Plasmon wave propagates for 80 µm with no diffraction

A needlelike beam of light that propagates for an unprecedented distance of 80 µm without spreading could greatly reduce signal loss for on-chip optical systems and might eventually aid development of more powerful microprocessors.

The cosine-Gauss plasmon beam, caused by quasiparticles called surface plasmons, remains very narrow and controlled along an unprecedented distance, say 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 at Harvard SEAS have characterized and created a “needle beam,” or cosine-Gauss plasmon beam, which travels efficiently at the interface of gold and air. Top: simulated results; bottom: experimental results.

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

A fundamental problem that has hindered development of such optical interconnects is that all waves naturally spread laterally, a phenomenon known as diffraction, during propagation. This reduces the portion of the signal that can actually be detected.

“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, the 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.”

The researchers, led by Federico Capasso, have demonstrated that the needle beam propagates up to 80 µm without diffraction. The advance may help develop ultrafast, energy-efficient microprocessors.

To demonstrate, lead author Jiao Lin, a visiting postdoctoral fellow at SEAS from 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,” Genevet said. Capasso’s team hopes the findings will help develop microprocessors that are more powerful and energy-efficient.

The findings were published online in Physical Review Letters (doi: 10.1103/physrevlett.109.093904). The work was partially supported by the Air Force Office of Scientific Research.