Scientists Control Light Wakes For The First Time

Artist's rendition of the wake produced by a fast-moving charge in surface plasmons on gold. Daniel Wintz, Patrice Genevet, and Antonio Ambrosio

A wake has been created on a metal surface out of “light-like waves” known as plasmons. In the process, the creators proved that they can control the wake in ways that may allow the production of new types of holograms and focus light with a precision far smaller than a single wavelength.

When an object travels through a medium, or the medium flows past it, a wake is created. Even slow-moving objects can create wakes, but things get dramatic when the object is moving faster than the waves it generates, leading to the sonic booms of a bullwhip and the splashy lakes created by speedboats. The faster the object, relative to the speed of the medium, the wider the angle of the wake.

Moving faster than a water or sound wave is one thing, but surely you can't get this kind of effect in light, because nothing can move faster than light? Not so fast. Nothing can move faster than light in a vacuum, but light slows down in other media, leading to the bending effects of glass or water. Strange as it may seem, in these environments it is possible for objects to travel faster than the local speed of light.

Hollywood didn't invent the eerie blue glow associated with underwater nuclear reactors. This glow is the consequence of Chernekov radiation, where electrons or other charged particles travel faster than light's speed in water. Moving charges disrupt the local electromagnetic field, and at sufficient speeds this creates a shockwave that peaks in the ultraviolet but looks blue to our eyes.

Plasmons are the smallest unit of plasma oscillation, analogous to photons. They form a type of quasiparticle and shape the optical properties of metals. Surface plasmons, as the name suggests, are those that exist only at the surface of a material, and therefore interact with light falling on it.

In Nature Nanotechnology, Harvard University scientists have described creating wakes in the surface plasmons of a one-dimensional metallic nanostructure, formed from tiny rotated slits etched in gold. The speed was determined by the angle of incidence of incoming light and the angular momentum of the photons.

“By changing either one of these properties we demonstrate controlled steering of the Cherenkov surface plasmon wakes,” the paper reports.

"The ability to control light is a powerful one," said Professor Federico Capasso, in whose lab the work was done. "Our understanding of optics on the macroscale has led to holograms, Google Glass and LEDs, just to name a few technologies. Nano-optics is a major part of the future of nanotechnology and this research furthers our ability to control and harness the power of light on the nanoscale."

Lead author Daniel Wintz noted, "Being able to control and manipulate light at scales much smaller than the wavelength of the light is very difficult.” Consequently, the capacity to control the wakes both through the angles of the etched slits and the incident light could prove very useful.

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