Refraction in 2D Materials

Atoms in the crystal lattice of tantalum disulfide arrange themselves into six-pointed stars that can be manipulated by light. The phenomenon can be used to control the material’s refractive index. It could become useful for 3D displays, virtual reality and in lidar systems for self-driving vehicles. (Source: W. Li, Rice U.)

Micro­scopic crystals in tantalum disulfide have a starring role in what could become a hit for 3D displays, virtual reality and even self-driving vehicles. A two-dimen­sional array of the material has unique optical charac­teristics that can be controlled in ambient conditions and under general illu­mination, according to engineer Gururaj Naik and graduate student Weijian Li of Rice’s Brown School of Engi­neering. When they pull a two-dimensional sliver off a bulk sample and shine light on it, the layered material rearranges the charge density waves of electrons that flow through, altering its refrac­tive index. Light emitted along the affected axis changes its color depending on the strength of the light that goes in.

“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 strength of light input also changes its refractive index, which quanti­fies the speed at which light travels through. 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 its c-axis. It turns out that also deter­mines its 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 stimulus is 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,” he said.

The phenomenon appears to work in tantalum disulfide as thin as 10 nano­meters and as thick as a milli­meter, he said. “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.” (Source: Rice U.)

Reference: W. Li & G. V. Naik: Large Optical Tunability from Charge Density Waves in 1T-TaS2 under Incoherent Illumination, Nano. Lett., online 20 August 2020; DOI: 10.1021/acs.nanolett.0c02234

Link: Electrical & Computer Engineering, Rice University, Houston, USA

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