Twist on Electrons Graphic
Beams of circularly polarized light (shown as blue spirals) can have two different mirror-image orientations, as shown here. When these beams strike a sheet of titanium diselenide (shown as a lattice of blue and silver balls), the electrons (aqua dots) in the material take on the handedness of the light's polarization. (Image: Ella Maru Studio)

Some molecules, including most of the ones in living organisms, have shapes that can exist in two different mirror-image versions. The right- and left-handed versions can sometimes have different properties, such that only one of them carries out the molecule’s functions. Now, a team of physicists has found that a similarly asymmetrical pattern can be induced and measured at will in certain exotic materials, using a special kind of light beam to stimulate the material.

In this case, the phenomenon of “handedness,” known as chirality, occurs not in the structure of the molecules themselves, but in a kind of patterning in the density of electrons within the material. The researchers found that this asymmetric patterning can be induced by shining a circularly polarized mid-infrared light at an unusual material, a form of transition-metal dichalcogenide semimetal called TiSe2, or titanium diselenide. The findings could open up new areas of research in the optical control of quantum materials.

The team found that while titanium diselenide at room temperature has no chirality to it, as its temperature decreases it reaches a critical point where the balance of right-handed and left-handed electronic configurations gets thrown off and one type begins to dominate. They found that this effect could be controlled and enhanced by shining circularly polarized mid-infrared light at the material, and that the handedness of the light (whether the polarization rotates clockwise or counterclockwise) determines the chirality of the resulting patterning of electron distribution.

“It’s an unconventional material, one that we don’t fully understand,” says Jarillo-Herrero. The material naturally structures itself into “loosely stacked two-dimensional layers on top of each other,” sort of like a sheaf of papers, he says.

Within those layers, the distribution of electrons forms a “charge density wave function,” a set of ripple-like stripes of alternating regions where the electrons are more densely or less densely packed. These stripes can then form helical patterns, like the structure of a DNA molecule or a spiral staircase, which twist either to the right or to the left.

Ordinarily, the material would contain equal amounts of the right- and left-handed versions of these charge density waves, and the effects of handedness would cancel out in most measurements. But under the influence of the polarized light, Ma says, “we found that we can make the material mostly prefer one of these chiralities. And then we can probe its chirality using another light beam.” It’s similar to the way a magnetic field can induce a magnetic orientation in a metal where ordinarily its molecules are randomly oriented and thus have no net magnetic effect.

After inducing the particular directionality using the circularly polarized light, “we can detect what kind of chirality there is in the material from the direction of the optically generated electric current,” Xu adds. Then, that direction can be switched to the other orientation if an oppositely polarized light source shines on the material.

Gedik says that although some previous experiments had suggested that such chiral phases were possible in this material, “there were conflicting experiments,” so it had been unclear until now whether the effect was real. Though it’s too early in this work to predict what practical applications such a system might have, the ability to control electronic behavior of a material with just a light beam, he says, could have significant potential.

While this study was carried out with one specific material, the researchers say the same principles may work with other materials as well. The material they used, titanium diselenide, is widely studied for potential uses in quantum devices, and further research on it may also offer insights into the behavior of superconducting materials.

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