Optical loss turned into a beneficial tool for controlling light direction

The interaction between light and materials is complex, involving absorption, reflection, and conversion into thermal energy. When light encounters a metallic surface, it also interacts with the electrons within the metal, resulting in what we call “optical loss.”

Traditionally, producing ultra-small optical components that utilize light has been challenging due to increased optical loss as the size decreases. However, in recent years, researchers have started exploring the non-Hermitian theory in optics, which takes a different perspective on optical loss. Instead of considering it as a drawback, this theory embraces optical loss and seeks to harness its potential. This unconventional approach has led to groundbreaking discoveries in physics, turning what was once perceived as an imperfection into a hidden opportunity. It’s like finding a “blessing in disguise” – an unexpected stroke of luck in the world of physics.

A team of researchers, including Prof. Junsuk Rho from POSTECH and Ph.D. candidates Heonyeong Jeon and Seokwoo Kim, along with Prof. Yongmin Liu from Northeastern University, collaborated on a study that demonstrated the control of light beams using non-Hermitian meta-grating systems. Their work was recently published in Science Advances, garnering significant attention in the scientific community.

Visualization of light incident on a metagrating and its conversion into unidirectional SPPs. (Simulation). Credit: POSTECH

When light interacts with a metal surface, it induces collective oscillations of electrons known as surface plasmon polaritons (SPPs). To manipulate the direction of SPPs, a common approach involves using a “grating coupler” as an auxiliary device. However, these couplers have limitations as they unintentionally convert incident light at right angles into SPPs in unintended directions.

In this study, the research team employed the non-Hermitian theory to address this limitation. They began by calculating the exceptional point, a theoretical concept where a specific optical loss occurs. Subsequently, they conducted experiments using their specially designed non-Hermitian meta-grating coupler to validate its efficacy. Remarkably, the meta-grating coupler demonstrated exceptional control over SPPs, enabling unidirectional manipulation that was nearly impossible with conventional grating couplers. By adjusting the size and spacing of the meta-gratings, the researchers even achieved the propagation of light and SPPs in opposite directions. Moreover, they successfully reversed the conversion of incident light into SPPs back to normal light using the same meta-grating device.

Observation of the interference pattern between the SPP propagating to the right and the SPP reflected by the metagrating. Due to the unidirectionality of the metagrating, the SPP does not transmit through the metagrid in the opposite direction. Credit: POSTECH

The research outcomes hold significant potential for quantum sensor research across diverse domains, including the detection of disease antigens for diagnostics or harmful gases in the atmosphere. When combined with engineering, these findings can unlock a multitude of applications. Prof. Junsuk Rho, the team leader, expressed the importance of the research by stating that it has extended the realm of non-Hermitian optics into the nano-scale domain. This advancement will undoubtedly contribute to the advancement of future plasmonic devices, characterized by exceptional control over directionality and enhanced performance.

Source: Pohang University of Science and Technology

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