Graphene Utilized to Compress Infrared Light by 1000 Times

An international team of researchers led by Professor Min Seok Jang at KAIST has obtained the first optical images of mid-infrared light waves that are trapped inside a few-nanometer-thick waveguide and compressed by 1000 times.

Article | Fall 2021

Recently, a new type of graphene plasmons – collective oscillations of free electrons coupled to the electromagnetic waves – was discovered in a very thin dielectric layer separating graphene from a metallic sheet. In such a configuration, graphene electrons are “reflected” as image charges in the metal, so when the light waves “push” the electrons in graphene, they oscillate together with their images. This new type of electronic oscillations is called “image” or “acoustic” graphene plasmons (AGPs).

The existence of AGPs has been demonstrated via indirect methods such as far-field infrared spectroscopy and photocurrent mapping. The indirect observation is the price that researchers had to pay for the strong compression of electromagnetic fields inside very thin structures. It was believed that the field intensity outside the device was insufficient for direct optical probing.

Three research groups collaborated to overcome these limitations by combining a unique experimental technique and advanced nanofabrication methods. A KAIST research team led by Professor Min Seok Jang at the School of Electrical Engineering used a highly sensitive scattering-type scanning near-field optical microscope (s-SNOM) to directly measure the electromagnetic fields of the AGP waves propagating in a nanometer-thin waveguide, visualizing a thousand-fold compression of mid-infrared light for the first time. The nanostructures were created by the team of Professor Sang-Hyun Oh from the University of Minnesota in the U.S., and graphene was synthesized at the Center for Integrated Nanostructure Physics of the Institute of Basic Science at Sungkyunkwan University led by Professor Young Hee Lee.

Despite earlier presumptions, Professor Jang and his then-postdoc Sergey Menabde successfully obtained direct images of AGP waves by taking advantage of their rapidly decaying yet always present electric field above graphene. They showed that AGP is detectable even when most of its energy is flowing inside the dielectric below graphene. When the AGP launched by the microscope’s nano-tip is reflected by the graphene edge, the resulting interference pattern can be detected by the s-SNOM that maps the distribution of the field amplitude s under the nano-tip (Figure 2a). The interference pattern reveals information about the plasmon wavelength and propagation loss.

Researchers measured the interference of AGP near-fields in the dielectric layer as thin as 8 nm, while the excitation free-space wavelength was 8.7 um (Figure 2b), thereby demonstrating the compression of infrared light by more than 1000 times. Such ultra-compressed electromagnetic fields could provide strong light-matter interactions at the microscopic level, significantly improving the interaction scale down to a single molecule.

The research paper authored by Sergey Menabde and In-Ho Lee et al., titled “Real-space imaging of acoustic plasmons in large-area graphene grown by chemical vapor deposition,” was published in Nature Communications on February 19, 2021 (article number 938).