the procedure, the transverse-longitudinal phase space coupling is introduced into the electron beam in addition to the energy modulation and momentum compaction processes. As a result, the peak current of the electron beam will be strengthened significantly within a narrow width of the electron bunch, which can be used to generate ultrashort FEL pulses. Comparing to a conventional ESASE scheme, the proposed scheme can reduce the required modulation laser power by at least one order of magnitude. The signal-to-noise ratio of the final FEL radiation pulses can also be improved. In addition, we discuss the possibility of using this scheme to generate two-pulse two-color x-ray FEL with tunable central wavelengths and time delays for a conceivable x-ray pump/x-ray probe experiment.

Probing space-time distortion via the interaction of multi-PW class laser pulses with underdense plasmas

Petawatt class femtosecond lasers and x-ray free electron lasers (XFEL) open up a new page in research fields related to space and vacuum physics. One of fundamental principles can be explored by these new instruments is the equivalence principle, saying that gravitation and acceleration should be treated equivalently. If it is true this must lead to the appearance of the Unruh effect analogous to Hawking’s black hole radiation. It says that a detector moving with a constant acceleration w sees a boson’s thermal bath with its temperature Tw=ℏw/2πkBc. Practical detection of the Unruh effect requires extremely strong acceleration and fast probing sources. Here we demonstrate that x-rays scattering from highly accelerated electrons can be used for such detection. We present two, feasible for the Unruh effect, designs for highly accelerated systems produced in underdense plasma irradiated by high intensity lasers pulses. The first is Thomson scattering of a XFEL pulse from plasma wave excited by an intense laser pulse. Properly chosen observation angles enable us to distinguish the Unruh effect from the normal Doppler shift with a reasonable photon number. The second is a system consisting of electron bunches accelerated by a laser wake-field, as sources and as detectors, which move in a constantly accelerating reference frame (Rindler space) and are probed by x-ray free electron laser pulses. The numbers of photons is shown to be enough to observe reproducible results.

Photoelectron spectroscopy of Rydberg excited states in singly charged molecular ions CS2+ by NIR laser pulses

We performed photoelectron spectroscopy using femtosecond extreme-ultraviolet (XUV) pulses from a free-electron laser and femtosecond near-infrared (NIR) pulses from a synchronized laser, and succeeded in measuring photoelectrons from highly excited Rydberg states of molecular ions Such excited states are prepared via a sequential process of photoionization and subsequent photoexcitation during a short pulse of XUV, and then a band of the Rydberg excited states is probed with a synchronized NIR pulse. The photoelectron spectrum has been well explained by our hydrogenic Rydberg orbital model. The present result demonstrates that the two-color experiments of XUV free-electron lasers and NIR laser makes it possible to observe highly excited states of molecular ions. This implies that the present approach provides ones a new spectroscopic tool for studying excited states of various molecular ions.

An Experimental Protocol for Femtosecond NIR/UV - XUV Pump-Probe Experiments with Free-Electron Lasers

This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a free-electron laser. This includes methods to establish the spatial and temporal overlap between the optical and free-electron laser pulses during the experiment, as well as important aspects of the data analysis, such as corrections for arrival time jitter, which are necessary to obtain high-quality pump-probe data sets with the best possible temporal resolution. These methods are demonstrated for an exemplary experiment performed at the FLASH (Free-electron LASer Hamburg) free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques.

An Experimental Protocol for Femtosecond NIR/UV - XUV Pump-Probe Experiments with Free-Electron Lasers.

This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a free-electron laser. This includes methods to establish the spatial and temporal overlap between the optical and free-electron laser pulses during the experiment, as well as important aspects of the data analysis, such as corrections for arrival time jitter, which are necessary to obtain high-quality pump-probe data sets with the best possible temporal resolution. These methods are demonstrated for an exemplary experiment performed at the FLASH (Free-electron LASer Hamburg) free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques.

Damage accumulation in thin ruthenium films induced by repetitive exposure to femtosecond XUV pulses below the single-shot ablation threshold

The process of damage accumulation in thin ruthenium films exposed to multiple femtosecond extreme ultraviolet (XUV) free-electron laser (FEL) pulses below the critical angle of reflectance at the FEL facility in Hamburg (FLASH) was experimentally analyzed. The multi-shot damage threshold is found to be lower than the single-shot damage threshold. Detailed analysis of the damage morphology and its dependence on irradiation conditions justifies the assumption that cavitation induced by the FEL pulse is the prime mechanism responsible for multi-shot damage in optical coatings.

Probing electron dynamics in the double photoionization process of two-valence electron systems with extreme UV and soft X-ray free-electron laser pulses

We numerically investigate the removal of the two outer electrons in the X = He, Li+, C4+, Be, B+ and Ne6+ targets by the absorption of a single photon of energy in the extreme UV/soft X-ray spectral region. Our theoretical method to describe the behavior of the electrons in the presence of the laser field is based on a spectral approach of configuration interaction type to solve the time-dependent Schrödinger equation. The probe of the electron dynamics in the different systems shows that the way the outer electrons will leave their parent X system is systematically dictated by the amount of excess photon energy available to them relative to the ionization potential of the corresponding X+ ion.

Vacuum birefringence in the head-on collision of x-ray free-electron laser and optical high-intensity laser pulses

The focus of this article is on providing compact analytical expressions for the differential number of polarization flipped signal photons constituting the signal of vacuum birefringence in the head-on collision of x-ray free electron (XFEL) and optical high-intensity laser pulses. Our results allow for unprecedented insights into the scaling of the effect with the waists and pulse durations of both laser beams, the Rayleigh range of the high-intensity beam, as well as transverse and longitudinal offsets. They account for the decay of the differential number of signal photons in the far-field as a function of the azimuthal angle measured relative to the beam axis of the probe beam in forward direction, typically neglected by conventional approximations. Moreover, they even allow us to extract an analytical expression for the angular divergence of the perpendicularly polarized signal photons. We expect our formulas to be very useful for the planning and optimization of experimental scenarios aiming at the detection of vacuum birefringence in XFEL/high-intensity laser setups, such as the one put forward at the Helmholtz International Beamline for Extreme Fields (HIBEF) at the European XFEL.

Observation of Ultrafast Solid-Density Plasma Dynamics Using Femtosecond X-Ray Pulses from a Free-Electron Laser

The complex physics of the interaction between short pulse high intensity lasers and solids is so far hardly accessible by experiments. As a result of missing experimental capabilities to probe the complex electron dynamics and competing instabilities, this impedes the development of compact laser-based next generation secondary radiation sources, e.g. for tumor therapy [Bulanov2002,ledingham2007], laboratory-astrophysics [Remington1999,Bulanov2015], and fusion [Tabak2014]. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small angle X-ray scattering of femtosecond X-ray free-electron laser pulses facilitates new capabilities for direct in-situ characterization of intense short-pulse laser plasma interaction at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short pulse high intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the pre-inscribed grating, due to plasma expansion, which is an hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets.

Hard X-ray induced fast secondary electron cascading processes in solids

Recent studies confirmed that the materials used in the extreme UV and soft X-ray regime for precise characterization of intense free-electron laser pulses (e.g., Si3N4) do not work efficiently in the hard X-ray regime, which is due to the fact that the impact of a hard X-ray photon is followed by a series of electron cascading processes. Following theoretical indication, we show that this limitation can be circumvented and the cascading time can be significantly reduced if the X-ray photon energy is double the ionization energy. We investigate an alternative material for pulse diagnostics, SnO2, using the Linac Coherent Light Source at photon energies of 5 keV and 9 keV. We prove the validity of the concept and show that it has a large potential for practical applications. By applying the proposed criteria, the temporal accuracy of the non-invasive pulse diagnostic tools can be improved in current and emerging hard X-ray facilities.

Control of FEL radiation properties by tailoring the seed pulses

Seeded free-electron lasers (FELs) produce intense, ultrashort and fully coherent X-ray pulses. These seeded FEL pulses depend on the initial seed properties. Therefore, controlling the seed laser allows tailoring the FEL radiation for phase-sensitive experiments. In this contribution, we present detailed simulation studies to characterize the FEL process and to predict the operation performance of seeded pulses. In addition, we show experimental data on the temporal characterization of the seeded FEL pulses performed at the sFLASH experiment in Hamburg.

Overview of Undulator Concepts for Attosecond Single-Cycle Light

The production of intense attosecond light pulses is an active area in accelerator research, motivated by the stringent demands of attosecond science: (i) short pulse duration for resolving the fast dynamics of electrons in atoms and molecules; (ii) high photon flux for probing and controlling such dynamics with high precision. While the free-electron laser (FEL) can deliver the highest brilliance amongst laboratory x-ray sources today, the pulse duration is typically 10–100 femtoseconds. A major obstacle to attaining attosecond duration is that the number of optical cycles increases with every undulator period. Hence, an FEL pulse typically contains tens or hundreds of cycles. In recent years, several novel concepts have been proposed to shift this paradigm, providing the basis for single-cycle pulses and paving the way towards high-brilliance attosecond light sources. This article gives an overview of these concepts.

Femtosecond single-shot timing and direct observation of subpulse formation in an infrared free-electron laser

We experimentally demonstrate a single-shot arrival time monitor for short picosecond infrared free-electron laser (IR FEL) pulses based on balanced optical cross-correlation with a synchronized fs table-top laser. Employing this timing tool at the Fritz Haber Institute IR FEL, we observe a shot-to-shot timing jitter of only 100 fs and minute-scale timing drifts of a few picoseconds, the latter being strictly correlated with the electron beam energy of the accelerator. We acquire sum-frequency cross-correlation data with micropulse resolution, providing full access to the IR FEL pulse shape evolution within the macropulse. These measurements provide unprecedented insights into the occurrence of limit-cycle oscillations of the FEL intensity as a consequence of subpulse formation. Our experimental results are complemented by four-dimensional simulations of the nonlinear pulse dynamics in a low-gain FEL oscillator based on Maxwell-Lorentz theory.

Hydrogen resist lithography and electron beam lithography for fabricating silicon targets for studying donor orbital states

Recently, phosphorous structures in silicon have been of interest theoretically and experimentally due to their relevance in the field of quantum computing. Coherent control of the orbital states of shallow donors in silicon has been demonstrated in bulk doped samples. Here we discuss the fabrication techniques required to 1) obtain patterned two dimensional dilute sheets of impurities in silicon of controlled doping densities 2) get them to act as targets for a terahertz laser. Scanning tunnelling microscope hydrogen lithography enables patterning of impurity features in silicon with a resolution from 1nm to tens of nm. Molecular beam epitaxy is used for a protective thin-film crystalline silicon growth over the impurity sheet. Electron beam lithography coupled with reactive ion etching allows features from tens to hundreds of microns to be etched into the silicon with 10 to 20nm resolution. The experimental readout is achieved via illumination of the silicon target by terahertz light and subsequent electrical detection. The electrical signal comes from coherent and non-linear excitations of the impurity electrons. This detection technique enables the precision condensed matter samples to remain intact after exposure to the free electron laser pulse.

Mechanism of single-shot damage of Ru thin films irradiated by femtosecond extreme UV free-electron laser.

Ruthenium is a perspective material to be used for XUV mirrors at free-electron laser facilities. Yet, it is still poorly studied in the context of ultrafast laser-matter interaction. In this work, we present single-shot damage studies of thin Ru films irradiated by femtosecond XUV free-electron laser pulses at FLASH. Ex-situ analysis of the damaged spots, performed by different types of microscopy, shows that the weakest detected damage is surface roughening. For higher fluences we observe ablation of Ru. Combined simulations using Monte-Carlo code XCASCADE(3D) and the two-temperature model reveal that the damage mechanism is photomechanical spallation, similar to the case of irradiating the target with optical lasers. The analogy with the optical damage studies enables us to explain the observed damage morphologies.

Growth of Cuprous Oxide Particles in Liquid-Phase Synthesis Investigated by X-ray Laser Diffraction.

Cuprous oxide (Cu2O) particles obtained by surfactant-assisted liquid-phase synthesis have cuboid shapes but the internal structures are difficult to be visualized by electron microscopy. Herein, we investigated the internal structures of numerous individual Cu2O particles with submicrometer dimensions by X-ray diffraction imaging (XDI) using X-ray free-electron laser (XFEL) pulses. The reconstructed two-dimensional electron density maps, which displayed inhomogeneous internal structures, were divided into five classes characterized by the positions and shapes of high and low electron density areas. Further analysis of the maps in each class by a manifold learning algorithm revealed that the internal structures of Cu2O particles varied in correlation with total electron density while retaining the characteristics within each class. On the basis of the analyses, we proposed a growth mechanism to yield the inhomogeneity in the internal structures of Cu2O particles in surfactant-mediated liquid-phase synthesis.