Newswise — Brightness is an extremely fragile and exposed attribute. Brightness may undergo absorption or reflection on a material's surface, contingent upon the properties of the substance, or undergo transformation and convert into thermal energy. When encountering the surface of a metallic material, brightness also has a tendency to dissipate energy to the electrons within the metal, encompassing a wide array of occurrences referred to as "optical loss."

Generating ultra-compact optical elements that employ light in diverse manners poses significant challenges due to the inverse relationship between the size of an optical component and the magnitude of optical loss. Nonetheless, in recent times, the non-Hermitian theory, which harnesses optical loss in a fundamentally distinct manner, has been employed in the realm of optics exploration. Novel insights in physics are emerging as the non-Hermitian theory, which embraces optical loss, is embraced, investigating avenues to leverage this phenomenon. Unlike conventional physics, where optical loss is perceived as an imperfect aspect of an optical system, this research presents a "blessing in disguise" — an initially perceived disaster that ultimately leads to fortuitous outcomes.

Prof. Junsuk Rho from POSTECH, along with PhD candidates Heonyeong Jeon and Seokwoo Kim, both from the Department of Mechanical Engineering at POSTECH, collaborated with Prof. Yongmin Liu from Northeastern University (NEU) in Boston and their joint research team. In their remarkable achievement, they successfully manipulated the trajectory of light beams by employing non-Hermitian meta-grating systems. The groundbreaking research conducted by the team was highlighted in Science Advances, a prestigious international academic journal.

When light interacts with a metallic surface, the electrons within the metal collectively oscillate in unison with the incident light wave, giving rise to a phenomenon known as surface plasmon polariton (SPP). To manipulate the direction of SPPs, a commonly employed auxiliary device is the "grating coupler." However, the efficiency of this device is constrained by its tendency to unintentionally convert right-angle incident light into SPPs propagating in undesired directions.

The research team tackled this limitation by employing the non-Hermitian theory. Initially, they conducted calculations to determine the theoretical location of an exceptional point, which is associated with a specific optical loss. Subsequently, they conducted experiments using a specially designed non-Hermitian meta-grating coupler to validate its efficacy. The meta-grating coupler exhibited remarkable effectiveness in enabling precise control over the direction of SSPs, a feat that was nearly unattainable with conventional grating couplers. By manipulating the size and spacing of the meta-gratings, the team successfully achieved the propagation of light and SPPs in opposite directions. Moreover, they demonstrated the capability to revert the conversion of incident light into SSPs back to normal light, utilizing the same meta-grating device.

The research outcomes hold significant potential for advancing quantum sensor research across multiple domains, including the detection of disease antigens for diagnostic purposes and harmful gases in the atmosphere. When combined with engineering, these findings pave the way for a diverse range of applications. Prof. Junsuk Rho, the team leader, expressed the significance of the research, stating that it has effectively extended the realm of non-Hermitian optics to the nano-scale domain. The implications are expected to contribute to the advancement of future plasmonic devices, characterized by exceptional direction control and performance capabilities.

 The research was funded by the US National Science Foundation, Samsung Science and Technology Foundation, and the National Research Foundation of Korea.

Journal Link: Science Advances