X-ray free-electron laser (XFEL) facilities have been excellent light sources to explore ultrafast phenomena in the fields of physics, chemistry, biology and material science by supplying intense femtosecond X-ray pulses with a few tens of gigawatt power. However, shorter pulse lengths have been desired to trace the electron dynamics in atomic and molecular systems, theoretical and experimental studies also have been devoted to how to generate attosecond X-ray pulses from XFEL facilities. In this talk, I will discuss the following issues to generate isolated terawatt attosecond pulses from a free-electron laser with simulation results: (1) the generation of the current spike in the electron beam with optimal width and peak current; (2) the amplification of a photon pulse to the terawatt-level in the attosecond time domain with variable-spaced current spikes; (3) the enhancement of the signal-to-noise ratio of the attosecond pulse with optimized undulator layout. Also, the ongoing project to demonstrate the generation of attosecond pulse at Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) will be briefly introduced.
This paper presents an experimental method to realize the best high-order harmonics generation (HHG) phase matching in the interaction of strong optical field with gas target. By studying the effects of the relative location between gas target source and the Gaussian-shaped driving femtosecond laser field focus on the harmonics yield, conclusions are obtained that the optimum position of gas target for phase matching is always behind the of the focal point of the driving field, with much lower HHG yield before the focus caused by serious harmonics phase mismatch. Meanwhile, with optimum harmonics phase matching, the high-order harmonics field that resulted has the similar Gaussian-shaped spatial distribution characteristics with the driving field, verifying experimentally the commonly used assumptions for attosecond laser pulse based on HHG. This optimization method is also suitable both for other driving field with different spatial distribution of light intensity and other type of target source. The results here have important guiding significance for high harmonic generation and high harmonic isolated attosecond pulse technology.
High-repetition-rate, femtosecond x-ray lasers would be useful for dynamical study of ultra-fast phenomena in nature. One of routes to get fs x-ray lasers is to use inner-shell processes in atomic and ionic systems. In this paper, the two inner-shell schemes recently proposed will be reviewed and compared in detail. One of important issues using inner-shell schemes is fast and intense x-ray pumping source. One of good candidate sources for that purpose is Larmor radiation produced by electrons under an intense fs laser field. The relativistic, nonlinear Thomson scattering by an electron of an intense laser field is investigated in computer simulation. Under a laser field with a pulse duration of 20 fs Full Width Half Maximum and an intensity of 1020 W/cm2, the motion of an electron is highly relativistic and generates an ultra-short radiation of 2 attoseconds with photon energies of 100 to 600 eV. An interesting modulated structure of the spectrum is observed and analyzed. A radiation produced by the zigzag motion of an electron under a linearly polarized laser has better characteristics than by a helical motion under a circularly polarized laser pulse in terms of an angular divergence and an energy spectrum.
The manipulation of magnetic anisotropy in a Co/Pt nano- multilayer(nano-ML) system with particles being embedded is reported. The samples, fabricated by a newly-developed normal incidence pulsed laser deposition (NIPLD) method, have salient magnetic properties, different from particle- free samples of almost the same structure: (1) they exhibit bi-axial magnetic anisotropies and (2) there exists a critical field at which the change in easy direction from a parallel direction to a perpendicular direction and vice versa. The careful manipulation of particles and nano- layers has also allowed us to control the degree of magnetic anisotropy by embedding particles in a well-defined nano- multilayer system: uni-axial anisotropy to bi-axial one and vice versa. This work indeed clearly shows that the integration of nano-building blocks into nano-structures can tailor properties of nano-materials.
The development of keV X-ray lasers based on inner-shell atomic transitions requires extremely fast energy deposition on a target in order to effectively compete with the inherently fast (0.1 - 20 fs) atomic decay processes. The duration of ultrahigh peak power laser systems is now reaching this timescale. In principle, these systems can be used to produce sufficient duration and energy X-rays or electrons for pumping inner-shell transitions. In this paper X-ray laser schemes are described in which the Coster-Kronig Auger-process is the dominant lower level decay mechanism. Such systems have inherently short lower level lifetimes and it is shown that under certain conditions they can be inverted with excitation by energetic electrons as well as X-rays. They are therefore relatively immune to secondary electron ionization.
Molybdenum-silicon multilayer soft x-ray mirrors have been fabricated using a magnetron sputtering system. Their structures have been characterized by x-ray diffraction (XRD) and reflectivities at normal incidence have been measured by using monochromatized synchrotron radiation in the 18-24 nm region. A normal incidence reflectivity as high as 40% at 20.8 nm was achieved.
Molibdenum-silicon multilayer structure as a soft x-ray mirror fabricated using a magnetron sputtering system has been studied through x-ray diffraction (XRD) of Cu K(alpha ) (1.54 angstrom) radiation. The angular positions, the relative intensities, the widths of Bragg primary and secondary peaks in XRD pattern are sensitive to structural parameters such as unit thickness (bi-layer thickness), thickness ratio of composite materials, diffusion lengths, and roughness. Effects of each structural parameter on XRD pattern have been investigated. The results show that the angular positions of Bragg primary peaks are sensitive to the bi-layer thickness, the pattern of secondary peaks is sensitive to the thickness ratio. It is also found that the major effect of interfacial diffusion is the reduction of the intensities of high-order Bragg primary peaks, and roughness is responsible for the broadening of the widths of Bragg primary peaks and the increase of the intensities of Bragg secondary maxima. Using the above results, we have analyzed experimental XRD data to draw the structural parameters of Molibdenum-silicon multilayer structure.
An overview of the X-ray Laser project at Princeton University will be given. Emphasis will be on improvements being made to the small scale soft x-ray laser (SXL). New target designs to enhance cooling and others to reduce losses due to beam refraction have been introduced though results are stilt preliminary. Proof of principle experiments for the application of the SXL to both transmission and reflection microscopy have been performed. To generate shorter wavelength x-ray lasers on a reasonable scale-size, a high power, 300 fsec pulse duration, ultraviolet KrF laser system (the PSP-laser) has been developed. Of the several theoretical schemes which exist, the two-laser approach and a newly proposed recombination approach will be described. The new approach proposes to scale the existing 18.2 nm recombination x-ray laser to shorter wavelength (<4 nm) by making use of the short-pulse pump laser and rapid cooling associated with the adiabatic expansion of µsphere targets.
A program aimed at the development of a table-top soft X-ray laser on the CVI 18.22 nm transition is reported. A gain-length product of GL = 2.5-3.5 has been observed for a variety of target geometries using a 3-ns 6-15 J Nd:glass driving laser emitting at 1.05 micron. Plans for future experiments are outlined.
In the 1980's many exciting advances in the field of soft x-ray lasers took place. A novel
experimental set-up was developed at Princeton to improve the performance and extend the
wavelength range of existing soft x-ray lasers. In th1s paper a new target manipulation system
capable of remotely aligning multiple targets in a vacuum chamber within a magnetic field is
described Alignment to the magnetic field orientation, incident pump laser and the two axial
diagnostics: a high resolution spectrometer equipped with a multichannel array and a time
resolved imaging spectrometer is presented. We include a brief explanation of the Macintosh-
Camac based data acquisition system. In addition we discuss the possibifity to increase in the
near future the brightness of the x-ray laser beam by several orders of magnitude by developing a
cavity with multilayer mirrors.
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