Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.

https://www.engr.colostate.edu/ece//faculty/rocca/pdf/journals/rohringer_etal_2012.pdf

Atomic inner-shell X-ray laser at 1.46 nanometres pumped by an X-ray free-electron laser

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.

/pdf/table-top-soft-x-ray-lasers-1xt6argbcz.pdf

Table-top soft x-ray lasers

In this paper, the author reviews the plasma-based x-ray lasers which we have already demonstrated saturated amplification whose wavelengths are between 50 and 6?nm. Section?1 describes the motivation of this review paper which includes basic ideas, developments and their applications of x-ray lasers. In section?2, the author describes the early x-ray laser researches on the recombination and the electron collisional excitation schemes including the hydrogen-like and lithium-like ion recombination schemes and the electron collisional excitation scheme. Section?3 describes the first demonstration of significant lasing at Livermore for the electron collisional excitation scheme of neon-like selenium ions at a wavelength of 20.6 and 20.9?nm and Princeton for the recombination scheme of hydrogen-like carbon ions at a wavelength of 18.2?nm. In section?4, the author describes the electron collisional excitation type soft x-ray lasers which are at present the most successful x-ray lasers. The subjects with which the author deals are saturated amplification neon-like soft x-ray lasers, improvement of neon-like soft x-ray laser performance using multi-layer mirrors, atomic physics issues of the neon-like soft x-ray lasers, gain guiding of the x-ray laser beam propagation, discharge-pumped compact repetitive neon-like ion soft x-ray lasers, the collisional excitation nickel-like ion soft x-ray lasers, high gain and saturated amplification nickel-like soft x-ray lasers at wavelengths as short as 7?nm, the short wavelength nickel-like x-ray lasers whose wavelengths are close to the longest wavelength edge the water window of 4.4?nm, transient collisional excitation scheme which is currently the most popular soft x-ray lasers pumped by short-pulse compact lasers with a laser energy of a few J to a few tens of?J. In section?5, the author describes various plasma-based x-ray laser schemes other than the recombination and the collisional schemes, such as the optical field ionization schemes and inner-shell ionization schemes. Section?6 includes soft x-ray laser applications such as soft x-ray holography, soft x-ray interferometers, soft x-ray microscopy and other applications. In section?7, the author summarizes this review paper and he proposes a future direction for x-ray laser researches.

https://iopscience.iop.org/article/10.1088/0034-4885/65/10/204/pdf

Review of soft x-ray laser researches and developments

Capillary discharge extreme ultraviolet lamp sources for EUV microlithography and other applications. The invention covers operating conditions for a pulsed capillary discharge lamp for EUVL and other applications such as resist exposure tools, microscopy, interferometry, metrology, biology and pathology. Techniques and processes are described to mitigate against capillary bore erosion, pressure pulse generation, and debris formation in capillary discharge-powered lamps operating in the EUV. Additional materials are described for constructing capillary discharge devices fore EUVL and related applications. Further, lamp designs and configurations are described for lamps using gasses and metal vapors as the radiating species.

Capillary discharge extreme ultraviolet lamp source for EUV microlithography and other related applications

An EUV photon source includes a plasma chamber filled with a gas mixture, multiple electrodes within the plasma chamber defining a plasma region and a central axis, a power supply circuit connected to the electrodes for delivering a main pulse to the electrodes for energizing the plasma around the central axis to produce an EUV beam output along the central axis, and a preionizer for ionizing the gas mixture in preparing to form a dense plasma around the central axis upon application of the main pulse from the power supply circuit to the electrodes. The EUV source preferably includes an ionizing unit and precipitator for collecting contaminant particulates from the output beam path. A set of baffles may be disposed along the beam path outside of the pinch region to diffuse gaseous and contaminant particulate flow emanating from the pinch region and to absorb or reflect acoustic waves emanating from the pinch region away from the pinch region. A clipping aperture, preferably formed of ceramic and/or Al2O3, for at least partially defining an acceptance angle of the EUV beam. The power supply circuit may generates the main pulse and a relatively low energy prepulse for homogenizing the preionized plasma prior to the main pulse. A multi-layer EUV mirror is preferably disposed opposite a beam output side of the pinch region for reflecting radiation along the central axis for output along the beam path of the EUV beam. The EUV mirror preferably has a curved contour for substantially collimating or focusing the reflected radiation. In particular, the EUV mirror may preferably have a hyperbolic contour.

Method and apparatus for generating high output power gas discharge based source of extreme ultraviolet radiation and/or soft x-rays

A soft-x-ray-pumped inner-shell photoionization laser has been produced in Cd vapor at 4416 and 3250 A. A gain of 5.6 cm−1 has been measured at 4416 A, and a reasonably high-energy storage of 0.2 mJ/cm3 in the upper laser states has been obtained.

High-gain inner-shell photoionization laser in Cd vapor pumped by soft-x-ray radiation from a laser-produced plasma source.

A segmented plasma excitation and recombination (SPER) device is employed in a deposition scheme in which the plasma generated in the gap between adjacent electrodes is formed into a beam of ions/atoms by flowing a background gas through the gap. The beam strikes a workpiece and deposits a layer of the vaporized electrode material thereon. Also described are techniques where the ions react with workpiece or the background gas to form a layer, as well as where the ions bombard the workpiece to etch away a layer.

SPER Device for material working

Large inversion densities in Zn+ have been produced by photoionizing inner #x2010;shell d electrons with the broadband soft #x2010;x #x2010;ray flux from a 106 #x2010; #x3bc;m laser #x2010;produced plasma This pumping scheme is an effective technique for rapidly producing large population densities (5 #xd7;1013 cm-3) in the 3d9 #x2009;4s2 #x2009;2D5/2 upper laser level These large densities (a) produced laser action on a two #x2010;electron transition from the d #x2010;electron manifold to the outer #x2010;electron manifold at 7478 #xc5;, (b) allowed the isotope shifts on this relatively weak transition to be measured, and (c) may eventually be transferred to other high lying states in Zn+ to produce vacuum ultraviolet lasers

/pdf/high-gain-soft-x-ray-pumped-photoionization-laser-in-zinc-djrh5ypsiq.pdf

High‐gain soft‐x‐ray‐pumped photoionization laser in zinc vapor

The soft-x-ray and XUV emission from laser-produced plasmas produced by 300-mJ pulses from 1.06- and 10.6-μm lasers was compared both by measuring the absorption due to inner-shell photoionization in Cd vapor in the 10–71-nm region and by directly observing the plasma emission in the 35–75-nm region. The results indicate that a 10.6-μm laser can be a more efficient pumping source for a photoionization laser than a 1.06-μm laser in the nanosecond pulse regime at low laser intensities.

Comparison of extreme-ultraviolet flux from 1.06- and 10.6-Mum laser-produced plasma sources for pumping photoionization lasers.

Stimulated emission on three UV transitions in Cd+ has been observed by transferring population from inner-shell d-electron states, populated by photoionization, to outer-shell p-electron states using the output from a narrow-frequency dye laser. The use of similar techniques in other elements could eventually result in a number of new lasers in the UV and VUV.

/pdf/stimulated-emission-in-the-ultraviolet-by-optical-pumping-d973m4ml9n.pdf

J. J. Macklin

Papers

High-gain inner-shell photoionization laser in Cd vapor pumped by soft-x-ray radiation from a laser-produced plasma source.

SPER Device for material working

High‐gain soft‐x‐ray‐pumped photoionization laser in zinc vapor

Comparison of extreme-ultraviolet flux from 1.06- and 10.6-Mum laser-produced plasma sources for pumping photoionization lasers.

Stimulated emission in the ultraviolet by optical pumping from photoionization-produced inner-shell states in Cd +