X-Ray Transparency

A method to achieve electromagnetically induced transparency (EIT) for x rays has been worked out. Normally a gas of atoms will absorb light at a certain frequency if that frequency corresponds to the energy needed to bridge the gap between two internal quantum levels in the atoms in the gas.

If, however, a third quantum level exists, it might be possible to set up the quantum phenomenon called electromagnetically induced transparency. To bring this about, one laser beam, the pump beam, creates a coherent superposition of levels 2 and 3, which become so-called “dressed states.”

If tuned properly, the transition pathways from level 1 to the two dressed states will interfere destructively. After that, over a narrow range of frequencies, absorption of a probe beam is suppressed at the frequency corresponding to the transition from level 1 to level 2.

It just so happens that this selective transparency causes the index of refraction to vary rapidly near that frequency, a development which has, in turn, been exploited in slowing optical light pulses. Getting all this to work at x-ray frequencies has been difficult since the energy levels are broad, corresponding to very short-lived vacancies in inner electron shells lodged in somewhat heavy atoms; in neon atoms, for example, the lifetime is about 2 femtoseconds.

Theorists calculated that to get EIT to work for x rays, a very powerful pump beam (10^12 W/cm^2) would be needed. A new study of the problem, undertaken by Christian Buth, Robin Santra, and Linda Young at Argonne National Lab, shows that such a powerful beam will not necessarily undo the fragile troika of states needed for EIT to work.

Santra (rsantra@anl.gov) says that induced transparency for x rays would help in the shaping of x-ray pulses in pending x-ray laser facilities where biomolecule imaging and molecular movies are envisioned. Tests of the new x-ray transparency scheme might be made soon at Berkeley's Advanced Light Source and at Argonne's Advanced Photon Source. (Physical Review Letters, upcoming article