The complexity of exploring the extreme conditions of a planet's interior, or the conditions that lead to a fusion reaction, has baffled scientists for many years. But thanks to a powerful X-ray laser at the European X-Ray Free-Electron Facility (XFEL), an international group of researchers has investigated a state of matter that resembles these environments: warm dense matter (WDM). The findings will aid the advancement of inertial confinement fusion, an experimental technology that could lead to an abundance of clean energy.
Details of their findings were published online in the journal Nature Physics on July 29, 2024.
Heat drastically changes the state of matter. Depending on the temperature, substances are either a solid, liquid, or gas. But at a certain temperature range, matter assumes the WDM state - too hot to be described as dense matter, but at the same time too dense for the physics of weakly coupled plasma. Its enigmatic nature means the boundary between WDM and other states of matter have not been defined. WDM is not stable in our daily environment, making it difficult to produce or examine in a laboratory. Typically, scientists compress samples in diamonds anvil cells to reach high pressures, or use powerful optical lasers to turn solids into WDM for a tiny fraction of a second.
"The capabilities of a free-electron laser, however, have enabled us to observe phenomena in matter that were previously impossible to detect," points out Zhong Yin, an associate professor at SRIS, Tohoku University. By employing the intense X-ray pulses of the European XFEL, Zhong and his colleagues could generate and analyze WDM, using copper as the sample material. The intensity of the pulses excites the electrons in the copper foil so that it switches to a WDM state, a change which was observed via light transmission.
A metal that is irradiated by an intense X-ray pulse becomes transparent if the electrons in the metal absorb X-ray energy so fast that there are no electrons left to excite. The remaining tail of the pulse can then penetrate the material unhindered. This is known as saturable absorption (SA).
Conversely, a metal can become increasingly opaque if the front of the pulse creates excited states that have higher absorption coefficient than the cold metal. The tail of the pulse is then absorbed stronger, an effect known as reverse saturable absorption (RSA). Both processes are routinely used in optics to generate a specific pulse length with lasers.
The researchers irradiated focused, 15 femtosecond-short X-ray pulses onto a 100 nanometre-thick copper film, analysing the transmitted signal using a grating spectrometer. "The spectrum depends on the intensity of the X-ray pulse," explains Laurent Mercadier, a scientist based at the European XFEL who led the experiment. "At low-to-moderate X-ray intensity, copper becomes more and more opaque to the X-ray beam and exhibits RSA. However, at higher intensities, absorption saturates, and the foil becomes transparent."