An artistic representation of the nano-tip used for the ultra-high-resolution imaging of the image phonon-polaritons in hBN at the gold crystal edge. Image: Jang Research Group.
An artistic representation of the nano-tip used for the ultra-high-resolution imaging of the image phonon-polaritons in hBN at the gold crystal edge. Image: Jang Research Group.

Researchers at the Korea Advanced Institute of Science and Technology (KAIST), together with collaborators, have successfully demonstrated a new platform for guiding compressed light waves in very thin van der Waals crystals. Their method for guiding mid-infrared light with minimal loss will provide a breakthrough for the practical application of ultra-thin dielectric crystals in next-generation optoelectronic devices based on strong light-matter interactions at the nanoscale.

Phonon-polaritons are collective oscillations of ions in polar dielectrics coupled to electromagnetic waves of light, but the electromagnetic field of phonon-polaritons is much more compressed than the original light wavelength.

Recently, scientists demonstrated that the phonon-polaritons in thin van der Waals crystals can be compressed even further when the material is placed on top of a highly conductive metal. In such a configuration, charges in the polaritonic crystal are ‘reflected’ in the metal, and their coupling with light results in a new type of polariton called image phonon-polaritons. Highly compressed image modes provide strong light-matter interactions, but are very sensitive to the substrate roughness, which hinders their practical application.

Challenged by these limitations, four research groups combined their efforts to develop a unique experimental platform using advanced fabrication and measurement methods. The groups report their findings in a paper in Science Advances.

A KAIST research team, led by Min Seok Jang from the School of Electrical Engineering, used a highly sensitive scanning near-field optical microscope (SNOM) to directly measure the optical fields of the hyperbolic image phonon-polaritons (HIP) propagating in a 63nm-thick slab of hexagonal boron nitride (h-BN) on a monocrystalline gold substrate. The team found that the mid-infrared light waves in the dielectric crystal are compressed 100 times.

Jang and a research professor in his group, Sergey Menabde, successfully obtained direct images of these HIP waves propagating for many wavelengths, and detected a signal from the ultra-compressed high-order HIP in regular h-BN crystals for the first time. They showed that the phonon-polaritons in van der Waals crystals can be significantly more compressed without sacrificing their lifetime.

This was possible due to the atomically smooth surfaces of the home-grown gold crystals used as a substrate for the h-BN. At mid-infrared frequencies, there is practically zero surface scattering and extremely small ohmic loss on gold, which provides a low-loss environment for HIP propagation. The HIP mode probed by the researchers was 2.4 times more compressed and yet exhibited a similar lifetime to the phonon-polaritons in a low-loss dielectric substrate, resulting in a figure of merit that was twice as high in terms of the normalized propagation length.

The ultra-smooth monocrystalline gold flakes used in the experiment were chemically grown by the team of Asger Mortensen from the Center for Nano Optics at the University of Southern Denmark.

The mid-infrared spectrum is particularly important for sensing applications, as many important organic molecules have absorption lines in the mid-infrared, but a large number of molecules are required by conventional detection methods for successful operation. In contrast, the ultra-compressed phonon-polariton fields can provide strong light-matter interactions at the microscopic level, thus significantly improving the detection limit down to a single molecule. The long lifetime of the HIP on monocrystalline gold will further improve the detection performance.

Furthermore, Jang and his team also demonstrated the striking similarity between the HIP and image graphene plasmons. Both image modes possess a significantly more confined electromagnetic field, yet their lifetime remains unaffected by the shorter polariton wavelength. This observation provides a broader perspective on image polaritons in general, and highlights their superiority for nanolight waveguiding compared to the conventional low-dimensional polaritons in van der Waals crystals on a dielectric substrate.

“Our research demonstrated the advantages of image polaritons, and especially the image phonon-polaritons,” said Jang. “These optical modes can be used in future optoelectronic devices where both the low-loss propagation and the strong light-matter interaction are necessary. I hope that our results will pave the way for the realization of more efficient nanophotonic devices such as metasurfaces, optical switches, sensors, and other applications operating at infrared frequencies.”

This story is adapted from material from KAIST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.