Plasmon Wave Propagates for 80 µm with No Diffraction

A needle-like beam of light that propagates for an unprecedented distance of 80 µm without spreading could greatly reduce signal loss for on-chip optical systems.

The cosine-Gauss plasmon beam, caused by quasiparticles called surface plasmons, remains very narrow and controlled along an unprecedented distance, said a Harvard University-led American and French team. The surface plasmons travel in tight confinement with a nanostructured metal surface. The metallic stripes that carry these plasmons have the potential to replace standard copper electrical interconnects in microprocessors, enabling ultrafast on-chip communications.

Researchers led by Federico Capasso at Harvard SEAS demonstrated a cosine-Gauss plasmon beam, dubbed a "needle beam," that propagates without diffraction. The advance may help develop ultrafast, energy-efficient microprocessors. (Image courtesy of Patrice Genevet)

Based at the Harvard School of Engineering and Applied Sciences (SEAS) and the Laboratoire Interdisciplinaire Carnot de Bourgogne, CNRS, in France, the applied physicists both characterized and created this needle beam, which travels efficiently at the interface of gold and air.

A fundamental problem that has so far hindered the development of such optical interconnects is that all waves naturally spread laterally, or diffract, during propagation. This reduces the portion of the signal that can actually be detected.

Federico Capasso (left), Patrice Genevet (right) and an international team of colleagues have demonstrated a new type of tightly controlled lightwave that could eliminate signal loss in on-chip optical devices. (Photo by Eliza Grinnell, Harvard SEAS Communications)

"We have made a major step toward solving this problem by discovering and experimentally confirming the existence of a previously overlooked solution of Maxwell's equations that govern all light phenomena," said principal investigator Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS. "The solution is a highly localized surface plasmon wave that propagates for a long distance — approximately 80 microns in our experiments — in a straight line without any diffraction."

Top: Simulated results; bottom: experimental results. (Image courtesy of Patrice Genevet)

To demonstrate the propagation, lead author Jiao Lin, a visiting postdoctoral fellow at SEAS from the Singapore Institute of Manufacturing and Technology, and co-author Patrice Genevet, a research associate in Capasso's group, sculpted two sets of grooves into a gold film that was plated onto the surface of a glass sheet. These tiny grooves intersect at an angle to form a metallic grating. When illuminated by a laser, the device launches two tilted, plane surface waves, which interfere constructively to create the nondiffracting beam.

"Our French colleagues did a beautiful experiment, using an ultrahigh-resolution microscope to image the needle-shaped beam propagating for a long distance across the gold surface," said Genevet.

Top: A micrograph and diagram of the metallic gratings that produce the needle beam. Bottom: An approximation of the experimental setup. A laser is focused from the glass substrate side onto the device. Once the nondiffractive surface wave is created, detailed information on its intensity distribution is gathered using an ultrahigh-spatial-resolution near-field scanning optical microscope. (Images courtesy of Patrice Genevet)

Capasso's team hopes the findings will help develop microprocessors that are more powerful and energy-efficient.

Their findings were published online Aug. 31 in Physical Review Letters. The work was partially supported by the US Air Force Office of Scientific Research.

For more information, visit: seas.harvard.edu