Polarized Light can Induce Asymmetrical Pattern in Exotic Materials

The shapes of certain molecules, including most of the molecules in living organisms, can occur in two different mirror-image versions. At times, the right-handed and left-handed versions can have varying properties, where only one of the versions performs the functions of the molecule.

Physicists have now discovered that when the material is activated by a unique kind of light beam, an analogously asymmetrical pattern can be induced and quantified on demand, in specific unusual materials.

In such a situation, the phenomenon of “handedness,” called chirality, takes place in a kind of patterning in the density of electrons inside the material and not in the molecular structure themselves.

The scientists discovered that shining a circularly polarized mid-infrared light at an exotic material could induce this asymmetric patterning. This material is a type of transition-metal dichalcogenide semimetal known as titanium diselenide (TiSe₂).

These latest discoveries, which could pave the way for new avenues of research in the optical regulation of quantum materials, was recently described in the Nature journal in a paper written by Suyang Xu and Qiong Ma, both postdocs from MIT; professors Nuh Gedik and Pablo Jarillo-Herrero; and 15 collaborators at MIT and other universities in the United States, Japan, China, Singapore, and Taiwan.

The researchers discovered that TiSe₂ lacks chirality at room temperature; however, when the temperature of TiSe₂ reduces, it achieves a critical point where the balance of left-handed and right-handed electronic configurations becomes disrupted and one type of configuration takes over.

The scientists observed that when circularly polarized mid-infrared light is shined at the material, this effect could potentially be managed and improved. They also found that the light’s handedness (whether the polarization rotates in a counterclockwise or clockwise fashion) establishes the chirality of the ensuing patterning of the distribution of electrons.

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

The electron distribution inside those layers creates a “charge density wave function,” which can be described as a set of ripple-like stripes of alternating areas in which the electrons are less densely or more densely packed together. These ripple-like stripes can subsequently form helical patterns, similar to the structure of a spiral staircase or a DNA molecule, which bend either to the left or to the right.

The material would normally contain the same amounts of the left-handed and right-handed versions of these charge density waves, and the impacts of handedness would be nullified in a majority of the measurements. However, this was different under the effect of the polarized light.

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.

Qiong Ma, Postdoc, MIT

It is just like the way a magnetic field causes a magnetic orientation in a metal where normally its molecules are arbitrarily oriented and hence do not have any net magnetic effect. However, using light to induce this effect in the chirality inside a solid material is something “nobody ever did before,” explained Gedik.

Once the specific directionality was induced 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,” added Xu.

If an oppositely polarized light source is illuminated on the material, that direction can be subsequently switched to the other orientation.

According to Gedik, while some experiments done in the past had proposed that these chiral phases could possibly be achieved in this material, “there were conflicting experiments,” so until now, it had not been clear whether the effect was indeed real.

While it is too early in this analysis to expect the kind of practical applications that could be provided by such a system, the ability to regulate a material’s electronic behavior with only a beam of light could have major potential, Gedik added.

Although this analysis was performed with one particular material, the same principles may also work with other kinds of materials, stated the researchers.

TiSe₂—the material used by the researchers—is extensively studied for its promising applications in quantum devices, and additional studies on this material may provide a better understanding of the behavior of superconducting materials.

According to Gedik, this method of causing changes in the material’s electronic state is a novel tool that can perhaps be used more widely.

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.

Nuh Gedik, Professor, MIT

While chirality is widespread and popular in biological molecules and in certain magnetic phenomena, “this is the first time we’ve shown that this is happening in the electronic properties of a solid,” added Jarillo-Herrero.

“The authors found two new things,” stated Jasper van Wezel, a professor at the University of Amsterdam who was not part of the research group.

According to him, the latest discoveries are “a new way of testing whether or not a material is chiral, and a way of enhancing the overall chirality in a big piece of material. Both breakthroughs are significant. The first as an addition to the experimental toolbox of materials scientists, the second as a way of engineering materials with desirable properties in terms of their interaction with light.”

The study was supported by the Gordon and Betty Moore Foundation, the U.S. Department of Energy, and the National Science Foundation.

The research team included scientists at Drexel University, Carnegie Mellon University, and MIT; National Cheng Kung University, National Sun Yat-Sen University, and Academia Sinica based in Taiwan; Northeastern University, the National University of Singapore, Shenzen University in China, the National Institute for Materials Science in Japan, and Cornell University.