Quantum materials for nonlinear terahertz photonics?

Several years ago, Klaas-Jan Tielrooij, a researcher at the Catalan Institute of Nanoscience and Nanotechnology in Spain, became intrigued by the fundamental properties of graphene and similar quantum materials—a bandgap of zero and massless charges. It turns out these properties are highly useful for electronic and optical devices.

The absence of a bandgap has rather unique implications for interactions between these materials and terahertz light.

“Terahertz light has a very small photon energy, so it doesn’t get absorbed by typical semiconductors, which have a bandgap larger than the photon energy,” says Tielrooij. “This inspired me to explore the suitability of these materials toward novel applications using terahertz light.”

At CLEO 2022, Tielrooij presented his work within this realm. It’s based on the concept that graphene and related materials like topological insulators conduct electricity in a special way and are highly efficient in upconverting terahertz photons to a higher photon energy—for example, creating 1.5 THz light from incident light at 0.5 THz.

The underlying mechanism is that these materials’ massless charges efficiently absorb terahertz light.

“Energy of this absorbed light is rapidly shared among all charges, which establish a hot carrier distribution, followed by subsequent cooling,” Tielrooij explains. “These processes take place on the femtosecond to picosecond timescale.”

These materials also exhibit terahertz nonlinearity: after terahertz absorption, they absorb less terahertz radiation. “As a result of the ultrafast heating-cooling dynamics of the electrons and their nonlinearity, terahertz light is generated with an increased photon energy, in particular 3-, 5-, or 7-times larger,” he says. “This is the generation of the third, fifth, seventh, etc. harmonic signal.”

Graphene and topological insulators not only exhibit intriguing physical properties, Tielrooij points out, but are also making their way into useful applications that exploit these properties not present in other materials systems. Terahertz harmonic generation is an important example of this trend because it can lead to electronic light sources with unprecedented frequencies in the terahertz regime (see figure).

“I find it fascinating that a single layer of electrons—either in graphene or at the surface of topological insulators—can lead to upconverting terahertz photons with a field conversion efficiency for third-harmonic generation on the order of 10% and the generation of 1 mW of third harmonic power,” Tielrooij says.

The next step is to “keep improving the efficiency, while also designing devices that exploit the exceptional properties of these quantum materials toward nonlinear terahertz photonic applications,” he adds. “One example is the possibility of electrically generating terahertz signals with carrier frequencies that were previously not attainable, such as for 6G wireless network systems.”

RELATED READING

1. J. C. Deinert et al., ACS Nano, 15, 1145 (2021); https://doi.org/10.1021/acsnano.0c08106.

2. S. Kovalev et al., npj Quantum Mater., 6, 84 (2021); https://doi.org/10.1038/s41535-021-00384-9.