Researchers at Stanford University have taken a significant step closer to the creation of a Harry Potter-style invisibility cloak with the development of a broadband metamaterial capable of manipulating nearly the entire visible spectrum.
Scientists have long believed that the creation of an invisibility cloak capable of successfully hiding objects from sight requires the use of artificially-engineered materials—metamaterials—with a negative refractive index over the entire visible spectrum. While previous experimentation has yielded cloaking devices capable of camouflaging objects from microwave, near-infrared and terahertz waves, devices capable of cloaking obkects over broad bandwidths of visible light have remained only a concept.
Now, researchers at Stanford University have succeeded in developing a broadband metamaterial that exhibits a negative refractive index more than double that of any previously-developed metamaterial. According to the research team, the new material exhibits a refractive index—the degree to which a material bends light’s path—significantly less than anything found in nature.
“The library of refractive indexes that nature gives us is limited,” Jennifer Dionne, an assistant professor of materials science and engineering and an affiliate member of the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory, said. “All natural materials have a positive refractive index.”
In contrast, artificially-made metamaterials offer the ability to alter the refractive index to near-zero or negative values. While the optical properties of natural materials depend on the chemistry of the constituent atoms, the optical properties of a metamaterial are derived from the geometry of its nanoscale unit cells, or “artificial atoms,” which can be altered to change the refractive index of the artificial material.
However, Dionne explains, such a material needs to interact with both the electric and magnetic fields of light in order to successfully disguise an object. Previous metamaterial research efforts have created artificial atoms comprised of two constituents—one that interacts with the electric field, and a second one that interacts with the magnetic field.
Dionne says this approach presents a drawback, however. Because the individual constituents interact with different colors of light, it is often difficult to make them overlap over a broad range of wavelengths to successfully shield an object.
To bypass this issue, Dionne and her colleagues developed a single metamaterial “atom” with characteristics that enable it to interact with both the electric and magnetic components of light. The new metamaterial reportedly exhibits “a negative refractive index over a wavelength range of roughly 250 nanometers in multiple regions of the visible and near-infrared spectrum.” Researchers said the addition of slight changes to its structure would make it useful across the entire visible spectrum.
