Novel research led by Vanderbilt and Fritz Haber Institute successfully confines powerful, long-wavelength light to the nanoscale that could enable advances in terahertz optics and optoelectronic devices

Josh Caldwell, professor of mechanical engineering and Director of the Interdisciplinary Materials Science graduate program at Vanderbilt University, and Alex Paarmann of the Fritz Haber Institute, led an international collaborative research project that successfully demonstrated the confinement of terahertz (THz) light to nanoscale dimensions using a new type of layered material. This could lead to improvements in opto-electronic devices such as infrared emitters used in remote controls and night vision and terahertz optics desired for physical security and environmental sensing.

The research, Ultraconfined terahertz phonon polaritons in hafnium dichalcogenides, led by Caldwell and Paarmann in collaboration with Prof. Lukas M. Eng from the Technische Universität Dresden (TUD), Germany, was published in Nature Materials on Sept. 15, 2025.

While THz technology promises high-speed data processing, integrating it into compact devices has been challenging due to its long wavelength. Traditional materials have struggled to confine THz light effectively, limiting the potential for miniaturization.

To address this, the research team used hafnium dichalcogenides, a type of layered material composed of hafnium and chalcogen elements like sulfur or selenium. By employing phonon polaritons (a type of quasiparticle resulting from the coupling of photons with lattice vibrations in a crystal). They achieved extreme confinement of THz light, compressing the THz wavelengths of over 50 microns in length to dimensions that were less than 250 nanometers. This was accomplished with minimal energy loss, paving the way for more energy-efficient THz devices.

“A collaborator, Artem Mishchenko, put this advance in context, making the analogy that the over 200-fold compression of light waves is akin to taking ocean waves and confining them to a teacup,” Caldwell noted.

The team’s collaborative research has focused on understanding how light and matter interact at the nano- to atomic scale, their influence on nonlinear optics, and how such changes differ from bulk materials. This involves the sub-diffractional confinement of light using polaritons within the optical spectral domain (primarily the infrared), the design of nanoscale optical components, and identifying and characterizing novel optical, electro-optical, and electronic materials.

“This started as a summer research project for a high school student but quickly expanded into an exciting observation of unprecedented level of optical confinement,” Caldwell said.

The study emerged from a long-standing collaboration between the Berlin-based FHI, Vanderbilt, and TU Dresden, using the near-field optical microscopy end station installed by the Eng-group at the Free-Electron Laser user facility FELBE at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Germany. This end station has been developed and maintained as a user laboratory for the last 15 years in tight collaboration between TU Dresden and HZDR.

“Exploring the ultrahigh THz light compression via phonon polaritons, e.g. in hafnium dichalcogenides, requires the extreme nanoscale imaging capabilities of our near-field microscope at the HZDR free-electron laser,” said Lukas Eng of TU Dresden.

The results could lead to the development of ultra-compact THz resonators and waveguides, essential for applications in environmental sensing and security imaging. The integration of these materials into van der Waals heterostructures (structures made by stacking layers of two-dimensional materials with weak vertical interaction) could further enhance the capabilities of 2D materials research, offering new opportunities for nanoscale opto-electronic integration.

The researchers said that the study not only highlights hafnium dichalcogenides as a promising platform for THz applications but also sets the stage for exploring new physics through ultra-strong or even deep-strong light-matter coupling. The findings suggest a future where high-throughput materials screening could identify even more effective materials for THz technology, driving innovation in this critical field.

“Our work with hafnium dichalcogenides shows how we can push the boundaries of THz technology, potentially transforming how we approach opto-electronic integration,” Paarmann said.

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