A research team led by Assistant Professor of Materials Science and Engineering Jennifer Dionne has made a significant step towards the eventual creation of an invisibility cloak, having designed a metamaterial that interacts with a wide range of wavelengths of light.
Dionne collaborated with fourth-year doctoral student Ashwin Atre, fifth-year doctoral student Hadiseh Alaeian and postdoctoral scholar Aitzol García-Etxarri on a study published in Advanced Optical Materials detailing the team’s findings.
While the researchers’ design is new, scientists have worked with metamaterials — artificial materials that interact with light, magnetism or other natural phenomena in unusual ways — since 1999.
“A regular material derives its optical properties from the chemical nature of its constituent atoms,” Atre said. “In a metamaterial, we design the structure of these artificial atoms such that they interact with light in unnatural or extraordinary ways.”
According to García-Etxarri, Dionne’s team focused on developing a metamaterial with a negative refraction index in order to manipulate the path of light to “refract the wrong way,” which he said is not possible with regular matter.
“The index dictates how light will travel through a material, so if we can change that index or that property, we can now change how light propagates through a material,” Atre said. “That’s sort of the basis for the idea of an invisibility cloak, in that you can control light and force it to move around an object so it never interacts with the object itself.”
The research team worked on the theoretical metamaterial design for about two years, according to Atre, and the project was born out of the group’s investigation into how a crescent shape interacts with light.
Atre said that he and García-Etxarri focused on the use of a crescent shape in the design of meta-atoms, while Alaeian primarily investigated how transformation optics could be applied to the metamaterial.
“The crescent shape has been proposed in previous work because it has sharp tips and localizes the field of energy at the tips,” Alaeian said. “It utilizes the highly confined and large gradient field at the tips of the crescent.”
According to Alaeian, previously demonstrated metamaterials have mostly worked in regions of light with longer wavelengths, such as the microwave region. The metamaterial proposed by Dionne’s researchers would work with a much larger range of light wavelengths and colors.
“We were interested in creating something to work in the visible range of frequency,” she said. “Through this paper we tried to propose the idea, based on transformation optics, for the methodology of the design of these materials.”
Atre agreed that the primary challenge in developing metamaterials is determining how to allow for the material’s interaction with a wide range of electric and magnetic wavelengths, which is often prohibited by design complexities.
“To get a metamaterial to interact with everything from blue light through to red light and really cloak an object to visible light that we can see, [doing] that has remained a challenge and still is a challenge,” he said. “Our design is one step closer to that goal in that we have increased the bandwidth that it can interact with light so now it covers a bandwidth that is twice that of previous metamaterials.”
Moving forward, the group hopes to apply their theoretical design to the fabrication of an actual metamaterial. Researchers emphasized, however, that there are many barriers to be overcome before a real invisibility cloak can be created, such as fabricating the material over a large area and preventing the light that passes through the material from being diminished.
“What we did is [take] one step forward in one of the main challenges in metamaterials, which is their ability to work on a wide wavelength,” García-Etxarri said. “The applications are still very far away in terms of feasibility, but this is one step forward and we are happy with it.”