High-speed electron camera reveals new ‘light twisting’ behavior in ultrathin materials

SLAC's high-speed electron camera discovers new 'light-twisting' behavior in ultrathin material

An image taken by SLAC’s high-speed electron camera, the Ultrafast Electron Diffraction (MeV-UED) instrument, showing evidence of circular polarization of terahertz light by an ultrathin sample of tungsten ditelluride. Credit: Nano Letters (2024). DOI: 10.1021/acs.nanolett.4c00758

While taking images with a high-speed electron camera at the Department of Energy’s SLAC National Acceleratory Laboratory, scientists have discovered a new behavior in an ultrathin material that offers a promising approach to manipulating light that will be useful in devices that detect, control, or emit light, collectively known as optoelectronic devices, and study how light is polarized in the material. Optoelectronic devices are used in many technologies that touch our everyday lives, including light-emitting diodes (LEDs), fiber optics, and medical imaging.

As stated in Nano LettersA team led by SLAC and Stanford University professor Aaron Lindenberg has discovered that when aligned in a specific direction and exposed to linear terahertz radiation, an ultrathin layer of tungsten ditelluride, which has the desired light-polarizing properties used in optical devices, circularly polarizes incident light.

Terahertz radiation lies between microwaves and infrared regions of the electromagnetic spectrum and enables new ways of characterizing and controlling the properties of materials. Scientists would like to find a way to use this light to develop future optoelectronic devices.

To capture the behavior of a material in terahertz light, you need a sophisticated instrument capable of capturing interactions at very high speeds. SLAC’s world-leading ultrafast electron diffraction (MeV-UED) instrument at the Linac Coherent Light Source (LCLS) is adept at doing just that.

While MeV-UED is typically used to visualize the motion of atoms by measuring the scattering of electrons after a sample is struck by an electron beam, this new work used femtosecond electron pulses to visualize the electric and magnetic fields of the incoming terahertz pulses that caused the electrons to move back and forth. In the study, circular polarization was indicated by images of the electrons that showed a circular pattern rather than a straight line.

High-speed electron camera reveals new 'light twisting' behaviour in ultra-thin material

This illustration shows how electrons moved in a circular pattern (right) after a thin material (center) was hit by linearly polarized terahertz radiation (left). Source: Nano Letters (2024). DOI: 10.1021/acs.nanolett.4c00758

The ultrathin material was just 50 nanometers thick. “That’s 1,000 to 10,000 times thinner than what we typically need to trigger this type of response,” Lindenberg said.

Scientists are excited about the possibility of using these ultrathin materials, known as two-dimensional (2D) materials, to make optoelectronic devices smaller and more capable of performing more functions. They imagine building devices from layers of 2D structures, like stacking Lego blocks, Lindenberg said. Each 2D structure would be composed of a different material, precisely aligned to generate a specific type of optical response. These different structures and functionalities could be combined into compact devices that could find potential applications—for example, in medical imaging or other types of optoelectronic devices.

“This work adds another layer to our toolkit for manipulating terahertz light fields, which in turn could enable interesting new ways of controlling materials and devices,” Lindenberg said.

More information:
Edbert J. Sie et al., Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal, Nano Letters (2024). DOI: 10.1021/acs.nanolett.4c00758

Provided by SLAC National Accelerator Laboratory

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