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Ballistic electrons chart a new course for next-gen terahertz devices

In a world increasingly driven by high-speed communication and low-power electronics, a team of researchers from the National University of Singapore (NUS) and Singapore University of Technology and Design (SUTD) has proposed a fundamentally new way of manipulating light using the geometry of matter itself.

Their research, "Nonlinear Optical Resonances from Ballistic Electron Funneling," which was in ACS Nano, offers a fresh take on how ballistic electrons—those that travel freely like tiny billiard balls—can generate nonlinear optical signals without needing intense laser power or exotic materials.

"The idea is simple but powerful: by designing the nanoscale geometry just right, we can steer electrons in a way that doubles the frequency of incoming light," said Dr. Hue T.B. Do, lead author.

Instead of relying on traditional nonlinear materials, which often require high-intensity lasers to operate, this new method uses optical resonators shaped like bow ties to funnel electrons through narrow junctions.

The trick lies in how these electrons scatter off the walls of the resonator. When surfaces are smooth, electrons reflect in a predictable manner—a phenomenon known as specular scattering—enabling them to move asymmetrically and generate a second harmonic signal, or light at twice the frequency.

Plasma physics meets nanophotonics

To study this intricate dance of electrons and light, the team turned to an unconventional simulation method: Particle-in-Cell (PIC) modeling, originally developed in plasma physics to track high-energy particles in space and fusion devices.

"Unlike conventional simulations that treat electrons like a fluid, our method tracks each electron individually," explained Professor Wu Lin, co-corresponding author at SUTD. "This kinetic approach is essential to capture how electron-surface interactions produce nonlinear effects."

Their simulations revealed that second-harmonic generation—a cornerstone of optical technologies like lasers and sensors—can occur at field intensities 1,000 to 10,000 times lower than those required by standard methods. This means frequency-doubling could be achieved using compact, low-power infrared sources, such as those used in portable sensors or next-generation wireless systems.

Toward tunable terahertz photonics

Beyond its theoretical elegance, the team's work offers practical design guidelines for building nanoscale terahertz (THz) rectifiers and photodetectors. Their proposed devices operate without any applied voltage and can be finely tuned by adjusting the angle and width of the bow tie geometry.

Graphene—a 2D material known for its exceptional conductivity and tunability—is a strong candidate for realizing such devices experimentally. Other high-mobility semiconductors may also benefit from the same principles.

"Our study shows that even without traditional junctions or doping, you can achieve rectification and upconversion simply by controlling the shape of the structure," said Professor Michel Bosman, co-corresponding author from the NUS Department of Materials Science and Engineering.

A roadmap for experiment

The researchers are now calling on experimentalists to join the effort. They believe the conditions for observing the effect are within reach. Specular scattering can be engineered through clean etching techniques or electrostatic doping, and the required light intensity is already achievable using existing mid-infrared sources.

"This work opens the door to efficient, passive optical devices that could one day power wearable sensors, infrared energy harvesters, or low-power wireless receivers," said Dr. Do. "And perhaps just as exciting, it shows how revisiting tools from plasma physics can yield surprising insights in nanophotonics."