Transparency Spans into X-Ray Domain

Researchers have recently taken electromagnetically induced transparency (EIT) into another realm. A group led by Robin Santra  theoretically studied the way laser fields affect X-ray absorption. Their studies concluded that EIT exists in the X-ray domain. This discovery might help expand uses of X-ray technology in several different scientific disciplines.

EIT, a process that enables control over absorption and dispersion of a gaseous medium, has already been known to exist in the optical domain, or visible light. It involves one laser field interacting with the atoms coupling two quantum levels to allow a gas to become transparent at a specific wavelength determined by a third quantum level. Researchers have previously used EIT in the visible domain to slow down the speed of light to that of a bicycle.

But whereas visible light interacts with outer electron shells, X-rays interact only with the inner-most shells, causing a different sort of effect on the atom with regard to EIT. When an X-ray comes into contact with an atom, the inner-most electron is transferred to a higher-lying, “excited” quantum state, and then the atom decays in a femtosecond, or one millionth of a nanosecond. The excited electron must now be “laser-dressed” within this minuscule timeframe, which means a strong laser field must be applied to alter the electronic structure of the absorbing material before the atom collapses. Therefore, in order for EIT to remain possible in the X-ray domain, the dressing laser must be much more intense than usual for EIT.

 Santra’s team theorized that EIT was still possible, in spite of the strong distortion an atom might experience in such an intense laser field. After testing their theory through various computer calculations, they were able to describe X-ray absorption in strong laser fields and verify EIT’s existence in the X-ray domain. Their work is published in Physical Review Letters (C. Buth, R. Santra, and L. Young, “Electromagnetically induced transparency for x-rays,”  Physical Review Letters 98, 253001, 2007). 

This research can be applied to shape X-ray pulses arbitrarily on a femtosecond timescale. According to Santra, the theory also suggests a method for producing short X-ray pulses using existing laser technology. Short X-ray pulses are useful for future pump-probe experiments, which will allow movies of molecular motion on ultra-fast timescales.

Collaborators on this research include Robin Santra, Christian Buth and Linda Young of Argonne. Young and the Atomic Physics group are planning an experimental investigation of EIT at the Advanced Light Source at Lawrence Berkeley National Laboratory.

Modified from Argonne News, Vol. 60, No. 16, August 15, 2007