‘Plasmonic Nanoantennas’ Promising for Optics

Arrays of tiny “plasmonic nanoantennas” have been developed that can precisely manipulate light in new ways, potentially enabling optical innovations such as more powerful microscopes, computers and telecommunications.

Researchers at Purdue University have used the nanoantennas to abruptly change the phase of light. Their findings were published online Dec. 22 in Science.

“By abruptly changing the phase, we can dramatically modify how light propagates, and that opens up the possibility of many potential applications,” said Vladimir Shalaev, scientific director of nanophotonics at Purdue’s Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering.

The nanoantennas are V-shaped gold structures formed on top of a silicon layer. They are an example of metamaterials, which typically include so-called plasmonic structures that conduct clouds of electrons called plasmons. The antennas are 40 nm wide, and researchers have demonstrated that they can transmit light through an ultrathin plasmonic nanoantenna layer about 50 times smaller than the wavelength of light it is transmitting.

(Upper left) A schematic for an array of gold plasmonic nanoantennas that can precisely manipulate light in new ways, a technology that could make possible optical innovations including more powerful microscopes, telecommunications and computers. (Upper right) A scanning electron microscope image of the structures. (Bottom) The experimentally measured refraction angle versus incidence angle for light, demonstrating how the nanoantennas alter the refraction. (Image: Purdue University Birck Nanotechnology Center)

“This ultrathin layer of plasmonic nanoantennas makes the phase of light change strongly and abruptly, causing light to change its propagation direction, as required by the momentum conservation for light passing through the interface between materials,” Shalaev said.

The work extends findings conducted by scientists led by Federico Capasso, the Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at Harvard School of Engineering and Applied Sciences (SEAS). In that work, described in an October Science paper, the Harvard researchers modified Snell’s law, a long-held formula used to describe how light reflects and refracts while passing from one material into another. (Other research by Capasso can be seen here).

Until now, Snell’s law had implied that when light passes from one material to another, there are no abrupt phase changes along the interface between the materials. The Harvard researchers’ experiments, however, showed that the phase of light and the propagation direction could be changed dramatically by using metamaterials, which in this case were based on an array of antennas.

The Purdue researchers took the work a step further, creating arrays of nanoantennas and changing the phase and propagation direction of light over a broad range of near-infrared light. The wavelength size manipulated by the antennas in the Purdue experiment ranged from 1 to 1.9 µm.

“The near-infrared, specifically a wavelength of 1.5 microns, is essential for telecommunications,” Shalaev said. “Information is transmitted across optical fibers using this wavelength, which makes this innovation potentially practical for advances in telecommunications.”

The Harvard researchers predicted how to modify Snell’s law and demonstrated the principle at one wavelength.

“We have extended the Harvard team’s applications to the near-infrared, which is important, and we also showed that it’s not a single frequency effect, it’s a very broadband effect,” Shalaev said. “Having a broadband effect potentially offers a range of technological applications.”

The innovation could bring technologies for steering and shaping laser beams for military and communications applications, nanocircuits for computers that use light to process information, and new types of powerful lenses for microscopes.

Critical to the advance is the ability to alter light so that it exhibits “anomalous” behavior: It bends in ways not possible using conventional materials by radically altering its refraction, a process that occurs as electromagnetic waves bend when passing from one material into another. Scientists measure this bending of radiation by its index of refraction. All natural materials, such as glass, air and water, have positive refractive indices.

However, the nanoantenna arrays can cause light to bend in a wide range of angles, including negative angles of refraction.

“Importantly, such dramatic deviation from the conventional Snell’s law governing reflection and refraction occurs when light passes through structures that are actually much thinner than the width of the light’s wavelengths, which is not possible using natural materials,” Shalaev said. “Also, not only the bending effect, refraction, but also the reflection of light can be dramatically modified by the antenna arrays on the interface, as the experiments showed.”

For more information, visit: www.purdue.edu or to see Shalaev’s presentation on the future of optics, it is available on demand here.