Scientists working at the National Institute of Standards and Technology (NIST) have demonstrated for the first time a new type of lens that bends and focuses ultraviolet (UV) light in such an unusual way that it can create ghostly, 3D images of objects that float in free space.* The easy-to-build lens could lead to improved photolithography, nanoscale manipulation and manufacturing, and even high-resolution three-dimensional imaging, as well as a number of as-yet-unimagined applications in a diverse range of fields.
"Conventional lenses only capture two dimensions of a three-dimensional object," says NIST's Ting Xu. "Our flat lens is able to project three-dimensional images of three-dimensional objects that correspond one-to-one with the imaged object."
The new lens is formed from a flat slab of metamaterial with special characteristics that cause light to flow backward—a counterintuitive situation in which waves and energy travel in opposite directions, creating a negative refractive index. The new material is a realization of a theory first proposed in 1967 by Russian physicist Victor Veselago. In addition to describing how a negative refraction index could occur, Veselago reasoned that a material with a refractive index of -1 could be used to make a lens that is flat, as opposed to traditional refractive lenses. A flat lens with a refractive index of -1 could be used to directly image three-dimensional objects, projecting a three-dimensional replica into free space.
A negative-index flat lens like this also has been predicted to enable the transfer of image details substantially smaller than the wavelength of light and create higher-resolution images than are possible with lenses made of positive-index materials such as glass.
Researchers have managed to create a number of metamaterials, which are engineered on a subwavelength scale, to achieve Veselago's vision. But until now, making metamaterials that work in the UV has been impossible because it required making structures with features as small as 10 nanometers, or 10 billionths of a meter.
Researchers working at NIST took inspiration from a theoretical metamaterial design recently proposed by a group at the FOM Institute for Atomic and Molecular Physics in Holland. Aside from achieving record-short wavelengths, their metamaterial lens is inherently easy to fabricate. It doesn't rely on nanoscale patterns, but instead is a simple sandwich of alternating nanometer-thick layers of silver and titanium dioxide, the construction of which is routine.
Their lens has a focal length of about half a millionth of a meter—challenging to achieve with conventional refractive optics such as glass lenses—and the metamaterial can be turned on and off using higher frequency light as a switch, allowing the flat lens to also act as a shutter with no moving parts.
"Our lens will offer other researchers greater flexibility for manipulating UV light at small length scales," says Lezec. "With its high photon energies, UV light has a myriad of applications, including photochemistry, fluorescence microscopy and semiconductor manufacturing. That, and the fact that our lens is so easy to make, should encourage other researchers to explore its possibilities."
The new work was performed in collaboration with researchers from the Maryland NanoCenter at the University of Maryland, College Park; Syracuse University; and the University of British Columbia, Kelowna, Canada.
For more details, see the May 23, 2013, NIST news announcement, "The Better to See You With: Scientists Build Record-Setting Metamaterial Flat Lens."