High-speed electron camera discovers new ‘light-distorting’ behavior in ultra-thin material

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

As reported in Nano lettersThe team, led by SLAC and Stanford professor Aaron Lindenberg, found that when oriented in a specific direction and exposed to linear terahertz radiation, an ultrathin film of tungsten ditelluride, which has desirable properties for polarizing light used in optical devices, circularly polarizes the incoming light.

Terahertz radiation lies between the microwave and infrared regions of the electromagnetic spectrum and enables new ways to characterize and control the properties of materials. Scientists are eager to find a way to harness that light to develop future optoelectronic devices.

To capture the behavior of a material under terahertz light, a sophisticated instrument is needed that can record the interactions at ultra-high speed. SLAC’s flagship instrument for ultrashort electron diffraction (MeV-UED) at the Linac Coherent Light Source (LCLS) can do just that.

While the MeV-UED is normally used to visualize the motion of atoms by measuring how they scatter electrons after hitting a sample with a beam of electrons, this new work used the femtosecond electron pulses to visualize the electric and magnetic fields of the incoming terahertz pulses, which caused the electrons to wiggle back and forth. In the study, circular polarization was indicated by images of the electrons that showed a circular pattern instead of a straight line

The ultrathin material was just 50 nanometers thick. “This is 1,000 to 10,000 times thinner than what we normally need to have to cause this type of response,” Lindenberg said.

Researchers are excited about using these ultrathin materials, known as two-dimensional (2D) materials, to make optoelectronic devices smaller and more functional. They envision creating devices from layers of 2D structures, like stacking Legos, Lindenberg said. Each 2D structure would be made 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 is a new element in our toolbox for manipulating terahertz light fields, which in turn could open up new ways to control materials and devices in interesting ways,” Lindenberg said.