Refraction at the interface between two materials is essential to the cooperation of light with photonic devices and the propagation of light through the atmosphere at large. Supporting the traditional rules for the refraction of an optical field is the implicit assumption of the distinctness of its spatial and temporal degrees of freedom.
Scientists at the University of Central Florida have devised a new type of laser beam that doesn’t follow long-held principles about how light refracts and travels. It has unique properties that are not shared by common laser beams.
The beams called spacetime wave packets to follow different rules when they refract. Normally, light slows down when it travels into a denser material.
Ayman Abouraddy, a professor at UCF’s College of Optics and Photonics and the study’s principal investigator, said, “In contrast, spacetime wave packets can be arranged to behave in the usual manner, not to change the speed at all, or even to speed up in denser materials anomalously. As such, these pulses of light can arrive at different points in space at the same time.”
“Think about how a spoon inside a water-filled glass looks broken at the point where the water and air meet. 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.”
However, Snell’s Law still applies, the underlying change in velocity of the pulses is no longer applicable for the new laser beams. These abilities are counter to Fermat’s Principle that says light always travels such that it takes the shortest path.
Abouraddy says, “What we find here, though, is no 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. 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 farther away will incur a longer delay than that’s close by. We find that we can arrange for our pulses to propagate such that they arrive at the two submarines at the same time. Now the person sending the pulse 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 simultaneously so you can blindly synchronize them without knowing where they are.”
Scientists created spacetime wave packets using a device called a spatial light modulator. Through this, they reorganized the energy of a pulse of light so that its properties in space and time are no longer separate.
This allows them to control the “group velocity” of the pulse of light, roughly the speed at which the peak of the pulse travels.
Abouraddy said, “This new field that we’re developing is a new concept for light beams. As a result, everything we look into using these beams reveals new behavior. All the behavior we know about light takes an underlying presumption tacitly that its properties in space and time are separable. So, all we know in optics is based on that. It’s a built-in assumption. It’s taken to be the natural state of affairs. But now, breaking that underlying assumption, we’re starting to see new behavior all over the place.”
Basanta Bhaduri, lead author and a former research scientist with UCF’s College of Optics and Photonics, said, “The study results are important in many ways, including the new research avenues it opens.”
“Space-time refraction defies our expectations derived from Fermat’s principle and offers new opportunities for molding the flow of light and other wave phenomena.”
Murat Yessenov, a doctoral candidate in the college, said, “We believe that spacetime wave packets have more to offer, and many more interesting effects can be unveiled. The next steps for the research include studying the interaction of these new laser beams with devices such as laser cavities and optical fibers and applying these new insights to matter rather than to light waves.”
- Basanta Bhaduri et al., Anomalous refraction of optical spacetime wave packets, Nature Photonics (2020). DOI: 10.1038/s41566-020-0645-6