Scientists have developed new methods using laser pulses to control electrons in graphene with unprecedented speed and precision, opening the door for a new generation of ultra-fast electronic devices. These breakthroughs, from two independent research teams, demonstrate the ability to both guide electrons without scattering and to generate them at precise locations within the wonder material.
Graphene, a single layer of carbon atoms arranged in a honeycomb lattice, has long been hailed for its remarkable electronic properties. Its potential for revolutionizing electronics has been extensively studied since its discovery, which was awarded the Nobel Prize in Physics in 2010. However, controlling the behavior of electrons within this material has been a significant challenge. Now, new research shows how ultrafast lasers can overcome this hurdle, paving the way for nanoelectronics that could operate at speeds far beyond current technologies.
Observing Ballistic Electron Motion
In a significant advance, a team at the University of Kansas has, for the first time, directly observed the “ballistic” motion of electrons in graphene. This phenomenon, where electrons move without colliding with other particles, could be the key to creating faster and more energy-efficient electronics. The findings, published in the journal ACS Nano, detail how researchers tracked electrons moving at an astonishing 22 kilometers per second.
Normally, electron movement in solids is chaotic, characterized by frequent collisions with other particles—as many as 10 to 100 billion times per second. These collisions slow the electrons, generate waste heat, and limit the speed of current silicon-based devices to mere centimeters per second. “Without collisions, an electron would move uninterrupted within a solid, similar to cars on a freeway or ballistic missiles through theair,” explained Ryan Scott, a doctoral student and lead author of the study. “We refer to this as ‘ballistic transport.'”
An Innovative Four-Layer Structure
To observe this ballistic motion, the University of Kansas team, led by Professor Hui Zhao, had to overcome a major challenge: electrons in graphene remain mobile for only a trillionth of a second before being recaptured by the “holes” they leave behind. To extend this timeframe, the researchers engineered a novel four-layer structure, sandwiching two layers of graphene between two single-layer materials, molybdenum disulfide and molybdenum diselenide. This design effectively separated the electrons from the holes, keeping the electrons mobile for about 50 trillionths of a second—long enough to be tracked by their ultrafast laser setup.
Pump-Probe Laser Technique
The researchers used an all-optical, ultrafast laser technique to both initiate and observe the electron’s movement. A “pump” laser pulse energized the electrons, setting them in motion, while a “probe” pulse, arriving a fraction of a second later, detected the subtle changes in light reflectance caused by the electrons’ presence. This allowed the team to map the electrons’ path in real-time and real space. Their measurements revealed that the electrons moved ballistically for approximately 20 trillionths of a second. Scott noted that this extended mobility was crucial for their observations. “Separating them with two layers of molecules, with a total thickness of just 1.5 nanometres, forces the electrons to stay mobile for about 50-trillionths of a second, long enough for the researchers, equipped with lasers as fast as 0.1 trillionths of a second, to study how they move.”
Pinpointing Electron Generation
In a parallel breakthrough, researchers at Kiel University in Germany have demonstrated that laser pulses can be used to generate electrons at specific, predetermined locations within graphene nanostructures. This level of control, detailed in the journal Physical Review Research, could lead to the development of electronic components that operate at petahertz speeds—a thousand times faster than the gigahertz speeds of modern computers.
The team, led by Dr. Jan-Philip Joost and Professor Michael Bonitz, used sophisticated computer simulations to model the behavior of electrons in graphene nanoribbons when subjected to a spatially uniform laser pulse. Their work showed that by carefully selecting the laser’s properties, such as its frequency, polarization, and phase, it is possible to excite electrons in a highly localized manner, effectively turning parts of the material on and off.
Harnessing Graphene’s Unique Properties
This remarkable effect is rooted in the unique, topology-based electronic structure of graphene nanoribbons. The Kiel team’s simulations, based on a theoretical framework known as the nonequilibrium Green functions approach, revealed that a uniform laser field could induce ultrafast charge separation on a subnanometer scale within a few femtoseconds. This means that even though the laser shines on the entire nanostructure, electrons are generated only in specific, targeted areas. This innovative approach translates the concept of energy absorption to a spatial domain, allowing for precise control over electron placement.
Implications for Future Electronics
Together, these two advancements mark a new era in the manipulation of electrons in graphene. The ability to guide electrons without scattering and to create them at will in precise locations opens up a vast design space for novel electronic devices. The University of Kansas team’s work on ballistic transport directly addresses the issues of speed and energy efficiency. “Ballistic transport of electrons in graphene can be utilized in devices with fast speed and low energy consumption,” said Professor Zhao. By minimizing the collisions that cause heat and slow down current electronics, devices based on ballistic transport could be significantly more powerful.
The research from Kiel University, on the other hand, offers a pathway to incredibly fast switching speeds. The ability to control electron localization with femtosecond precision could enable the development of petahertz-scale transistors and other nanoelectronic components. This level of control is a fundamental requirement for pushing the boundaries of information processing.
Overcoming Technical Hurdles
Both research teams faced significant technical challenges. The University of Kansas group had to contend with extremely weak optical signals, requiring them to average over 80 million measurements for each data point to obtain clear results. Professor Zhao acknowledged the difficulty, stating, “This requires the experimental setup to be stable for a long period of time. It took some tricks and tedious work to get it done.” For the Kiel team, the challenge lay in the immense computational cost of their simulations. Traditional methods scaled poorly with time, making their femtosecond-scale simulations unfeasible. They developed a new “G1–G2 scheme” to overcome this limitation, enabling them to model the complex quantum dynamics of the system accurately.
The next steps for both teams involve refining their techniques and exploring new device designs. The University of Kansas researchers plan to investigate how to prolong the ballistic motion of electrons, possibly by cooling their samples to lower temperatures. The Kiel team will likely continue to explore the possibilities of their spatial control method in more complex nanostructures. These pioneering efforts in controlling electrons in graphene are laying the groundwork for a future where electronics are not only faster and more efficient but also built on entirely new principles of quantum control.
