Turning solid aluminium transparent by intense soft X-ray photoionization

Abstract

Saturable absorption is a phenomenon readily seen in the optical and infrared wavelengths. It has never been observed in core-electron transitions owing to the short lifetime of the excited states involved and the high intensities of the soft X-rays needed. We report saturable absorption of an L-shell transition in aluminium using record intensities over 10 16 W cm −2 at a photon energy of 92 eV. From a consideration of the relevant timescales, we infer that immediately after the X-rays have passed, the sample is in an exotic state where all of the aluminium atoms have an L-shell hole, and the valence band has approximately a 9 eV temperature, whereas the atoms are still on their crystallographic positions. Subsequently, Auger decay heats the material to the warm dense matter regime, at around 25 eV temperatures. The method is an ideal candidate to study homogeneous warm dense matter, highly relevant to planetary science, astrophysics and inertial confinement fusion. Saturable absorption, the decrease in the absorption of light with increasing intensity, is a well-known effect in the visible and near-visible region of the electromagnetic spectrum 1 , and is a widely exploited phenomenon in laser technology. Although there are many ways to induce this effect, in the simplest two-level system it will occur when the population of the lower, absorbing level is severely depleted, which requires light intensities sufficiently high to overcome relaxation from the upper level. Here, we report on the production of saturable absorption of a metal in the soft X-ray regime by the creation of highly uniform warm dense conditions, a regime that is of great interest in high-pressure science 2,3 , the geophysics of large planets 4,5 , astrophysics 6 , plasma production and inertial confinement fusion 7 . Furthermore, the process by which the saturation of the absorption occurs will lead, after the X-ray pulse, to the storage of about 100 eV per atom, which in turn evolves to a warm dense state. This manner of creation is unique as it requires intense, subpicosecond, soft X-rays. As such, it has not hitherto been observed in this region of the spectrum, owing both to the lack of high-intensity sources, and the rapid recombination times associated with such high photon energies. However, with the advent of new fourth-generation X-ray light sources, including the free-electron laser in Hamburg 8 (FLASH), soft X-ray intensities that have previously remained the province of high-power optical lasers can now be produced. Experiments at such high intensities using gas jets have already exhibited novel absorption phenomena 9 , and the possibility of irradiating solid samples with intense soft and hard X-rays has aroused interest as a possible means of producing warm dense matter (WDM) at known atomic densities 10,11 .We present the first measurements of the absorption coefficient of solid samples subject to subpicosecond soft X-ray pulses with intensities up to and in excess of 10 16 W cm −2 , two orders of magnitude higher than could ...

Saturated ablation in metal hydrides and acceleration of protons and deuterons to keV energies with a soft-x-ray laser

“…1). The resulting submicron spot with a power density above 10 17 W/cm 2 at best focus [5,22] produced well-defined craters in the solid samples [Figs. 2(a), 2(b), and 2(c)].…”

“…Breakthrough results have been reported in coherent diffractive imaging [1], femtosecond x-ray holography [2], cluster physics [3], and high-energy density science [4,5]. Parallel developments with table-top x-ray lasers demonstrate new capabilities at a significantly smaller laboratory scale [6][7][8][9].…”

Time resolved X-ray absorption spectroscopy in condensed matter: A road map to the future

“…This is a mandatory operation in order to avoid sample radiation damage and/or to avoid the warm matter density regime that can be easily reached with SASE FELs. Nonetheless, there are examples of high fluence XAS at VUV FEL (FLASH) and hard X-rays FELS (LCLS and SACLA) on aluminum and iron samples [58][59][60]. This set ups are all built in transmission.…”

An extreme ultraviolet Michelson interferometer for experiments at free-electron lasers

“…27,28 To assess the transient and possible permanent changes in material properties of the beam splitter under intense free-electron-laser radiation, one-dimensional radiation hydrodynamics simulations using HELIOS-CR 29 performed. 30 We model the multilayer structure and the underlying membrane, and start the simulation at ambient temperature (25 meV).…”