This self-contained, comprehensive book describes the fundamental properties of soft x-rays and extreme ultraviolet (EUV) radiation and discusses their applications in a wide variety of fields, including EUV lithography for semiconductor chip manufacture and soft x-ray biomicroscopy. The author begins by presenting the relevant basic principles such as radiation and scattering, wave propagation, diffraction, and coherence. He then goes on to examine a broad range of phenomena and applications. The topics covered include EUV lithography, biomicroscopy, spectromicroscopy, EUV astronomy, synchrotron radiation, and soft x-ray lasers. He also provides a great deal of useful reference material such as electron binding energies, characteristic emission lines and photo-absorption cross-sections. The book will be of great interest to graduate students and researchers in engineering, physics, chemistry, and the life sciences. It will also appeal to practicing engineers involved in semiconductor fabrication and materials science.

X-Rays and Extreme Ultraviolet Radiation: Principles and Applications

Since the middle of the 1990s, X-ray phase imaging including phase tomography has been attracting increasing attention. The advantage of X-ray phase imaging is that an extremely high sensitivity is achieved for weak-absorbing materials, such as biological soft tissues, which generate a poor contrast by conventional methods. Medical and biological imaging is the main target of X-ray phase imaging, and several trials using synchrotron radiation sources and laboratory sources have been made. Measuring and controlling the X-ray phase are also significant for X-ray microscopy with a high spatial resolution, and innovative techniques are attracting intense interest. The progress of X-ray phase imaging is supported by the developments in X-ray sources such as third-generation synchrotron radiation sources, optical elements, and image detectors. This article describes the advantages of using X-ray phase information and reviews various techniques studied for X-ray phase imaging.

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Recent Advances in X-ray Phase Imaging

X-ray interferometry for imaging applications is discussed with a review of X-ray interferometric imaging activities reported to date. Phase measurement and phase tomography based on X-ray interferometry are also presented. Finally the advantage of X-ray interferometric imaging in comparison with other phase-sensitive X-ray imaging methods is discussed.

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Phase-sensitive imaging and phase tomography using X-ray interferometers

This article reviews the progress in the development of practical table-top sources of soft x-ray laser radiation. The field is rapidly approaching the stage at which soft x-ray lasers sufficiently compact to fit onto a normal optical table will be routinely utilized in science and technology. This is the result of recent advances in the amplification of soft x-ray radiation in both compact laser-pumped and discharge-pumped devices. The use of excitation mechanisms that take full advantage of new ultrafast high power optical laser drivers and multiple pulse excitation schemes has resulted in the demonstration of saturated soft x-ray amplification at wavelengths as short as 14 nm using several Joule of laser-pump energy. Moreover, several schemes have demonstrated significant gain with only a fraction of a Joule of laser-pump energy. In addition, the demonstration of saturated table-top soft x-ray lasers pumped by very compact capillary discharges has shattered the notion that discharge-created plasmas are insufficiently uniform to allow for soft x-ray amplification, opening a route for the development of efficient, high average power soft x-ray lasers. Recently, a table-top capillary discharge laser operating at 46.9 nm has produced millijoule-level laser pulses at a repetition rate of several Hz, with a corresponding spatially coherent average power per unit bandwidth comparable to that of a beam line at a third generation synchrotron facility. This review summarizes fundamental and technical aspects of table-top soft x-ray lasers based on the generation of population inversions in plasmas, and discusses the present status of development of specific laser systems.

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Table-top soft x-ray lasers

New methods have recently been developed to measure the carrier-envelope phase evolution of ultrashort optical pulses. These methods utilize a powerful combination of time-domain and frequency-domain techniques. The resulting ability to stabilize the carrier-envelope phase of the pulse train emitted by a mode-locked laser means that the absolute frequencies in the optical spectrum can be determined, which has had an immediate impact on optical frequency metrology and optical clocks. In the time domain, this establishes the basis for controlling and tailoring the electric field of ultrashort pulses, which will affect applications in extreme nonlinear optics and coherent control. This paper provides a detailed look at the correspondence between time and frequency that underlies these results and reviews the recent progress in the field.

https://iopscience.iop.org/article/10.1088/0022-3727/35/8/201/pdf

Phase stabilization of ultrashort optical pulses

We report the demonstration of an amplitude-division soft-x-ray interferometer that can be used to generate high-contrast interferograms at the wavelength of any of the saturated soft-x-ray lasers (5.6-46.9 nm) that are available at present. The interferometer, which utilizes grazing-incidence diffraction gratings as beam splitters in a modified Mach-Zehnder configuration, was used in combination with a tabletop 46.9-nm laser to probe a large-scale (~2.7-mm-long) laser-created plasma.

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Dense plasma diagnostics with an amplitude-division soft-x-ray laser interferometer based on diffraction gratings.

Interferometry using optical lasers is a powerful technique for the diagnostics of plasmas that provides time-resolved two-dimensional maps of the electron density. However, free‐free absorption and refraction of the probe beam limit the maximum electron density, plasma size, and plasma density gradients that can be probed with optical wavelengths. The development of soft-x-ray lasers has opened the possibility of extending plasma interferometry to shorter wavelengths, significantly expanding the range of plasma parameters that can be probed. Recently, a 15.5-nm Ne-like Y laser pumped by the Nova laser at Lawrence Livermore National Laboratories was used in combination with a skewed Mach‐Zehnder interferometer to probe very high-density laser-created plasmas. 1,2 The amplitude-division interferometer used in those pioneering experiments was implemented by use of thin multilayer beam splitters and multilayer-coated mirrors developed for that wavelength. The advent of gain-saturated discharge-pumped tabletop soft-x-ray lasers 3,4 and the development of several saturated 5 or nearly saturated 6,7 soft-x-ray lasers pumped by relatively compact optical lasers will allow the probing of a great variety of dense plasmas. However, multilayer beam splitters cannot be developed for the wavelengths corresponding to some of these lasers at present, owing to the lack of materials with adequate optical constants. Alternatively, amplitude-division interferometers based on diffraction gratings 8 and wavefront-division interferometers 9,10 have been discussed.

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Soft-x-ray laser interferometry of a plasma with a tabletop laser and a Lloyd's mirror.

Soft-x-ray laser interferograms of laser-created plasmas generated at moderate irradiation intensities $(1\ifmmode\times\else\texttimes\fi{}{10}^{11}\char21{}7\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\ensuremath{-}2})$ with $\ensuremath{\lambda}=1.06\ensuremath{\mu}\mathrm{m}$ light pulses of \ensuremath{\sim}13-ns-FWHM (full width at half maximum) duration and narrow focus (\ensuremath{\sim}30 \ensuremath{\mu}m) reveal the unexpected formation of an inverted density profile with a density minimum on axis and distinct plasma sidelobes. Model simulations show that this strong two-dimensional hydrodynamic behavior is essentially a universal phenomena that is the result of plasma radiation induced mass ablation and cooling in the areas surrounding the focal spot.

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Two-dimensional effects in laser-created plasmas measured with soft-x-ray laser interferometry.

We have used a tabletop soft-x-ray laser and a wave-front division interferometer to probe the plasma of a pinch discharge. A very compact capillary discharge-pumped Ne-like Ar laser emitting at 46.9 nm was combined with a wave division interferometer based on Lloyd's mirror and Sc-Si multilayer-coated optics to map the electron density in the cathode region of the discharge. This demonstration of the use of tabletop soft-x-ray laser in plasma interferometry could lead to the widespread use of these lasers in the diagnostics of dense plasmas.

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Soft-x-ray laser interferometry of a pinch discharge using a tabletop laser.

Summary form only given. The development of gain-saturated soft X-ray lasers has opened the possibility to extend laser interferometry to large-scale high-density plasmas that can not be probed with optical lasers. The recent advent of gain-saturated tabletop soft X-ray lasers creates the opportunity of developing portable soft X-ray tools that will allow detailed maps of the electron density evolution in a great variety of very dense plasmas. Recently, our group has demonstrated plasma interferometry combining a 46.9-nm capillary discharge tabletop laser with a wavefront-division interferometer based on Lloyd's mirror. The Lloyd's mirror has the advantage of constituting the simplest possible soft X-ray interferometer.

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K. Kanizay

Papers

Dense plasma diagnostics with an amplitude-division soft-x-ray laser interferometer based on diffraction gratings.

Soft-x-ray laser interferometry of a plasma with a tabletop laser and a Lloyd's mirror.

Two-dimensional effects in laser-created plasmas measured with soft-x-ray laser interferometry.

Soft-x-ray laser interferometry of a pinch discharge using a tabletop laser.

Dense plasma diagnostics with an amplitude division soft X-ray laser interferometer based on diffraction gratings