2D Material Takes on Chirality of Circularly Polarized Light

Physicists at MIT have uncovered a new way to test whether or not a material is chiral, and have also found a way to enhance the overall chirality in a large piece of material. The material they used, titanium diselenide (TiSe₂), is a transition-metal dichalcogenide (TMD) semimetal that has potential use in quantum devices.

The researchers achieved optical chiral induction in TiSe₂ by shining light on the material while cooling it below a critical temperature. They observed that although TiSe₂ at room temperature had no chirality, as its temperature decreased it reached a point where the balance of right-handed and left-handed electronic configurations was thrown off in favor of one chiral domain. They further 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 (i.e., whether the polarization rotated clockwise or counterclockwise) determined the chirality of the resulting patterning of electron distribution.

Professor Pablo Jarillo-Herrero said that TiSe₂ naturally structures itself into “loosely stacked two-dimensional layers on top of each other,” similar to a sheaf of papers. 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. The stripes can then form helical patterns that twist either to the right or to the left.

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. Courtesy of Ella Maru Studio.

Ordinarily, the material would contain equal amounts of the right- and left-handed versions of charge density waves, and the effects of handedness would cancel out in most measurements. But under the influence of the polarized light, researcher Qiong Ma said, “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.”

After inducing a 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,” researcher Suyang Xu said. That direction can be switched to the other orientation if an oppositely polarized light source shines on the material.

Although this study was carried out with one specific material, the researchers believe that the same principles could work with other materials, and that this approach to inducing changes in the electronic state of a material could potentially be applied more broadly. “This interaction with light is a phenomenon which will be very useful in other materials as well, not just chiral material, but I suspect in affecting other kinds of orders as well,” professor Nuh Gedik said.

The research was published in Nature (www.doi.org/10.1038/s41586-020-2011-8).