Sub-picosecond X-ray Pulses Research Articles

Ultrafast time-resolved structural changes of thin-film ferromagnetic metal heated with femtosecond optical pulses

As a classic ferromagnetic material, nickel has been an important research candidate used to study dynamics and interactions of electron, spin, and lattice degrees of freedom. In this study, we specifically chose a thick, 150 nm ferromagnetic nickel (111) single crystal rather than 10-20 nm thin crystals that are typically used in ultrafast studies, and we revealed both the ultrafast heating within the skin depth and the heat transfer from the surface (skin) layer to the bulk of the crystal. The lattice deformation after femtosecond laser excitation was investigated by means of 8.04 keV subpicosecond x-ray pulses, generated from a table-top laser-plasma based source. The temperature evolution of the electron, spin, and lattice was determined using a three temperature model. In addition to coherent phonon oscillations, the blast force and sonic waves, induced by the hot electron temperature gradient, were also observed by monitoring the lattice contractions during the first couple of picoseconds after laser irradiation. This study further revealed the tens of picoseconds time required for heating the hundred nanometer bulk of the Ni (111) single crystals.

Short X-ray pulses from third-generation light sources

High-brightness X-ray radiation produced by third-generation synchrotron light sources (TGLS) has been used for numerous time-resolved investigations in many different scientific fields. The typical time duration of X-ray pulses delivered by these large-scale machines is about 50-100 ps. A growing number of time-resolved studies would benefit from X-ray pulses with two or three orders of magnitude shorter duration. Here, techniques explored in the past for shorter X-ray pulse emission at TGLS are reviewed and the perspective towards the realisation of picosecond and sub-picosecond X-ray pulses are discussed.

Ultrafast structural dynamics studied by kilohertz time-resolved x-ray diffraction**Project supported by the National Natural Science Foundation of China (Grant Nos. 61222509 and 11421064) and the W. M. Keck Foundation.

Ultrashort multi-keV x-ray pulses are generated by electron plasma produced by the irradiation of femtosecond pulses on metals. These sub-picosecond x-ray pulses have extended the field of x-ray spectroscopy into the femtosecond time domain. However, pulse-to-pulse instability and long data acquisition time restrict the application of ultrashort x-ray systems operating at low repetition rates. Here we report on the performance of a femtosecond laser plasma-induced hard x-ray source that operates at 1-kHz repetition rate, and provides a flux of 2.0 × 1010 photons/s of Cu Kα radiation. Using this system for time-resolved x-ray diffraction experiments, we record in real time, the transient processes and structural changes induced by the interaction of 400-nm femtosecond pulse with the surface of a 200-nm thick Au (111) single crystal.

A new possibility for production of sub-picosecond x-ray pulses using a time dependent radio frequency orbit deflection

It is shown that two radio frequency deflecting cavities with slightly different frequencies can be used to produce time-dependent orbit deflection to a few special electron bunches circulating in a synchrotron without affecting the majority of the electron bunches. These special bunches produce an x-ray pulse in which transverse position or angle, or both, are correlated with time. The x-ray pulse is then shortened, either with an asymmetrically cut crystal that acts as a pulse compressor, or with an angular aperture such as a narrow slit positioned downstream. The implementation of this technique creates a highly flexible environment for synchrotrons in which users of most beamlines will be able to easily select between the x-rays originated by the standard electron bunches and the short x-ray pulses originated by the special electron bunches carrying a time-dependent transverse correlation.

LOM and HOM damping study in a superconducting deflecting cavity for ALS at LBNL**Supported by NSFC (10775080)

Superconducting deflecting cavities can be used in synchrotron light source to generate subpicosecond X-ray pulses while the impedance of the lower order modes (LOM) and higher order modes (HOM) in the cavity should be kept below an accepted level to avoid beam instability. These modes can be damped by adding waveguide on beam pipe. Detailed simulation of Q in CST Microwave Studio is introduced and experiment results on an aluminum model cavity with damping waveguide are reported to make a comparison.

Femtosecond time-resolved powder diffraction experiments using hard X-ray free-electron lasers

In the next decade the scientific community expects a strong impact in physics, chemistry, biology, material research and life sciences by the availability of high-brilliance X-ray radiation from free-electron laser (FEL) sources. In particular, in the field of ultrafast science these new sources will allow new types of experiments, enabling new phenomena to be discovered. Whereas today ultrafast X-ray diffraction experiments are strongly restricted by the limited X-ray flux of current sources of sub-picosecond X-ray pulses, FELs will provide short pulses of typically 10(12) photons with a duration of the order of 100 fs and monochromaticity of 10(-3). Here, the feasibility of time-resolved single-shot powder diffraction experiments using these intense pulses, and the requirements of these experiments, are discussed. The detector count rates are estimated for diffraction from a model compound in a wide q-regime under the special consideration of high resolving power. In the case of LCLS radiation parameters, single-shot experiments will be feasible although high-resolution powder diffraction will require a reduction of the intrinsic FEL radiation bandwidth.

Evaluation and simulations of a Thomson scattering X-ray source based on ray tracing methods

We propose to build up a facility of sub-picosecond hard X-ray pulses based on Thomson scattering between femtosecond laser pulses and relativistic electrons which is a useful tool for the purpose of material investigation, plasma diagnostics, and shock wave measurement. This article reviews the principles and the development of X-ray sources based on Thomson scattering. ThenNew Light Source®, the Thomson scattering X-ray facility we will develop is introduced. The characteristics of a Thomson scattering X-ray source are analyzed. A computer model of the Thomson source to be developed is described in order to provide a source of the rays used in a ray tracing method, which has proved to be an essential computer tool for designing and optimizing the optical system of high brightness X-ray facilities. A code for the ray tracing source model is created based on the Monte Carlo approach. It is able to evaluate the properties and performances of the light source under development using this model. According to the simulation results, we discuss the dependence of imaging quality and source properties including spectral distribution, emittance, flux which depends on the laser and electron beam parameters, in order to check if operation performances are as expected. We also estimate the possibility of measuring the energy spectrum of a Thomson scattering source by using a crystal diffraction method. Ray tracing calculations are performed using SHADOW program package, and a new model of Thomson scattering X-ray source which can be processed in that program is established with additional code.

X-ray diffraction experiments with femtosecond time resolution

Intense ultrashort laser pulses enable the generation of subpico-second X-ray pulses in the multi-kilovolt range of photon energies. These X-ray pulses have opened the door to ultrafast time-resolved X-ray spectroscopy. Here, we report on the use of time-resolved X-ray diffraction to study ultrafast laser-induced order-to-disorder transitions and large amplitude picosecond acoustic transients that accompany the phase transitions.

Review of some selected experiments on laser electron–beam interaction

The third-generation 1.5 GeV electron storage ring DELTA, dedicated to synchrotron-radiation experiments, accelerator technology and UV FEL operation, was commissioned about 2 years ago at the Dortmund University. After the first successful lasing in the visible region (470 nm) beginning of this year, it is planned to obtain lasing also in the UV near 200 nm and then later to use the high average intracavity power and natural synchronization of electron and optical pulses to produce intense beams of high-energy γ-rays by Compton backscattering. Applying these γ-rays, a photo-fission experiment of importance for nuclear astrophysics is described. Since the time resolution of high-brightness synchrotron light sources is limited to more than 10 ps, it is envisioned to test a novel technique for generating sub-picosecond X-ray pulses based on the interaction of the electrons with a co-propagating beam of femtosecond photon pulses from an external TW-laser. This technique will be applied at DELTA by using its superconducting wiggler and a permanent-magnet undulator. Simulations show that a brilliance of 10 8 photons/(s mm 2 mrad 2 0.1% bandwidth) with pulse durations of the order of 100 fs can be achieved. Using the magnetic insertions of DELTA and an external TW-laser will further allow measurements for testing laser acceleration in the ultra-high vacuum of a storage ring and of radiation damping of the beam emittance on a fast time scale. First, results of laser-beam monitoring at the stretcher ring AmPS to determine the transverse beam emittance are presented and are intended to be performed also at DELTA. For these investigations the electron beam has been scanned at 90° with a high-power laser beam, and forward-scattered Compton photons were detected to derive the beam profile.

The phonon Bragg switch: a proposal to generate sub-picosecond X-ray pulses

We discuss a method to modulate an X-ray beam that relies on the scattering of hard X-ray photons by a superlattice of optical phonons generated by ultrafast laser pulses incident on a Bragg crystal. Using commercially available laser systems and well-developed techniques for pulse shaping, the crystal lattice motion can be controlled so that efficient Bragg diffraction is switched on and off in a time comparable to a phonon oscillation period (typically, 0.1–1 ps). A detailed proposal is given for the case of GaAs.

FEL simulations for the LCLS

A first design study report has recently been completed (The LCLS Design Study Group, LCLS Design Study Report, April 1998, SLAC-R-521) for the linac coherent light source (LCLS), a proposal to build an X-ray free electron laser (FEL) at the Stanford Linear Accelerator Center (SLAC) as a single pass self-amplified spontaneous emission (SASE) amplifier. The proposal includes the use of a very low emittance electron beam accelerated up to 15 GeV by the last third of the SLAC linac to produce sub-picosecond X-ray pulses with high brightness and full transverse coherence in a 112-m long undulator. Many aspects of the FEL design have been analyzed with FEL simulation codes. The paper discusses some of the results of these aspects, i.e. temporal X-ray pulse structure and power spectrum, trajectory errors and effects of undulator beam tube wakefields.

Generation of subpicosecond X-ray pulses using RF orbit deflection

A technique is proposed for producing high average intensity X-ray radiation from a storage ring for studies of the ultrafast phenomena on a subpicosecond time scale. Two RF cavity accelerating structures excited in the E 110 mode can be installed in a storage ring to create vertical displacements of electrons correlated with their longitudinal position in the bunch. The magnitude of these displacements can be sufficient for the X-ray radiation of the electron bunch between accelerating structures to be viewed as produced by a large number of independent sources, each of a subpicosecond duration.

Monochromatic focusing of subpicosecond x-ray pulses in the keV range

An effective x-ray optical method to focus keV x-ray pulses shorter than one picosecond by using spherically or toroidally bent crystals is presented. The spectral, spatial, and time-dependent properties of focusing by two-dimensional bent crystals are calculated by considering geometrical effects, physical limitation in high performance crystal optics, and reflectivities obtained by x-ray diffraction theory. These properties are compared with first experimental results of focusing x rays from a plasma created by a laser pulse with 4.5 mJ energy and 100 fs pulse length. The x-ray signals, simultaneously obtained from a von Hámos spectrometer and two-dimensional bent crystals are compared and found in good agreement with theoretical data. The possibilities and aspects of laser pump x-ray probe experiments using this type of x-ray optics system and currently available laser systems are discussed.

Pulse broadening in femtosecond x-ray diffraction

Advances in femtosecond laser technology make feasible the generation of femtosecond x-ray pulses. An analysis of Bragg diffraction of subpicosecond x-ray pulses is presented. It is shown that the three-dimensional nature of the diffracting crystal grating and the finite size of the x-ray beam will lead to a considerable broadening of the diffracted femtosecond x-ray pulse.

Subpicosecond X-ray Pulses

Short pulses of x-rays with a time duration comparable to the dynamics of molecules would make a variety of new structural studies possible. In their Perspective, Eisenberger and Suckewer discuss results reported by Schoenlein et al . ( p. 236 ) in the same issue on a new source of subpicosecond x-rays. A short-pulse infrared laser beam is scattered from a beam of high-energy electrons. The radiation scattered from the electrons is in the form of subpicosecond pulses at a wavelength of 0.4 angstrom.

Feasibility of generation of picosecond and subpicosecond x-ray pulses in thin films

The characteristics of a femtosecond laser plasma, formed by irradiation of a thin freely suspended carbon film, are investigated numerically. It is shown that the use of thin films can increase considerably the electron temperature of a femtosecond laser plasma and make it possible to generate x-rays of shorter wavelengths. This method can also be used to increase the efficiency of conversion of the energy of laser pulses into the radiation emitted by hydrogen-like carbon ions without a significant increase in the duration of x-ray pulses.

Picosecond X-ray pulses generated in a diode driven by 193-nm picosecond laser pulses

A new technique for the generation of picosecond x-ray pulses is presented. Picosecond laser pulses are used to generate ultrashort electron pulses in a conventional x-ray diode in which the cathode is replaced by a photocathode. In the experiments presented, 9-ps UV pulses are employed to produce characteristic x-ray pulses with duration in range of 20–90 ps at a repetition rate of 300 Hz. This technique is inherently capable of subpicosecond x-ray pulses and synchronization of the x-ray pulses with the laser pulses or other ultrafast devices with picosecond accuracy.

Picosecond X-Ray Pulses Generated in a Diode Driven by 193-nm Picosecond Laser Pulses

A new technique for the generation of picosecond x-ray pulses is presented. Picosecond laser pulses are used to generate ultrashort electron pulses in a conventional x-ray diode in which the cathode is replaced by a photocathode. In the experiments presented, 9-ps UV pulses are employed to produce characteristic x-ray pulses with duration in range of 20-90 ps at a repetition rate of 300 Hz. This technique is inherently capable of subpicosecond x-ray pulses and synchronization of the x-ray pulses with the laser pulses or other ultrafast devices with picosecond accuracy.

Generation of efficient ultrafast laser-plasma x-ray sources

When a subpicosecond laser is focused onto the surface of a solid, a high-density, short-lived plasma is created. Experimental observations demonstrate that subpicosecond laser-produced plasmas can generate subpicosecond x-ray pulses under the right conditions. The plasma x-ray emission extends over a broad spectral bandwidth and arises mainly from continuum emissions in the high-density plasma. Such a plasma source can be very efficient because almost 90% of the excitation laser energy can be coupled into the solid target and used to heat the target electrons to high temperature.

Generation of subpicosecond x-ray pulses in a hot solid-state plasma

Numerical modeling methods were used to investigate the radiation emitted by a plasma generated by high-power femtosecond laser pulses on the surface of a solid-state target. The results demonstrated that strong transfer of heat to the target should generate subpicosecond x-ray radiation. Calculations are reported of the time, spectral, and energy characteristics of such x-ray pulses.