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 ...
International audienceThe advent of X-ray free-electron lasers has granted researchers an unprecedented access to theultrafast dynamics of matter on the nanometre scale(1-3). Aside from being compact, seededplasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich(4) sources (up to 10(15)per pulse) that display high-quality optical properties(5,6). Hitherto, the duration of these sources waslimited to the picosecond range(7), which consequently restricts the field of applications. This bottleneckwas overcome by gating the gain through ultrafast collisional ionization in a high-density plasmagenerated by an ultraintense infrared pulse (a few 10(18) W cm(-2)) guided in an optically pre-formedplasma waveguide. For electron densities that ranged from 3 x 10(18) cm(-3) to 1.2 x 10(20) cm(-3), thegain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which pavesthe way to compact and ultrafast SXRL beams with performances previously only accessible inlarge-scale facilities
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.