image: Figure | Working principle. Illustrative scenario of propagation of DPP, at Terahertz frequencies, induced by the tip of a s-SNOM microscope in topological insulators coupled nano-antennas.
Credit: Leonardo Viti et al.
In the present era of modern nano-technologies, controlling light at the smallest scales is the key to faster communications, ultra-sensitive sensors, and revolutionary imaging systems. This is where Dirac plasmon polaritons (DPPs) come into play—exotic waves that blend light and electron motion in ultra-thin, two-dimensional materials.
Unlike ordinary light waves, which are limited by the speed of light in free space, DPPs can squeeze light into spaces a hundred times smaller than its natural wavelength. This makes them incredibly powerful tools for manipulating light at the nanoscale, far beyond what traditional optics can do.
What makes DPPs particularly exciting is their behaviour in Dirac materials, such as graphene and topological insulators, where electrons act as if they have no mass. This unique property allows DPPs to be highly tunable and responsive to changes in their environment, paving the way for next-generation nano-optoelectronic devices.
Their role becomes even more critical in the terahertz (THz) frequency range—a spectral region between microwaves and infrared light that remains one of the most underutilized in science and technology. The THz gap holds tremendous potential for security scanning, wireless communications, and medical diagnostics, but controlling light at these frequencies has been a persistent challenge.
DPPs offer a solution. Because they can confine and guide THz waves at the nanoscale, they could lead to compact and efficient THz photonic components, such as detectors, modulators, and waveguides. The ability to tune and direct these waves opens the door to reconfigurable photonic circuits, with applications ranging from quantum technologies to ultra-fast computing.
In a new paper published in Light: Science & Applications, a team of scientists led by prof. Miriam Serena Vitiello have developed a new method to precisely control the behavior of Dirac plasmon polaritons (DPPs)—collective oscillations of massless charge carriers—in two-dimensional materials, opening new possibilities for advanced nanophotonic technologies.
DPPs are critical for manipulating light at the nanoscale, but their high momentum and rapid signal loss at terahertz (THz) frequencies have made them difficult to harness.
Now, a research team has demonstrated a novel approach using topological insulator metamaterials made from epitaxial Bi₂Se₃. By designing and fabricating laterally coupled nanostructures—called metaelements—with specific spacing, they were able to tune the wavevector of DPPs through geometric control.
Using advanced phase-sensitive near-field microscopy, the team successfully launched and imaged DPP propagation in these nanostructures.
The study revealed that adjusting the spacing between coupled metaelements could increase the polariton wavevector by up to 20% and extend the attenuation length by more than 50%.
“These findings represent a significant step toward the development of tunable THz optical devices with lower energy loss and enhanced performance. This breakthrough could open a new venue for THz nanophotonics, non-linear optics and energy efficient photovoltaic devices” the scientists forecast.
Journal
Light Science & Applications
Article Title
Tracing terahertz plasmon polaritons with a tunable-by-design dispersion in topological insulator metaelements