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2D Material Takes on Chirality of Circularly Polarized Light

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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.

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). 

Published: March 2020
Glossary
optoelectronics
Optoelectronics is a branch of electronics that focuses on the study and application of devices and systems that use light and its interactions with different materials. The term "optoelectronics" is a combination of "optics" and "electronics," reflecting the interdisciplinary nature of this field. Optoelectronic devices convert electrical signals into optical signals or vice versa, making them crucial in various technologies. Some key components and applications of optoelectronics include: ...
quantum
The term quantum refers to the fundamental unit or discrete amount of a physical quantity involved in interactions at the atomic and subatomic scales. It originates from quantum theory, a branch of physics that emerged in the early 20th century to explain phenomena observed on very small scales, where classical physics fails to provide accurate explanations. In the context of quantum theory, several key concepts are associated with the term quantum: Quantum mechanics: This is the branch of...
chirality
Chirality is a property of certain molecules and objects in which they are non-superimposable on their mirror images. In other words, a chiral object or molecule cannot be exactly superimposed onto its mirror image, much like a left and right hand. The term "chirality" comes from the Greek word cheir, meaning hand, emphasizing the handedness or asymmetry of the object or molecule. A molecule or an object with this property is said to be chiral, while its non-superimposable mirror image is...
Research & TechnologyeducationMassachusetts Institute of TechnologyMITLight SourcesMaterials2D materialsOpticsoptoelectronicsmid-infrared lightquantumQuantum MaterialschiralityCommunicationshandednessexotic materialsTech Pulse

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