Extreme ionization of gold atoms creates new universe of conditions inside CSU

An illustration of the laser interacting with gold atoms and subsequently heating them up (electrons flying off as they ionize). Credit: CSU alumnus Nick Adkins
An illustration of the laser interacting with gold atoms and subsequently heating them up (electrons flying off as they ionize. Credit: CSU alumnus Nick Adkins

Inside Colorado State University’s Advanced Beam Laboratory  (ABL), part of LaserNet US, researchers are using an ultra-intense laser system to recreate extreme conditions that only exist in the astrophysical world.

The team has performed record-setting ionization of gold atoms – a process that stripped these heavy atoms in solid matter of most of their electrons. These unprecedented degrees of ionization are a result of generating energy densities similar to those contained in the center of a star.

Their results, published in the October issue of Nature Photonics, could advance the basic understanding of atomic physics and the behavior of atoms in extreme environments, leading to the discovery of new phenomena and applications.

Turning up the heat

The lead author of the study is Reed Hollinger, who is a U.S. Department of Energy postdoctoral fellow who earned his Ph.D. in Electrical and Computer Engineering from CSU in 2018 under the guidance of University Distinguished Professor Jorge Rocca, the ABL lab director.

“The universe is capable of creating nearly unfathomable conditions,” said Hollinger, who also played a central role in the experiment that produced micro-scale nuclear fusion with record efficiency.  “Lasers are the perfect tool to help us reproduce these extreme environments in the laboratory. It is an impressive feat to create these conditions using relatively small lasers. It’s wild to think about how hot we’re making it inside that target chamber.”

Hollinger, Rocca and their collaborators achieved record ionization using a high-powered tabletop laser known as ALEPH, or Advanced Laser for Extreme Photonics. The team focused their laser into gold targets at an intensity greater than 1021 W/cm2 – a level at which the peak electric field exceeds an astronomical 100 trillion volts per meter. At the focus of this laser matter interaction, they generated an enormous energy density approaching 100 billion joules per cubic centimeter for a trillionth of a second.

The ultra-fast titanium sapphire laser operated with a pulse width on the order of tens of femtoseconds, or one quadrillionth of a second, and was converted from its normal operation at a central wavelength of 800 nanometers (red light) to 400 nanometers (blue light) through a second harmonic generation process in a large nonlinear crystal. Using very high-intensity blue light allowed the laser to deposit its energy at plasma densities close to solid density. This deposited energy superheated the gold atoms, stripping off an unprecedented 72 electrons, forming nitrogen-like gold atoms inside the hot, dense plasmas.

In addition to advancing the understanding of atomic processes in astrophysical and extreme laboratory environments, the study could also open the door to applications such as beams of highly charged particles and more efficient ultrafast x-ray radiation sources.

The research was supported by LaserNet US and the Department of Energy Fusion Energy Sciences (DOE FES) program.