Forget transparent aluminum: researchers make iron invisible to X-rays

By Matthew Francis




In practice, EIT is created in optical resonant cavities, in which laser light is reflected back and forth until it creates a standing wave. Much as a plucked string has places of total constructive interference (antinodes) and total destructive interference (nodes), the standing light wave will have points where the the photons reinforce each other or cancel each other out. By arranging materials at these points of interference, the transition between energy levels can be managed in such a way to ensure that the emission and absorption of photons are balanced.

Nucleons (protons and neutrons) within an atomic nucleus have energy levels available, just as electrons do within the larger atom. Because of the much stronger nuclear forces involved, the nucleus is a tightly confined space. That has two consequences: the nucleons need a lot of energy to move to a new level, and those energy levels are widely separated. Unlike electrons, where the energies are in the neighborhood of visible wavelengths, nuclear transitions correspond to X-ray energies.

In the case of nuclear EIT, two very thin sheets layers of iron (which normally absorbs X-rays) are placed between two platinum mirrors, all within a cavity. The iron is fixed in place by a layer of carbon, which is already transparent to some wavelengths of X-ray light. One iron sheet is placed at a node and the second at an antinode within the cavity; this particular arrangement creates the conditions necessary to drive the emission/absorption pattern needed to make the iron vanish. The X-rays behave as if the two sheets aren’t there.

(As a bonus, the intensity of light needed for this to happen is far lower than what’s usually needed for atomic EIT, a boon given the energies needed to drive X-ray lasers.)

As with similar atomic EIT, the speed of the X-ray photons is slowed drastically by all the absorption and emission. It ends up around 30 meters per second (as opposed to the vacuum light speed of 300 million meters per second)

The iron itself is of the relatively rare isotope 57Fe, which has a special nuclear transition that makes it suitable for this work. The cooperation between the layers is also critical; one must be precisely at the node of the standing wave of laser light, and the other at the antinode. Shifting or removing one layer even slightly makes the effect disappear, ruining the transparency. Airplanes and adolescent wizards will not be made to vanish by this technique, in other words.

Though exciting, the transparency itself is only one aspect of many, as there are many potential applications and experimental possibilities. By exploiting the resonance of the transition between energy levels in the nucleus, the researchers have achieved something new, opening up the possibility of manipulating X-ray light in the same way optical light is in standard quantum optics.