We see colour in objects because of the way light interacts with them. Different surfaces scatter or emit different wavelengths of light, giving us quantitative colour information in the process. So if colour is, arguably, a matter of perception, wasn't it only a matter of time before scientists figured out how to control the interaction between light and object to make those objects seem invisible?
A team of researchers, led by physicist Raiju Puthumpally-Joseph from the Université Paris-Sud, has developed a new way to manipulate the way light scatters, theoretically allowing us to make opaque objects seem transparent. It all hinges on a phenomenon called dipole-induced electromagnetic transparency, or DIET.
When dealing with quantum emitters like single atoms or molecules, the effects of light scattering are pretty straightforward. Photons, the quantized form of light, bounce off uneven or rough surfaces, causing the rays (particles) to be reflected away at odd angles.
The physics of light becomes significantly more complicated when dealing with two or more interacting emitters. In this case, it's impossible to separate the behaviour of emitters individually. The electromagnetic field of one emitter in a group doesn't only depend on the light beam striking a surface, it also depends on the electromagnetic field of all its neighbouring emitters, which are themselves also affected by the emitter in question. The result is a dynamic mess.
It gets even a little more complicated than that. Each of these quantum emitters can have a dipole, a positive side and a negative side, rooted in the uneven distribution of electrons within it. When a number of quantum emitters are combined, dipole-dipole couplings in a vapor can occur. This yields a complicated enhancement of the light-matter interaction.
It's these strong dipole-dipole interactions that can be manipulated to alter the spectral properties of scattered light. The frequency of light waves can be controlled such that the medium through which the light is traveling can be made to appear somewhat transparent.
This is dipole-induced electromagnetic transparency, DIET, and it's the result of the destructive interference between the electromagnetic waves emitted by quantum emitters. It's a phenomenon that could be used to generate slow light or even stopped light, purely by altering the way the light interacts with the medium through which it's traveling.
There are a number of uses for this phenomenon, though most are largely experimental. Puthumpally-Joseph's team expect DIET to be useful in exploring atomic vapors and ultracold dense atomic clouds, to start. The team also expects that series of oscillating dipoles might yield transparency windows, opening the door for even more interesting manipulations of light.
While this is super-neat physics, there's also a practical side. DIET could be used to slow down or even stop light. Slow light is an interesting phenomenon wherein light is slowed to a fraction of its normal speed through forced interactions with matter. This slow light can be used in transfer of information, switches, and high-resolution spectrometers.
Just what it sounds, stopped light is even stranger. This phenomenon essentially creates a light memory that could store large amounts of information in a cryogenically cooled crystal.
But these practical applications are pretty far off. For now, more work has to be done to confirm that DIET does exist in multilevel atomic or molecular systems. The whole paper describing this bizarre phenomenon is open-access and available online.