Atoms in the crystal lattice of tantalum disulfide arrange themselves into six-pointed stars that can be manipulated by light, offering a way to control the material's refractive index. Image: Weijian Li/Rice University.
Atoms in the crystal lattice of tantalum disulfide arrange themselves into six-pointed stars that can be manipulated by light, offering a way to control the material's refractive index. Image: Weijian Li/Rice University.

Microscopic crystals of tantalum disulfide have a starring role in what could become a hit for 3D displays, virtual reality and even self-driving vehicles. According to engineer Gururaj Naik and graduate student Weijian Li at Rice University's Brown School of Engineering, a two-dimensional array of tantalum disulfide has unique optical characteristics that can be controlled in ambient conditions and under general illumination.

The researchers have discovered that when they pull a two-dimensional sliver off a bulk sample of tantalum disulfide (with that tried-and-true tool, adhesive tape) and shine light on it, the layered material rearranges the charge density waves of electrons that flow through, altering its refractive index. This means that light emitted along the affected axis changes its color depending on the strength of the light that goes in. The researchers report their discovery in a paper in Nano Letters.

"We need an optical material that can change the refractive index for applications like virtual reality, 3D displays, optical computers and lidar, which is necessary for autonomous vehicles," said Naik, an assistant professor of electrical and computer engineering. "At the same time, it has to be fast. Only then can we enable these new technologies."

Tantalum disulfide, a semiconducting, layered compound with a prismatic metal center, appears to fit the bill. The material is already known for harboring charge density waves at room temperature that allow adjustments to its electrical conductivity. But the researchers have now found that the strength of light input can also change the material's refractive index, which determines the speed at which light travels through it. That makes it tunable, Naik said.

When exposed to light, the tantalum layer reorganizes into a lattice of 12-atom stars, like the Star of David or sheriff's badges, that facilitate charge density waves. How these stars are stacked determines whether the compound is insulating or metallic along one of its axes, known as the c-axis.

It turns out this also determines the material's refractive index. Light triggers the stars to realign, changing the charge density waves enough to affect the material's optical constants.

"This belongs to a class of what we call strongly correlated materials, which means the electrons strongly interact with each other," Li said. "In this case, we can predict the properties that show a strong response to some external stimulus."

That the external stimulus can be as mild as ambient white light is a plus, Naik added. "This is the first material we've seen where the interaction of light happens not just with single particles, but with a collection of particles together, at room temperature." The phenomenon appears to work in tantalum disulfide layers as thin as 10nm and as thick as 1mm.

"We think this is an important discovery for those who study strongly correlated materials for applications," Naik said. "We show light is a very powerful knob to change how correlation extends in this material."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.