Ultra fast laser tracking the "ballistic" motion of electrons in graphene

Figure 1. The setup of Hui Zhao and his team at the University of Kansas Ultra Fast Laser Laboratory.
A team of researchers from the University of Kansas's ultrafast laser laboratory recently managed to capture real-time ballistic transmission of electrons in graphene, which could lead to faster, more powerful, and more energy-efficient electronic devices in the future.

The motion of electrons is often interrupted by collisions with other particles in the solid, approximately 10 to 100 billion times per second. This will slow down the speed of electrons, resulting in energy loss and generating excess heat. If these collisions can be prevented, then electrons can move unobstructed in solids similar to ballistic missiles as they propagate in the air.

"The ultrafast laser provides ultra-high time resolution and is one of the fastest experimental tools," said Hui Zhao, a professor of physics and astronomy. The ballistic transmission of electrons in solids occurs on a very short time scale, so studying ballistic transmission through ultrafast lasers to track the motion of electrons is a perfect match.

Previous electrical measurements have revealed the characteristics of ballistic transportation, "but in reality, tracking their ballistic motion in real-time and real space is cool," Zhao added. This provides a non-invasive and non-destructive tool for monitoring electrons in solids.

The research team observed the ballistic motion in graphene, which is composed of a single layer of carbon atoms forming a hexagonal lattice structure. Graphene is hailed as a magical material because of its unusual properties that enable faster and more efficient next-generation electronic devices.

"Light provides energy to electrons to release them, allowing them to move freely and leaving a 'hole' in their original position," Zhao said. However, electrons in graphene can only maintain a movement of about one trillionth of a second before falling back into the cavity, making tracking their motion a challenge.

To address this issue, the team designed and manufactured a four layer artificial structure, with two layers of graphene separated by molybdenum disulfide and molybdenum diselenide. "By inserting two single-layer semiconductors between two graphene layers, we separate electrons and holes, so that electrons do not fall back into holes quickly, providing us with enough time window to analyze the properties of electron motion," Zhao said.
A two-layer molecule with a total thickness of only 1.5 nanometers "forces electrons to keep moving for about 50 trillion parts of a second," said Dr. Ryan Scott, who conducted the experiment. For researchers equipped with lasers, this is enough to track the motion of electrons at a speed of 0.1 trillion parts per second.

The measurement device of the team is a transient absorption microscope based on ultrafast laser, which can analyze the motion of electrons at the nanoscale spatial resolution. "Our technology tracks moving electrons in graphene by affecting light reflection - they slightly increase the reflectivity of the sample at its position," Zhao said. This allows us to use laser pulses to track their movement.

In other words, they use a tightly focused laser pulse called a "pump pulse" to release electrons from the sample. They track the reflectivity of the sample by drawing another focused laser pulse called a "probe pulse", which reaches the sample at a later time.

In order to detect such small changes, they released 20000 electrons at once and used a probe laser to reflect the sample and measure this reflectivity. The team repeated this process 80 million times for each data point. Figure 2 shows an example of their key results in the displacement vs. time graph of small electrons, with straight lines representing uniform motion. Therefore, researchers have concluded that electrons move ballistic at an average speed of 22 kilometers per second, at a speed of approximately 20 trillion parts per second, and then encounter something that terminates their ballistic motion.

Figure 2. The relationship between electron displacement and time in graphene, plotted by ultrafast laser measurement.

Compared to electrical detection technology, their all optical ultrafast laser technology provides the high resolution required to explore electron transfer in ballistic and coherent states.

One of the most surprising aspects of the team's work was that their initial testing confirmed the effectiveness of their device structure design. "Electrons are indeed separated from holes by two monolayers and remain in motion for a longer period of time," Zhao said. So we know we have a great opportunity to track their ballistic motion. Our team has been studying charge transfer in van der Waals heterostructure types for 10 years, so we are pleased to see that we can use these artificial structures to fine tune electrons and keep them moving for longer periods of time.

What is the biggest challenge? Due to weak optical signals, the team had to average many measurement values to obtain conclusive features. "This requires the experimental setup to remain stable for a long period of time," Zhao said. "It requires some skills and tedious work to complete."

The real good news is that ballistic electronic transmission is fast and non scattering, so electronic devices using ballistic transmission may be faster, more powerful, and more energy-efficient, thereby reducing latency and heat issues.

"Now that we have a 'radar gun' that monitors ballistic electronic motion, we will attempt to use it as a tool to study how to control electronic motion using electric fields and other means," Zhao said. We also want to explore new device designs to extend the ballistic transmission length of electrons. The samples in this study were stored at room temperature. Cooling the samples to a lower temperature can also extend their ballistic length.

This project has received support from the US Department of Energy, and Ryan Scott's work has been supported by the Redeker Scholarship and Graduate Research Award from the University of Kansas.

Source: Laser Net