Rice Physicists Use Lasers of Varying Wavelengths to Create World’s First Laser-Cooled Neutral Plasma

Achieving a 20-year hunt that paves the way for simulators that re-create exotic states of matter seen inside white dwarf stars and Jupiter, physicists from the Rice University have produced the world’s first-ever laser-cooled neutral plasma.

The outcomes of the research have been described in the Science journal this week and include innovative methods for laser-cooling clouds of fast expanding plasma to temperatures around 50 times colder than deep space.

We don’t know the practical payoff yet, but every time physicists have laser cooled a new kind of thing, it has opened a whole world of possibilities. Nobody predicted that laser cooling atoms and ions would lead to the world’s most accurate clocks or breakthroughs in quantum computing. We do this because it’s a frontier.

Tom Killian, Lead Scientist, Professor of Physics and Astronomy, Rice University

Killian and graduate students Tom Langin and Grant Gorman used 10 lasers of different wavelengths to produce and cool the neutral plasma. To begin with, they vaporized strontium metal and trapped and cooled a puff of strontium atoms with the size similar to that a child’s fingertip by using one set of intersecting laser beams. Subsequently, the ultracold gas was ionized using a 10-ns blast from a pulsed laser. The pulse transformed the gas into a plasma containing ions and electrons by stripping one electron from each atom.

The newly formed plasma was made to expand quickly and dissipate within one-thousandth of 1 second by the energy from the ionizing blast. The important discovery this week is that it is possible to cool down the expanding ions using another set of lasers once the plasma is produced. In the new paper, Killian, Langin, and Gorman have given a detailed account of their method, thereby opening the door for their lab and others to create much colder plasmas with strange behavior in unexplained ways.

Plasma, one of four fundamental states of matter, is an electrically conductive mix of ions and electrons. However, in contrast to liquids, solids, and gases, which are well known in everyday life, plasmas tend to exist in extremely hot places such as a lightning bolt or the surface of the sun. Killian and his colleagues believe that the analysis of ultracold plasmas can offer answers to basic questions in relation to the way matter behaves under extreme conditions of low temperature and high density.

The team started laser cooling to make its plasmas, which is a technique for trapping and slowing down particles using intersecting laser beams. The energy an atom or ion is inversely proportional to how cold it is and directly proportional to how slow it moves about randomly. In the 1990s, laser cooling was developed to slow down atoms to the point where they are nearly motionless, or just a few millionths of a degree above absolute zero.

If an atom or ion is moving, and I have a laser beam opposing its motion, as it scatters photons from the beam it gets momentum kicks that slow it. The trick is to make sure that light is always scattered from a laser that opposes the particle’s motion. If you do that, the particle slows and slows and slows.

Tom Killian, Lead Scientist, Professor of Physics and Astronomy, Rice University

In 1999, Killian pioneered the ionization technique for producing neutral plasma from a laser-cooled gas while pursuing a postdoctoral fellowship at the National Institute of Standards and Technology in Bethesda, MD. Upon joining Rice’s faculty the following year, he started looking for a means to make the plasmas even colder. One impulse was to accomplish “strong coupling,” a phenomenon that occurs naturally in plasmas only in exotic places such as the center of Jupiter and white dwarf stars.

“We can’t study strongly coupled plasmas in places where they naturally occur,” stated Killian said. “Laser cooling neutral plasmas allows us to make strongly coupled plasmas in a lab, so that we can study their properties.”

In strongly coupled plasmas, there is more energy in the electrical interactions between particles than in the kinetic energy of their random motion. We mostly focus on the ions, which feel each other, and rearrange themselves in response to their neighbors’ positions. That’s what strong coupling means.

Tom Killian, Lead Scientist, Professor of Physics and Astronomy, Rice University

Since the ions possess positive electric charges, they repel each another by means of the same force that makes a person’s hair stand up straight upon getting charged with static electricity.

“Strongly coupled ions can’t be near one another, so they try to find equilibrium, an arrangement where the repulsion from all of their neighbors is balanced,” he stated. “This can lead to strange phenomena like liquid or even solid plasmas, which are far outside our normal experience.”

In the case of normal, weakly coupled plasmas, the impact of these repulsive forces on ion motion is very small since they are far outweighed by the effects of kinetic energy, or heat.

“Repulsive forces are normally like a whisper at a rock concert,” stated Killian. “They’re drowned out by all the kinetic noise in the system.”

However, at the center of a white dwarf star or Jupiter, ions are squeezed together by intense gravity so closely that repulsive forces, which considerably increase in strength at shorter distances, emerge victorious. Ions become strongly coupled despite the fact that the temperature is extremely high.

Killian and his colleagues produce plasmas with a density that is orders of magnitude lower than those of plasmas inside dead stars or planets; however, they increase the ratio of electric-to-kinetic energies by decreasing the temperature. Killian’s team observed that when the temperatures are as low as one-tenth of a Kelvin above absolute zero, the repulsive forces take over.

“Laser cooling is well developed in gases of neutral atoms, for example, but the challenges are very different in plasmas,” he stated.

We are just at the beginning of exploring the implications of strong coupling in ultracold plasmas. For example, it changes the way that heat and ions diffuse through the plasma. We can study those processes now. I hope this will improve our models of exotic, strongly coupled astrophysical plasmas, but I am sure we will also make discoveries that we haven’t dreamt of yet. This is the way science works.

Tom Killian, Lead Scientist, Professor of Physics and Astronomy, Rice University

The Air Force Office of Scientific Research and the Department of Energy’s Office of Science supported the study.

Video credit: Rice University