By Sally Pobojewski
News and Information Services
Using short pulses of intense laser light, U-M physicists have created “designer atoms”—new types of matter never before seen in nature.
Physics Prof. Philip H. Bucksbaum leads the research group that has used laser pulses to knock electrons away from the nucleus of an atom, produce atoms with hidden or “dark-state” electrons, rearrange the basic structure of the atom itself, and create atoms whose behavior changes over time.
“I call it quantum mechanical engineering,” Bucksbaum says. “Atoms and light are the raw materials we use to build basic quantum systems. By varying the coherence of the laser pulse, it’s possible to control the quantum nature of matter.”
An atom’s structure, motion and behavior are determined by the laws of quantum mechanics—the study of the indivisible bundles of energy that make up all light and all matter. At the quantum level, intense light and atoms interact in complex ways that scientists are just beginning to understand and control.
Bucksbaum will present details of his research at the American Physical Society (APS) meeting in Seattle this week. Results also were published in the March 1 issue of Physical Review Letters.
To perform the experiments, Bucksbaum’s group developed novel, intense single-cycle pulses of coherent light that turn on and off within a fraction of a picosecond—less than one-trillionth of a second. As these light pulses pass through atoms, they produce fundamental changes in the distribution of electrons that surround an atom’s nucleus, according to Bucksbaum.
“After the pulse passes, the atom is in a unique state,” Bucksbaum says. “Not only have the electrons been redistributed, the basic quantum structure that determines the properties of the atom has been changed.”
While these changes don’t last forever, Bucksbaum says the atom does retain its new structure long enough for it to interact with its environment or with other atoms in unusual, and potentially useful, ways.
“By adjusting the laser pulse, we can engineer these ‘designer atoms’ to exhibit all sorts of variable time-dependent behavior,” Bucksbaum explains. “They exist in a non-stationary state evolving and changing over time.”
Bucksbaum says the unusual properties of these engineered atoms may someday prove useful in atomic beam-related technologies, such as advanced semiconductor processing. Currently, Bucksbaum’s group is working with other scientists at the U-M’s NSF Center for Ultrafast Optical Science to explore how the techniques used to make ‘designer atoms’ could be used in the field of bond-selective chemistry to manipulate chemical reactions.
In his APS meeting presentation, Bucksbaum will describe how he used ultrashort laser pulses to ionize, or remove the electron from, a Rydberg atom—a loosely-bound type of atom where one electron travels around the nucleus in a wide-ranging elliptical track similar to a comet’s orbit around the sun. According to conventional theory, these atoms are very difficult to ionize with light, particularly when the electron is far from the nucleus.
“In quantum theory, photons [tiny bundles of energy that are the building blocks of light] cannot easily transfer energy to a nearly free electron,” Bucksbaum says. Without additional energy, the electron cannot escape from the atom.
“The theory holds true for ordinary light, but not for the very intense single cycles we can produce with ultra-short pulse lasers,” Bucksbaum says. “The laser pulse gives the electron an extra kick of energy, which is greater than the binding energy holding it to the nucleus of the atom.”
Bucksbaum adds that this project is only the latest example of ‘designer atoms’ devised by the U-M group. Last year, they showed how electrons in Rydberg atoms hide from light in new quantum states called “dark wavepackets.”
“Eventually, as technology develops shorter, more intense and more complex laser pulses, scientists should be able to control both excitation and ionization in any atom,” Bucksbaum says.
Bucksbaum believes the greatest contributions of this research will be increased understanding of the basic quantum mechanical forces that hold electrons within an atom, and advances in basic knowledge about what happens when light interacts with matter.
Others contributing to the U-M research program include Robert Jones, post-doctoral fellow in physics; Douglas Dykaar of AT&T Bell Labs in Murray Hill, N.J.; and Don You, graduate student in physics. The research is funded by the National Science Foundation.
