Doppler Effect Reversed

By demonstrating the reversal of the optical Doppler effect, researchers may have advanced invisibility cloak technology. They did this using a laser beam and a photonic crystal "superprism."

In physics, the Doppler effect describes the change in frequency of light or sound waves whenever there is a relative movement between an observer and a wave’s source.

Most people have experienced the Doppler effect in relation to sound: When a train is coming toward you, the sound frequency increases, whereas the frequency decreases as the train moves away.

Min Gu, director of Swinburne University's Centre for Micro-Photonics, in the lab. Gu and colleagues made the first experimental observation of an inverse Doppler shift at an optical frequency by refracting a laser beam in a photonic crystal prism that has the properties of a negative-index material. (Image: Paul Jones)

The situation is similar for light: When an object and an observer move closer together, light frequency increases from red wavelengths to blue ones. When they move farther apart, the light frequency decreases from blue to red.

In a paper published this week in Nature Photonics, Swinburne University researchers Dr. Baohua Jia, Dr. Xiangping Li and professor Min Gu and their collaborators at the University of Shanghai demonstrated the Doppler effect in reverse, which does not occur naturally. That is, when an object and a lightwave detector were moved closer together, a decrease in light frequency from blue wavelengths to red ones were shown.

“This is the first time in the world that the inverse Doppler effect has been demonstrated in the optical region,” said professor Gu, director of Swinburne’s Center for Micro-Photonics.

The researchers achieved this by creating an artificial nanostructured crystal — a photonic crystal — out of silicon.

By projecting a laser beam onto the unique superprism and changing the distance between it and the detector, the researchers created an inverse Doppler effect.

“In our superprism, the dispersion of light was twice the magnitude of a standard Newton prism. This large angle makes the prism’s refractive index — a property that determines how fast light travels through it — change to negative,” Gu said.

All materials that occur in nature have a refractive index greater than one. This means that whenever they move in respect to an observer, they will exhibit the standard Doppler effect.

“By creating this artificial material with a negative refractive index, we were able to reverse this natural phenomenon,” he said.

Being able to reverse the Doppler effect is a promising sign for the future development of invisibility cloaks. According to professor Gu, this technology, which has already been demonstrated on a microscale by US researchers, may be closer to becoming a reality than most people think.

Having a greater understanding of this phenomenon may also lead to a number of more immediate applications. Measuring the Doppler effect currently allows astronomers to detect the speed at which stars approach or recede from Earth, enables radars to measure the velocity of objects, and is used in medical imaging technology to measure blood flow in the human body.

For more information, visit: www.swinburne.edu