Professor Park Q Han’s research team was successful in identifying the principle behind the complete transmission of light
Now possible to develop fully anti-reflective coating and stealth technology

▲ From the left, Professor Park Q Han and co-authors Ji-Hun Kang and Ku Im
 

A research team led by Korea University Department of Physics Professor Park Q Han has identified the principle behind the complete transmission of light (i.e., with no reflection) into a medium and has succeeded in experimentally proving this with the use of metamaterials.

Until now, the complete transmission of light into a medium irrespective of that light’s incident conditions, such as wavelength, polarization, and incidence angle, has been a major challenge in optics and was once considered impossible. However, the team has proven that it can be achieved by introducing the theory of universal impedance matching. The research results were published online in the top scientific journal Nature Photonics on February 26, under the title “Universal Impedance Matching and the Perfect Transmission of White Light.”

The reflection of light is a fundamental phenomenon in optics and is the primary reason why we can actually see objects. However, at the same time, reflection indicates a loss of light, which can be disadvantageous because it decreases the efficiency of optical devices such as lenses and solar cells. Therefore, the effective control of reflection is considered to be a very important issue in optical technology, the energy industry, the development of military equipment, and other fields of science and technology. Since Newton first observed the interference of light in thin film, reflection and transmission control using interference has been a widespread subject of research. Currently, a variety of techniques have been proposed to reduce the reflection of light, such as anti-reflective coatings and moth-eye structures, but these techniques have limitations in that they operate only at certain angles of incidence or for a narrow range of wavelengths. Because the interference of light depends on the wavelength and the incident angle, it has been considered impossible to completely transmit light into a medium for all incident conditions (i.e., incident angle, wavelength, and polarization) using interference.

Beginning in 2014, Professor Park has addressed this challenge with support from the Basic Science Research for the Development of Korean Science and Technology, a public service offered by the Samsung Future Technology Promotion Foundation. Recently, by establishing the universal impedance matching theory, Professor Park and his research team have proven that the full transmission of light is possible for all incident conditions. In addition, the researchers have demonstrated through microwave experiments that metamaterials can be used to realistically implement this theory.

The universal impedance matching theory is based on the use of a special non-locality medium as an anti-reflection film. Usually the medium reacts only to light at the point where the light touches, but the non-locality medium also reacts to the light at points some distance away. The refractive index of the medium can also be altered according to the incidence angle of the light, so it is possible to achieve impedance matching that allows the complete transmission of light regardless of the incident conditions while utilizing interference. In general, materials obtained from nature show very weak non-localization, so it is difficult to meet the requirements of the universal impedance matching theory. However, the research team created a simple metamaterial using wave-structured plates as an alternative to materials with strong non-locality attributes and proved the theory to be sound based on microwave experiments. These results were presented in an introductory article in Nature Photonics to highlight the potential of this novel technique and suggest a new direction for future metamaterial research.

Research into anti-reflection technology, which began in earnest in the late 19th century, has recently focused on quartz-wave anti-reflection coatings, which have now been commercialized, and anti-reflection films that mimic multilayer thin film anti-reflection films and moth-eye structures. However, they all have limitations in that they cannot block light with a wide range of wavelengths, operate only at a certain incident angle, or have a very thick anti-reflection film. In previous work, the Park’s research team uncovered some of the fundamental principles of wavelength-independent anti-reflection and developed an anti-reflection technique that used very thin film. The latest research has since developed this method as a new way to completely block the reflection of light and enable 100 percent transmission in most common situations, including variation in wavelength, polarization, and incident angle. It is expected to bring about breakthroughs in solar cells and optical devices, where energy efficiency is important, and military technology, such as stealth technology.