Atomically Smooth Gold Crystals Enable Nanophotonic Applications

Researchers at KAIST and collaborators have demonstrated a platform to guide compressed lightwaves in extremely thin Van der Waals crystals. The researchers believe that the method, used to guide mid-infrared light with minimal loss, will enable practical applications of ultrathin dielectric crystals in next-generation optoelectronics devices based on strong light-matter interactions at the nanoscale.

The research targeted the practical use of phonon-polariton-based technology. Phonon-polaritons are collective oscillations of ions in polar dielectrics coupled to electromagnetic waves of light, whose electromagnetic field is much more compressed compared to the light’s wavelength. Recently, it was 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.

The scanning near-field optical microscope's nano-tip is used in the ultrahigh-resolution imaging of the image phonon-polaritons in hBN launched by the gold crystal edge. Courtesy of the Jang Research Group, KAIST.

In this context, charges in the polaritonic crystal are “reflected” in the metal, and their coupling with light results in a new type of polariton waves called the 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.

Seeking to overcome these limitations, four research groups combined efforts to develop an experimental platform using advanced fabrication and measurement methods.

A KAIST research team led by Min Seok Jang, a professor 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 63-nm-thick slab of hexagonal boron nitride (hBN) on a monocrystalline gold substrate, showing the mid-infrared lightwaves in dielectric crystal compressed by a hundred times.

Jang and research professor Sergey Menabde obtained direct images of HIP waves propagating for many wavelengths and detected a signal from the ultracompressed high-order HIP in ordinary hBN crystals, showing that the phonon-polaritons in van der Waals crystals can be significantly more compressed without sacrificing their lifetime.

The feat was made possible by the atomically smooth surfaces of the gold crystals used as a substrate for the hBN. Practically zero surface scattering and extremely small ohmic loss in gold at mid-infrared frequencies provide a low-loss environment for the HIP propagation. The HIP mode probed by the researchers was 2.4 times more compressed and yet exhibited a similar lifetime compared to the phonon-polaritons with a low-loss dielectric substrate, resulting in a twice higher figure of merit in terms of the normalized propagation length.

The ultrasmooth monocrystalline gold flakes used in the experiment were chemically grown by the team of N. Asger Mortensen, a professor in the Center for Nano Optics at the University of Southern Denmark.

The technology is expected to increase sensitivity to the mid-infrared portion of the spectrum, vitally important for sensing applications due to the number of organic molecules with absorption lines in the region. However, conventional detection methods need a large number of molecules to produce a reading. The ultracompressed 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.

The study also demonstrated the similarity between the HIP and image graphene plasmons. Both image modes possess a significantly confined electromagnetic field, though their lifetime remains unaffected by the shorter polariton wavelength. The observation provides a broader perspective on image polaritons more generally and highlights their utility in terms of nanolight waveguiding compared to the conventional low-dimensional polaritons in van der Waals crystals on a dielectric substrate.

Jang hopes that the work will pave the way for more efficient nanophotonic devices such as metasurfaces, optical switches, sensors, and others operating in the infrared region.

The research was published in Science Advances (www.doi.org/10.1126/sciadv.abn0627).