Vertical Hyperbolic Metamaterials Enhance Optical Spin Hall Effect
[The research team led by Professor Junsuk Rho of POSTECH demonstrated the optical spin Hall effect in a vertical hyperbolic metamaterial for the first time. Their research is featured as a cover story on American Chemical Society (ACS)]
When bowling, we see that a ball rolling in a curved pattern that looks like a bow. This curved ball has the strongest destructive power to strike pins but it is difficult to pitch a curved ball. Such phenomenon can also be found in the optics. Recently, the domestic research team has studied how the “optical spin Hall effect” can be augmented multiple times with a metamaterial.
Professor Junsuk Rho of Mechanical Engineering and Chemical Engineering Department at POSTECH and his PhD students Minkyung Kim and Dasol Lee first demonstrated large optical spin Hall effect with high efficiency with the use of artificially designed metamaterials and meta-surface. Their findings have been recently posted as a cover story on the international journal of optics, ACS Photonics.
When light is refracted, it transports transversely with respect to the plane of incidence and such a phenomenon is called the optical spin Hall effect. In the visible spectrum where human eyes can see, it is extremely small which is measured in nanometers and therefore difficult to observe.
So far, hyperbolic metamaterials that exhibit high level of anisotropy have been studied to augment the optical spin Hall effect. Hyperbolic metamaterial is a multi-layered structure of metals and nonmetals much thinner than wavelength. It has a property of metals in a certain direction and a property of nonmetals in other directions. Therefore, it can increase the optical spin Hall effect as it has both properties.
However, in the previous works, large optical spin Hall effect necessarily involves low efficiency which is why various experiments failed to obtain both high transmission and large optical spin Hall effect. Moreover, the transmission efficiency was usually lower than 0.1% when using hyperbolic metamaterials and its practical applications seemed to be impossible.
This time, the research team used vertically stacked hyperbolic metamaterials instead of horizontally stacked ones used before. By doing so, they were able to suggest a method to increase both the optical spin Hall effect and transmission efficiency at the same time. Also, they proved that the vertical hyperbolic metamaterials have thousands of times and more optical spin Hall effect and hundreds of times and more transmission efficiency than the horizontal hyperbolic metamaterial under the same conditions of material combinations and thickness.
Before their demonstration, it was theoretically known that the optical spin Hall effect exists but it had been difficult to observe. Now, their findings validated that the vertical hyperbolic metamaterials can enhance the optical spin Hall effect and this can be utilized for different purposes.
Practical applications in various fields are possible with the optical spin Hall effect observed in a vertical hyperbolic metamaterial which is suggested by the research team. Especially, it can be used in optical devices such as polarization-dependent functional filter, sensor, switch, beam splitter. It is anticipated that it will be utilized in the optical fields of integrated circuit and communications.
Professor Junsuk Rho showed his anticipation of this research in his comment, “Our experiment exhibited the optical spin Hall effect in a vertical hyperbolic metamaterial for the first time. We were also able to obtain the optical spin Hall effect with high transmission efficiency. I hope our findings will lead to further studies on a vertical hyperbolic metamaterial and the optical spin Hall effect.”
Their research was supported by the Ministry of Science and ICT of Korea and National Research Foundation of Korea.