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This graphic shows a so-called photonic Mach cone, which is sort of like a sonic boom; but in this instance, you can see the cone-shaped wake of light pulses.
Just as aircraft flying at supersonic speeds create cone-shaped sonic booms, pulses of light can leave behind cone-shaped wakes of light. Now, a superfast camera has captured the first-ever video of these events.
When an object moves through air, it propels the air in front of it away, creating pressure waves that move at the speed of sound in all directions. If the object is moving at speeds equal to or greater than sound, it outruns those pressure waves. As a result, the pressure waves from these speeding objects pile up on top of each other to create shock waves known as sonic booms, which are akin to claps of thunder.
Sonic booms are confined to conical regions known as "Mach cones" that extend primarily to the rear of supersonic objects. Similar events include the V-shaped bow waves that a boat can generate when traveling faster than the waves it pushes out of its way move across the water.
Previous research suggested that light can generate conical wakes similar to sonic booms. Now, for the first time, scientists have imaged these elusive "photonic Mach cones."
The fact that light can travel faster in one material than in another helped scientists to generate photonic Mach cones. First,study lead author Jinyang Liang, an optical engineer at Washington University in St. Louis, and his colleagues designed a narrow tunnel filled with dry ice fog. This tunnel was sandwiched between plates made of a mixture of silicone rubber and aluminum oxide powder.
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Then, the researchers fired pulses of green laser light — each lasting only 7 picoseconds (trillionths of a second) — down the tunnel. These pulses could scatter off the specks of dry ice within the tunnel, generating light waves that could enter the surrounding plates.
The green light that the scientists used traveled faster inside the tunnel than it did in the plates. As such, as a laser pulse moved down the tunnel, it left a cone of slower-moving overlapping light waves behind it within the plates.
Using a "streak camera," scientists have imaged a cone-shaped wake of light called a photonic Mach cone for the first time.
(Image credit: Liang et al. Sci. Adv.2017;3:e1601814)
Streak camera
To capture video of these elusive light-scattering events, the researchers developed a "streak camera" that could capture images at speeds of 100 billion frames per second in a single exposure. This new camera captured three different views of the phenomenon: one that acquired a direct image of the scene, and two that recorded temporal information of the events so that the scientists could reconstruct what happened frame by frame. Essentially, they "put different bar codes on each individual image, so that even if during the data acquisition they are all mixed together, we can sort them out," Liang said in an interview.
There are other imaging systems that can capture ultrafast events, but these systems usually need to record hundreds or thousands of exposures of such phenomena before they can see them. In contrast, the new system can record ultrafast events with just a single exposure. This lends itself to recording complex, unpredictable events that may not repeat themselves in precisely the same manner each time they happen, as was the case with the photonic Mach cones that Liang and his colleagues recorded. In that case, the tiny specks that scattered light moved around randomly.
The researchers said their new technique could prove useful in recording ultrafast events in complex biomedical contexts such as living tissues or flowing blood. "Our camera is fast enough to watch neurons fire and image live traffic in the brain," Liang told Live Science. "We hope we can use our system to study neural networks to understand how the brain works."
The scientists detailed their findings online Jan. 20 in the journal Science Advances.
Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.
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