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Exploiting nanostructured surfaces

Researchers are using a variety of new nanofabrication techniques to introduce pillars, columns, rods and other nanostructures on polymers, ceramics and metals.

Our increasing ability to access the nanoworld is profoundly affecting the way humans communicate, utilize energy and combat disease.

One way to create a nanostructure is to ‘cut it out’ of a larger piece of material; this is known as the ‘top-down’ approach. Two techniques commonly used to do this are electron-beam and ion-beam lithography.

Scientists are also having some success in scaling down manufacturing techniques, such as injection molding, to introduce nanoscale features onto a surface.

In three NSF-funded projects, Lehigh scientists are using nanofabrication techniques to introduce nanostructures onto the surface of a metal, a ceramic and a polymer.

Their goal is to develop technologies that could one day have an impact on our everyday lives.

Moving closer to an all-optical network
As consumers demand ever-faster communications and data processing, electrical engineers are seeking to replace today's electronics with photonic (or light-based) circuitry.

Electronic devices, however, contain nanoscale components, while the optical fibers in conventional microscale photonic circuits are orders of magnitude larger.

As photonic circuitry decreases in size, the dimensions of their optical components approach the wavelength of light (450 to 700 nanometers for visible light). This causes the light waves to diffract and the signal to be lost.

One way around this problem is to exploit a type of electromagnetic wave called a surface plasmon polariton (SPP), which forms when a light wave couples (or sets up a resonance) with collective oscillations of the electrons in the surface of a metal.

Because of the high electromagnetic fields associated with this resonance, these electromagnetic waves are confined to a very thin region along the metal surface.

“In order to build photonic devices that can store, reroute and transmit data, we must first find a way to trap these electromagnetic waves,” says Filbert J. Bartoli, professor of electrical and computer engineering. “One way to do this is to slow them down to a standstill.”

Light can be slowed down by passing it through a material with a high refractive index. And because different colors, or wavelengths of light, travel at different speeds, a prism can split a beam of white light into its separate colors.

Could this principle be applied to plasmonic waves?
Bartoli and Qiaoqiang Gan ‘10 Ph.D., now assistant professor of electrical engineering at the State University of New York in Buffalo, set out to find a way to control the refractive index of the interface along which a plasmonic wave travels.

After performing theoretical simulations, Gan predicted that the lightwaves could be trapped with a nanograting – a structure consisting of a series of nanogrooves cut into a metal surface. By gradually increasing the depth of these grooves along the grating, the frequency of the collective oscillations in metal's surface electrons decreases.

“At some point along the grating, light of a certain wavelength can no longer propagate, bringing the SPP to a standstill,” says Bartoli. “SPPs of different wavelengths would be trapped at different positions along the grating, enabling white light to be separated out into its separate colors.”

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A graded "nanograte" traps a "rainbow" of lightwaves at varying depths, heralding the development of all-optical devices.