Schematic of UV metamaterial flat lens showing alternating nanolayers of silver (green) and titanium dioxide (blue). Illuminated with UV light, a sample object placed on the slab is projected as a 3D image in free space on the other side of the slab. Credit: Lezec/NIST
In what is being called a first at any frequency beyond the microwave, scientists working at the National Institute of Standards and Technology have demonstrated a flat lens that bends and focuses ultraviolet light to create 3D images of objects that float in free space.
Developed in collaboration with researchers from the Maryland NanoCenter at the University of Maryland, Syracuse University, and the University of British Columbia and reported in an article in Nature (subscription required), the device is fabricated from alternating nanolayers of silver and titanium dioxide. The resulting metamaterial lens has a negative refractive index, enabling it to project a 3D image of any object placed on the lens. Scientists say the device is easy to build and could lead to improved photolithography, nanoscale manipulation and manufacturing, and high-resolution 3D imaging, among other applications.
The metamaterial has a negative index of refraction, meaning it causes light to essentially flow backward. Such a capability doesn’t exist in nature—naturally occurring materials such as air or water have positive refractive indexes—but Russian physicist Victor Veselago postulated in 1967 that materials with negative electrical permittivity and negative magnetic permeability would have a negative index of refraction. He also theorized that a material with a refractive index of -1 could be used to make a flat (as opposed to curved) lens, and that such a lens would be able to project 3D images into free space.
It took more than 30 years after Veselago’s prediction to develop Veselago lenses that worked at microwave, infrared and visible wavelengths; making lenses that work at shorter UV wavelengths requires features as small as 10 nm. The NIST researchers adapted a design proposed by a group at Holland’s FOM Institute for Atomic and Molecular Physics, producing a sandwich of alternating nanometer-thick layers of silver and titanium dioxide they say is easy to make and has a negative index of refraction regardless of the angle of incidence of incoming light. The scientists believe using other materials combinations may make similar lenses possible for use in other parts of the electromagnetic spectrum.
The article is, “All-angle negative refraction and active flat lensing of ultraviolet light,” T. Xu, A. Agrawal, M. Abashin, K.J. Chau and H.J. Lezec, Nature (doi:10.1038/nature12158).