Tag Archives: All-angle negative refraction and active flat lensing of ultraviolet light

Better photolithography and nanoscale manipulation and manufacturing (maybe) with flat lenses

A flat spray-on lens sounded only mildly intriguing as per the May 23, 2012 University of British Columbia news release (UBC engineer helps pioneer flat spray-on optical lens) on EurekAlert. It was the May 24, 2013 news item on ScienceDaily that provided more exciting possibilities,

For the first time, scientists working at the National Institute of Standards and Technology (NIST) have demonstrated 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.

The May 24, 2013 NIST news release, which originated the news item, describes some of the optical principles at work,

An article published in the journal Nature* explains that 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.

Naturally occurring materials such as air or water have a positive refractive index. You can see this when you put a straw into a glass of water and look at it from the side. The straw appears bent and broken as a result of the change in index of refraction between air, which has an index of 1, and water, which has an index of about 1.33. Because the refractive indices are both positive, the portion of the straw immersed in the water appears bent forward with respect to the portion in air.

The negative refractive index of metamaterials causes light entering or exiting the material to bend in a direction opposite what would occur in almost all other materials. For instance, if we looked at our straw placed in a glass filled with a negative-index material, the immersed portion would appear to bend backward, completely unlike the way we’re used to light behaving.

In 1967, Russian physicist Victor Veselago described how a material with both negative electric permittivity and negative magnetic permeability would have a negative index of refraction. (Permittivity is a measure of a material’s response to an applied electric field, while permeability is a measure of the material’s response to an applied magnetic field.)

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, which are curved. 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 has also 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.

It seems the metamateriels that solve the problem posed by lenses made of glass present a few problems of their own (from the NIST news release),

… 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 of light scientists want to manipulate, the smaller these features need to be, which makes fabricating the structures an increasingly difficult task. 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.

Moreover, because of limitations inherent in their design, 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 these problems, 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. They adapted the design to work in the UV—a frequency range of particular technological interest.

According to co-authors Xu, Amit Agrawal and Henri Lezec, 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. And because its unique design consists of a stack of strongly coupled waveguides sustaining backward waves, the metamaterial exhibits a negative index of refraction to incoming light regardless of its angle of travel.

This realization of a Veselago flat lens operating in the UV is the first such demonstration of a flat lens at any frequency beyond the microwave. By using other combinations of materials, it may be possible to make similarly layered metamaterials for use in other parts of the spectrum, including the visible and the infrared.

The metamaterial flat lens achieves its refractive action over a distance of about two wavelengths of UV light, about half a millionth of a meter—a focal length challenging to achieve with conventional refractive optics such as glass lenses. Furthermore, transmission through 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.”

I would have offered some information about what they are spraying onto the lens but neither the NIST nor the University of British Columbia (UBC) news releases provides any details about the ‘spray-on’ aspect of this flat lens. There is this from the UBC news release,

“The idea of a flat lens goes way back to the 1960s when a Russian physicist came up with the theory,” Chau [Kenneth Chau, an assistant professor in the School of Engineering at UBC's Okanagan campus] says. “The challenge is that there are no naturally occurring materials to make that type of flat lens. 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.”

The research team has developed a substance that can be affixed to surfaces like a glass slide and turn them into flat lenses for ultraviolet light imaging of biological specimens.

“Curved lenses always have a limited aperture,” he explains. “With a flat lens, suddenly you can make lenses with an arbitrary aperture size – perhaps as big as a football field.”

While the spray-on, flat lens represents a significant advancement in technology, it is only an important first step, Chau says.

“This is the closest validation we have of the original flat lens theory,” he says. “The recipe, now that we’ve got it working, is simple and cost-effective.

For those who want to pursue the research paper, here’s a link to and a citation for it,

All-angle negative refraction and active flat lensing of ultraviolet light by Ting Xu, Amit Agrawal, Maxim Abashin, Kenneth J. Chau, & Henri J. Lezec. Nature 497, 470–474 (23 May 2013) doi:10.1038/nature12158  Published online 22 May 2013

The paper is behind a paywall.