Plasmon Waves Control Light

A nanoscale device that converts optical signals into surface plasmon polaritons can distinguish between different movements of polarized light and precisely manipulate the signals in one direction or another without damaging them.

“If you want to send a data signal around on a tiny chip with lots of components, then you need to be able to precisely control where it’s going,” said Balthasar Müller, a graduate student at Harvard School of Engineering and Applied Sciences (SEAS) and co-lead author of a report on the research. “If you don’t control it well, information will be lost. Directivity is such an important factor.”

The coupler, developed by Müller and colleagues in collaboration with researchers from Singapore and China, transforms incoming light into waves called surface plasmon polaritons — rippling waves of electrons that slosh around the surface of metals. It could be used to steer the direction of surface plasmon polaritons by changing the movement of the light waves, paving the way for a new generation of on-chip optical interconnects that can efficiently funnel information from optical to electronic devices.

Two devices based on the herringbone pattern developed by a Harvard-led research team were presented in the Science paper: a rectangular array and a ring-shaped array (both interpreted in this illustration). Circularly polarized light with waves that wind in opposite directions is split by both devices, with its waves routed in opposite directions. For a ring-shaped coupler, this means that plasmons are channeled either toward or away from the center of the structure. Intensity at the center of the ring can therefore be switched on and off by manipulating the polarization of the incoming light. Courtesy of Jiao Lin and Samuel Twist.

Previously, the direction of the waves could be controlled by changing the angle at which light strikes the surface of the coupler, but “this was a major pain,” Müller said. “Optical circuits are very difficult to align, so readjusting the angles for the sake of routing the signal was impractical.”

For the new coupler to work, light must come in perpendicularly. Acting like a traffic controller, the device reads the polarization of the incoming light wave — which might be linear, left-hand circular or right-hand circular — and routes it accordingly. It also can split apart a light beam and send parts of it in different directions, allowing for information transmission on multiple channels.

An electron micrograph shows the nanoscale perforations at the surface of the plasmonic coupler. Courtesy of Jiao Lin and Balthasar Müller.

The patterning is what sets the device apart from others, the researchers say. It consists of a thin, gold sheet, peppered with tiny perforations in a herringbonelike-pattern.

“The go-to solution until now has been a series of parallel grooves known as a grating, which does the trick but loses a large portion of the signal in the process,” said principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. “Now, perhaps, the go-to solution will be our structure. It makes it possible to control the direction of signals in a very simple and elegant way.”

Because the new structure is so small — each repeating unit of the pattern is smaller than the wavelength of visible light — the investigators believe it should be easy to incorporate the design into novel technologies, such as flat optics.

These images, taken with a near-field scanning optical microscope, show plasmonic waves propagating across the surface of the coupler. In the central image, linearly polarized light is captured and converted into waves that travel both left and right. In the left image, left-hand circularly polarized light is routed only to the left; in the right image, right-hand circularly polarized light is routed only to the right. Courtesy of Jiao Lin and Balthasar Müller.

It also could be incorporated into future high-speed information networks that combine nanoscale electronics with optical and plasmonic elements on a single microchip.

“This has generated great excitement in the field,” Capasso said.

Müller and Capasso worked jointly with co-lead author Jiao Lin, a former SEAS postdoctoral fellow who is now at Singapore Institute of Manufacturing Technology; co-authors Qian Wang and Guanghui Yuan, both of Nanyang Technological University in Singapore; Nicholas Antoniou, principal FIB (focused ion beam) engineer at the Harvard Center for Nanoscale Systems; and Xiao-Cong Yuan, a professor at the Institute of Modern Optics at Nankai University in China.

Part of the work was performed at the Harvard Center for Nanoscale Systems.

Details of the study were published in Science (doi: 10.1126/science.1233746).

For more information, visit: www.seas.harvard.edu

Related research that uses light to control the traveling direction of electromagnetic waves in materials was released by scientists at King’s College London and Universitat Politècnica de València. (See: Waveguides Contain Spinning Photons)