Sonic booms of light captured for first time with ultrafast camera

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. The new technology used to make this discovery could one day allow scientists to help watch neurons fire and image live activity in the brain.

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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. Picture by Jinyang Liang and Lihong V. Wang

Science behind the tech

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.

Light sonic booms

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.”

Light travels at a speed of about 186,000 miles per second (300,000 kilometers per second) when moving through vacuum. According to Einstein’s theory of relativity, nothing can travel faster than the speed of light in a vacuum. However, light can travel more slowly than its top speed — for instance, light moves through glass at speeds of about 60 percent of its maximum. Indeed, prior experiments have slowed light down more than a million-fold.

The fact that light can travel faster in one material than in another helped scientists to generate photonic Mach cones:

  1. Scientists 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.
  2. Researchers fired pulses of green laser light 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 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.

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Using a “streak camera,” scientists have imaged a cone-shaped wake of light called a photonic Mach cone for the first time. Video by Jinyang Liang and Lihong V. Wang

The cone-shaped wake of light was captured for the first time using a “streak camera,” that could capture images at speeds of 100 billion frames per second in a single exposure. The new camera is fast enough to watch neurons fire and image live traffic in the brain

The researchers said their new technique could prove useful in recording ultrafast events in complex biomedical contexts such as living tissues or flowing blood. They hope they can use their system to study neural networks to understand how the brain works.

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