Experimental Demonstration of Negative Refraction at Visible Frequency
[POSTECH professor Junsuk Rho develops a negatively refractive super lens based on a vertical hyperbolic metamaterial that operates in the entire visible spectrum.]
[The new technology enables super high-resolution color imaging.]
What color is the coronavirus? Conventional optical microscopes are unable to generate high-resolution, colored images of viruses and bacteria. Expensive equipment is used to view them in a more real way, but post-processing often takes long and the resulting images are only available in black and white.
To this, a POSTECH research team led by Professor Junsuk Rho (Department of Mechanical Engineering and Chemical Engineering) and Ph.D. candidate Hanlyun Cho (Department of Mechanical Engineering) has developed a super lens based on a vertical hyperbolic metamaterial that exhibits a negative refractive index (negative refraction) in the entire visible domain for the first time. By using a hyperbolic metamaterial – a material that freely controls light – negative refraction can be achieved that cannot exist in natural materials.
When negative refraction occurs in the entire visible spectrum, high-resolution images can be generated beyond the diffraction limit*1. Original colors of viruses and bacteria can be viewed, which were previously only available in black and white.
However, when using a horizontal hyperbolic metamaterial with multilayered stacks of horizontal metal and dielectric layers, negative refraction was only possible in a narrow bandwidth. Theoretically, negative refraction is possible in a wide bandwidth by using a vertical hyperbolic metamaterial, but it has never been demonstrated due to its difficult fabrication process.
For this, the research team studied the relationship between the properties of hyperbolic metamaterials and the maximum layer thickness at which negative refraction is possible, and designed a hyperbolic metamaterial that can be sufficiently fabricated with the conventional nanoprocessing equipment.
The researchers experimentally verified that the newly developed metamaterial exhibits negative refraction in the wavelength band of 450 to 550 nanometers (nm). Theoretically, the vertical hyperbolic metamaterial can exhibit broadband negative refraction including the entire visible domain. However, there are limitations in materials and technology to achieve this goal, so 100nm bandwidth was used in this study.
As the design and processing method of vertical hyperbolic metamaterials are presented in this study, it shows promise that the metamaterials exhibiting broadband negative refraction can be designed and manufactured in the future. These metamaterials are also applicable in ultra-wideband ultra-high-resolution thin-film lenses, ultra-high-resolution full-color optical microscopes, and many others. For the first time in 50 years, this study has enabled the observation of negative refraction in the entire visible spectrum since the Russian mathematician Victor Veselago mathematically predicted negative refraction in 1968.
“We have confirmed the possibility of improving the narrow bandwidth – the biggest drawback of the conventional horizontal hyperbolic metamaterial – by experimentally verifying the vertical hyperbolic metamaterial in this study,” remarked Professor Rho who led the study. “It has important significance as it shows great potential for industrialization of nano-optics, such as generation of accurate images of virus and bacteria.”
The findings from this study were recently published in Nanophotonics, a prestigious journal that ranks in the top 10% in the field of optics according to the Journal Citation Report (JCR). The study was supported by the Mid-Career Researcher Program of the National Research Foundation of Korea.
1. Diffraction limit
The resolution limitation of an optical microscope where light appears superimposed as it disperses.