If you’ve had a typical childhood, at some point in time you’ve thrown a rock into a pond just to watch the little ripples that roll outwards. Anyone who has been to an ocean or a large lake has seen the wake caused by large ships and, if you are really lucky, you’ve seen an airplane create a sonic boom.
What do sonic booms, pond ripples, and wake from large ships all have in common? Wake occurs when an object travels through a medium faster than the wave it is creating. In water, this causes waves, and in the air, it causes compressive shockwaves that are otherwise known as sonic booms. Surprisingly enough, wake can exist wherever there is a wave. Researchers at Harvard’s John A. Paulson School of Engineering and Applied Sciences have recently discovered that a wake can exist even when the waves are made up of light.
Some may be quick to jab, citing my previous assertion of something travelling faster through a medium than its partner waves is impossible with light due to the physical impossibility to travel faster than the speed of light, C (~3.00 x 10^8 m/s). However, the speed of C is the speed of light in a vacuum, and it is true that it is impossible for an item to physically go faster than the speed of light due to the infinite mass and infinite energy paradox.
Contrary to what one would assume though, the phase velocity of light in a medium can be broken, and it will indeed produce a wake. Consider Vavilov-Cherenkov radiation, in which a particle emitted from a reactor, typically an electron, travels faster than the medium’s velocity of light. Light-speed in a vacuum is C, however, in water the speed of light is only 0.75 C. Matter during a nuclear reaction, or particle acceleration test, can easily be accelerated beyond the speed of light inside a medium.
It is even possible to achieve Cherenkov radiation effects with no minimum particle velocity by using a periodic medium. Specifically, a Bloch-wave phenomena occurring within a medium reduces the C value immensely. Federico Capasso and Vinton Hayes, the two researchers dedicated to the project, managed to create a nanostructure of rotated slits etched into a gold film, which altered the phase velocity of the surface plasmons at each slit relative to the other slits. Plasmons are the quantum unit of plasma oscillation, but in layman’s terms, this is simply the reaction that causes metallic items to be shiny in the visible spectrum, as well as reflect their specific colors. In the gold medium that Capasso and Hayes were using, it would originally appear, well, gold.
Capasso’s team utilized this faster-than-light wave of charge along the metamaterial to produce the wake they were looking for. The team also discovered that the angle of the light shining onto the metamaterial provided an additional measure of control onto the wake. Polarized light –when wave vibrations occur in a single plane– could even cause the wake to reverse itself. Since the actual surface plasmons are invisible to the naked eye, the team had to use an experimental technique that forced the plasmons from the surface, and collects the image through a fiber optic system. Despite the challenges, the team was able to glean the information they needed, as well as information leading to future research into nano-optics and nanotechnology as a whole.
Daniel Wintz, graduate student in Capasso’s lab states that “Being able to control and manipulate light at scales much smaller than the wavelength of the light is very difficult, it’s important that we not only observed these wakes but found multiple ways to control and steer them.”
I encourage anyone interested to look into the original research paper, published in Nature Nanotechnology, or to check out Harvard’s original press release.
This article originally appeared on TheMittani.com, written by Kristoff Merkas.
