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  • Spray-on flat lens works in the UV

Photonics Spectra
Aug 2013

GAITHERSBURG, Md. – A new, easily fabricated metamaterial-based flat lens that bends and focuses UV light could improve photolithography, nanoscale manipulation and manufacturing, and even high-resolution 3-D imaging, say its developers, scientists working at the National Institute of Standards and Technology (NIST).

The lens is formed by making a simple sandwich of alternating nanometer-thick layers of silver and titanium dioxide. The resulting slab is a metamaterial – a man-made substance with a negative index of refraction: Light entering or exiting the material bends in a direction opposite what would occur in almost all other materials. The lens is also flat, unique in that almost all lenses today – whether in an eye, a camera or a microscope – are curved.

“Curved lenses always have a limited aperture,” said team member Kenneth Chau of the University of British Columbia. “With a flat lens, suddenly you can make lenses with an arbitrary aperture size – perhaps as big as a football field.”

Russian physicist Victor Veselago first described the idea of a flat lens created from a negative-index material in a 1967 paper, but it took more than 30 years for such a material to be created.

“The challenge is that there are no naturally occurring materials to make that type of flat lens,” Chau said. “Through trial and error – and years of research – we have come up with a fairly simple recipe for a spray-on material that can act as that flat lens.”

For the past decade, scientists have made metamaterials that work at microwave, infrared and visible wavelengths by fabricating repeating metallic patterns on flat substrates. However, the smaller the wavelength that scientists want to manipulate, the smaller these features must be, making fabrication increasingly difficult. Until now, making metamaterials that work in the UV had been impossible because it required making structures with features as small as 10 nm.

A team at NIST created a UV metamaterial formed of alternating nanolayers of silver (green) and titanium dioxide (blue). When illuminated with UV light (purple), a sample object of any shape placed on the flat slab of metamaterial is projected as a 3-D image in free space on the other side of the slab. Here, a ring-shaped opening in an opaque sheet on the left of the slab is replicated in light on the right.

Because of their inherent limitations, metamaterials of this type designed for infrared and visible wavelengths have, so far, been shown to impart a negative index of refraction to light that is traveling only in a certain direction, making them hard to use for imaging and other applications that rely on refracted light.

To overcome this, the NIST researchers took inspiration from a theoretical metamaterial design recently proposed by a group at FOM Institute for Atomic and Molecular Physics in Amsterdam, Netherlands. They adapted the design to work in the UV – a frequency range of particular technological interest.

Because their lens doesn’t rely on nanoscale patterns, it is inherently easy to fabricate, the authors say.

“Our lens will offer other researchers greater flexibility for manipulating UV light at small-length scales,” said researcher Henri 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 lens achieves its refractive action over a distance of about two wavelengths of UV light – a focal length that is challenging to achieve with conventional refractive optics such as glass lenses. Also, transmission through the metamaterial can be turned on and off using higher-frequency light as a switch, also enabling the flat lens to act as a shutter with no moving parts.

The new work was performed in collaboration with researchers at the Maryland NanoCenter of the University of Maryland in College Park, Syracuse University in New York and the University of British Columbia in Kelowna, Canada.

“This is the closest validation we have of the original flat lens theory,” Chau said. “The recipe, now that we’ve got it working, is simple and cost-effective. Our next step is to extrapolate this technique further, explore the effect to the fullest and advance it as far as we can take it.”

The work appears in Nature (doi: 10.1038/nature12158).

An opening or hole through which radiation or matter may pass.
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
A material engineered from artificial matter not found in nature. The artificial makeup and design of metamaterials give them intrinsic properties not common to conventional materials that are exploited as light waves and sound waves interact with them. One of the most active areas of research involving metamaterials currently explores materials with a negative refractive index. In optics, these negative refractive index materials show promise in the fabrication of lenses that can achieve...
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