For the first time, an international group of scientists has managed to film light pulses forming a photonic Mach cone, the equivalent of a sonic boom but for light. The incredible feat was only possible thanks to an experimental camera that is capable of capturing 100 billion frames per second.
The Mach cone is created when a wave emitter moves faster than those waves. Supersonic jets routinely form these cones, but when it comes to light, things get a bit more complicated. Obviously, nothing moves faster than light in a vacuum, but light moves slower in other materials and that’s where these elusive phenomena can be studied.
The team, led by Dr Jinyang Liang and Dr Lihong Wang, shot brief laser pulses through a “source tunnel” sandwiched between two materials through which light would move more slowly. As the light goes through the tunnel, the pulses are scattered, creating wavelets that move at a superluminal speed.
The camera recorded a Mach cone that matches the theoretical predictions for it, with the results published in Science Advances. The superluminal pulse forms a triangular region that is dragged ahead across the material. When the same experiment was repeated with a subluminal pulse, no such pattern was visible.
Subluminal (top) and superluminal (pulse) being propagated in the source tunnel. The superluminal shows a clear Mach cone. Jinyang Liang and Lihong V. Wang
"We made the first-ever video recording of a propagating light-induced photonic Mach cone in real time," Dr. Liang told IFLScience. "This dynamic light scattering event was captured in a single camera exposure by the newly developed single-shot lossless-encoding compressed ultrafast photography (LLE-CUP) at 100 billion frames per second."
To snap such a fleeting event, the team had to design not only the experiment but also the camera, which was purposely constructed to record it. The LLE-CUP uses a three-in-one recording system. One channel works very much like a regular camera, while the other two record temporal information of the dynamic event. Combined together, they produce this incredible view on the phenomenon.
This observation might seem far removed from applied physics, but the technology could be used in many different applications, including observing neurons firing up and imaging how microstructures in living tissues change.
"We envision that the LLE-CUP technology will find widespread applications in both fundamental and applied sciences," Liang added. "Our camera is fast enough to watch neurons fire and image the 'live traffic' in the brain. We hope we can use our system to study neural networks to understand how the brain works."
