Creating And Controlling Light, All In One Quantum Material

New research shows that WTe₂ forms self-cavities that, without needing an external power source, can control light emission at terahertz frequencies.

In a new paper published in Nature Communications, a research team led by Columbia physicist James McIver revealed that a quantum material called tungsten ditelluride (WTe₂) can both create and control ultrafast light signals. The results pave the way to shrinking telecommunications, electronics, and imaging systems, while also speeding them up.

WTe₂ is a transition metal dichalcogenide (TMD) that can be peeled into thin layers with unique optical properties. One is a notably efficient nonlinear response to laser pulses: one color of light goes in, another comes out. “The way that light interacts with a material tells us important information about the material, like how its atoms are arranged and what governs its response to perturbations, while changing colors can be a way to store, manipulate, and transmit information on very fast time scales,” explained co-lead author Hope Bretscher, who recently joined Boston College as an assistant professor. Another is that where a laser is directed onto WTe2 matters: the resulting electrical current will change direction depending on where the light strikes the material.

Confining light within cavities can further enhance these effects. Cavities had conventionally been created with artificial structures, like mirrors, but last fall, McIver’s group showed that 2D devices can form them intrinsically. Since then, they have been investigating ways to use such “self-cavities” to manipulate energy moving through different materials, with a particular interest in ultrafast Terahertz (THz) frequencies, which are of interest for next-generation telecommunications.

In the current study, first author Xinyu Li, a graduate student working with McIver and Bretscher at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, used an on-chip THz spectroscopy technique to record picosecond changes in WTe_₂ flakes in response to laser pulses. “We measured the emitted light after photoexciting the sample at different positions and different intensities of the exciting light. We repeated these measurements on four different samples that had different geometries,” said Li.

When the laser struck the edges of the WTe_₂ flakes, it created an electrical current that became trapped in the cavity between the material and gold strips placed on top of it that serve as photodetectors. As the current bounced back and forth, it became concentrated at specific frequencies in the THz range—an example of what’s known as Purcell enhancement, caused by a self-cavity and without the need for an external power source.

“These findings establish WTe_₂ as a highly efficient, bias-free, coherent, and narrow-band THz emitter,” said Bretscher. All from a flake the size of a human blood cell.