Microscopy has come a long way in centuries since its inception, now allowing scientists to view specimens as tiny as single cells and viruses with unprecedented clarity. However, there are still limitations to how much can be seen with even the most advanced microscopes, and the potential to observe nanoscale particles up close and in living color has yet to be realized. Researchers at Pohang University of Science & Technology (POSTECH) have now shown that such a feat could one day be achieved through the use of new super lenses based on vertical hyperbolic metamaterials.
The vertical hyperbolic metamaterial (vHMM) developed at POSTECH exhibits a negative refractive index in the entire visible domain for the first time, according to the researchers. HMMs have unique optical properties that allow them to freely control light, but most study around these materials has been focused on horizontal HMMs, or hHMMs, as these are more easily fabricated that vHMMs. Horizontal HMMs, with multilayered stacks of metal and dielectric layers, only exhibit negative refraction within a narrow bandwidth. The researchers in this study used critical layer thickness analysis to design a vHMM by identifying the maximum layer thickness at which negative refraction was possible. Through this process, they were able to develop a material that could be fabricated with conventional nanoprocessing equipment.
The team tested their vHMM-based super lens, which was composed of gold and copolymer resist (EL8), and demonstrated negative refraction in a wavelength band of 450-550 nm. Theoretically, a vHMM-based lens can exhibit broadband negative refraction including the entire visible domain, but a bandwidth of 100nm was used for this study due to limitations in materials and technology. The fabrication and demonstration of this new vHMM shows promise for designing and manufacturing future materials exhibiting broadband negative refraction. This research was published in the journal Nanophotonics.
“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,” said engineering professor Junsuk Rho, who led the research. “It has important significance as it shows great potential for industrialization of nano-optics, such as generation of accurate images of virus and bacteria.”
These metamaterials could be applicable to technologies such as ultra-wideband ultra-high resolution thin-film lenses and ultra-high-resolution full-color optical microscopes. With a negative diffraction index in the entire visible range, these materials could one day reveal the colors of single-cell organism, bacteria and viruses, allowing researchers to view this specimens in whole new light.
Photo: A diagram showing negative refraction in a metamaterial (color is RGB of visible light). Credit: POSTECH