An artist's rendering of a running wave of polarization that excites the surface plasmon wakes. Courtesy of Daniel Wintz, Patrice Genevet and Antonio Ambrosio.
Capasso's team generated running waves of polarization that propagated faster than the phase velocity of the plasmons along a 1D metamaterial. This created wakes analogous to those responsible for Cherenkov radiation.
The metamaterial, a nanostructure of rotated slits etched into a gold film, changes the phase of the surface plasmons generated at each slit relative to each other, increasing the velocity of the running wave. The nanostructure also acts like a boat's rudder, allowing the wakes to be steered by controlling the speed of the running wave.
The angle of incidence and photon spin angular momentum of the light shining onto the metamaterial determines the speed of the running wave of polarization and thus provides an additional measure of control. The team also discovered that using polarized light can even reverse the direction of the wake relative to the running wave — like a wake traveling in the opposite direction of a boat.
"Being able to control and manipulate light at scales much smaller than the wavelength of the light is very difficult," said graduate student Daniel Wintz. "It's important that we not only observed these wakes but found multiple ways to control and steer them."
The observation itself was challenging, as "surface plasmons are not visible to the eye or cameras," said Antonio Ambrosio, a postdoctoral fellow at Harvard and researcher at the Italian Research Council. "In order to view the wakes, we used an experimental technique that forces plasmons from the surface, collects them via fiber optics and records the image."
This work could represent a new testbed for wake physics across a variety of disciplines, the researchers said.
"This research addresses a particularly elegant and innovative problem in physics which connects different physical phenomena, from water wakes to sonic booms, and Cherenkov radiation," said Patrice Genevet, a former Harvard postdoctoral scholar currently affiliated with the Singapore Institute of Manufacturing Technology.
Funding came from the National Science Foundation and U.S. Air Force Office of Scientific Research.
The research was published in Nature Nanotechnology (doi: 10.1038/nnano.2015.137).
For more information, visit seas.harvard.edu.