Ajou News

NEW Prof. Yeom Dong-il’s team uncovers new nonlinear optical qualities of graphene

  • 2021-03-19
  • 4434


A team of Ajou University researchers has uncovered a new optical phenomenon in graphene, widely regarded as a material that holds the key to the future. The team’s findings are expected to pave the way for developing new applications, including superspeed optical signal processing using two-dimensional materials, quantum communications, and quantum sensing.


Prof. Yeom Dong-il (Departments of Physics and Energy Systems Research, pictured left) and his team have discovered that interlaying two stacks of single-layer graphene can produce considerably more nonlinear optic signals. Their findings were published under the title, “Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene,” in the January 21 online issue of Light: Science & Applications (IF = 13.714, IF (%) = 2.577).


The paper lists two of Ajou’s doctoral students, Ha Seong-ju (pictured middle) and Park Nam-hun (pictured right), as first authors. The credit also goes to Profs. Lee Jae-hyun (Department of Materials Science and Engineering), Yoo Young-dong (Department of Chemistry), Park Ji-yong (Department of Physics), and Ahn Kwang-jun (Department of Energy Systems Research), as well as Prof. Jung Je-il (Department of Physics) from the University of Seoul.


Graphene, a two-dimensional material well known for its composition of pure carbon atoms, has been garnering much attention around the world for its high conductivity and transparency as well as flexibility—all the properties desired in next-generation materials for electronic and optical diodes. Stacking layers of single-layer graphene at an angle produces Moiré superlattices. Graphene has recently captured researchers’ attention as the angle at which these layers are stacked can produce dramatically different results, including both a superconductor with zero resistance and a perfect insulator. The Moiré pattern refers to superlattices that emerge with much greater frequency than with the original and denser patterns of nets or fabrics layered together at an angle.


Prof. Yeom’s team seized upon the nonlinear optic phenomenon that occurred when strong light was projected onto their twisted bilayer graphene (tBLG). The Moiré superlattices of the tBLG, in particular, gave rise to van Hove singularities (i.e., points at which the state density function of electrons peak-like discontinuities) with the state density of electrons. Moreover, the team discovered that, when the energy gap between the van Hove singularities tripled the energy of light being projected, significant enhancement occurred in the third-harmonic generation (THG) efficiency. Based on this finding, the team was able to increase the nonlinear optic signals of the electrically controlled tBLG by as much as 60 times those of single-layer graphene.


Prof. Yeom commented: “With this study, we became the first in the world to demonstrate that the angle at which graphene is stacked can be a new significant variable when it comes to controlling and enhancing the nonlinear optic properties of two-dimensional materials. I expect that our findings will contribute to the development of new superspeed optical signal processing techniques or quantum lighting sources that utilize the nonlinear optic properties of two-dimensional materials.”


The study was conducted with support from the National Research Foundation of Korea’s Experienced Researcher Support Program and Korea Energy Technology Evaluation and Planning’s Energy Workforce Development Program. 



Left: An optical microscope image of twisted bilayer graphene. The different colors represent the areas of graphene layers stacked together at different angles of rotation/twisting. Right: The third-harmonic generation image of bilayer graphene.