Researchers a the University of Central Florida (UCF) in the US have developed a new category of laser beams that could revolutionise optical communications and laser technologies. These new laser beams have properties that are unique and not possessed by standard laser beams.
The new laser beams are called ‘spacetime wave packets’. When they refract (which is what happens when light moves through different materials) they follow different rules to standard laser beams.
“Think about how a spoon inside a water-filled glass looks broken at the point where the water and air meet,” cited UCF College of Optics and Photonics professor and study principal investigator Ayman Abouraddy. “The speed of light in air is different from the speed of light in water. And so, the light rays wind up bending after they cross the surface between air to water, and so apparently the spoon looks bent. This is a well-known phenomenon described by Snell’s Law.”
The famous ‘speed of light’ that cannot be exceeded is actually the speed of light in a vacuum and is the fastest speed light can go. But light can travel more slowly than that. Light normally slows down when it enters a denser medium. “In contrast, spacetime wave packets can be arranged to behave in the usual manner, to not change speed at all, or even to anomalously speed up in denser materials,” he explained. “As such, these pulses of light can arrive at different points in space at the same time.”
It has traditionally been assumed that the properties of light in space and in time were separable. “[A]ll we know in optics is based on that. It’s a built-in assumption. It’s taken to be the natural state of affairs,” he pointed out. But the UCF team used a piece of equipment called a spatial light modulator to rearrange the energy of a pulse of light so that its properties were no longer separate in space and time. By this means they were able to control the light pulse’s ‘group velocity’ (approximately the speed of travel of the peak of the pulse). These reorganised pulses are the spacetime wave packets.
“[N]o matter how different the materials are that light passes through, there always exists one of our spacetime wave packets that could cross the interface of the two materials without changing its velocity,” he highlighted. “So, no matter what the properties of the medium are, it will go across the interface and continue as if it’s not there.”
“If you think of a plane trying to communicate with two submarines at the same depth but one is far away and the other one’s close by, the one that’s father away will incur a longer delay than the one that’s close by,” he elucidated. “We find that we can arrange our pulses to propagate such that they arrive at the two submarines at the same time. In fact, now the person doesn’t even need to know where the submarine is, as long as they are at the same depth. All those submarines will receive the pulse at the same time so you can blindly synchronise them without knowing where they are.”
This breakthrough does not violate Snell’s Law nor does it in any way contradict special relativity, because it does not apply to the underlying oscillations of the light wave; rather it applies to the propagation of the peak of the pulse. It does however run counter to Fermat’s Principle (that light always travels along the shortest path). “This new field that we’re developing is a new concept for light beams,” enthused Abouraddy. “As a result, everything we look into using these beams reveals new behaviour. … [W]e’re starting to see new behaviour all over the place.”
This breakthrough was based on previous research undertaken by the UCF team. Their results have been published in the journal Nature Photonics. The funds for the research were provided by the US Office of Naval Research.