30 May 2011

from NatureAsia Website

The inverse Doppler effect has been observed for the first time at optical wavelengths
 

In the normal Doppler effect (top), a wave appears compressed as it moves towards an observer.

In the inverse Doppler effect (bottom), the wave appears stretched as it moves toward the observer.
 

The Doppler effect is common in everyday life - the sound of a motor car has a higher frequency as it approaches quickly than when it passes and moves away.

Researchers from the University of Shanghai for Science and Technology in China, along with colleagues from Jiangxi Normal University in China and the Swinburne University of Technology in Australia, have now developed an experimental technique that for the first time at optical wavelengths has allowed them to observe the inverse Doppler effect, where an approaching optical wave is shifted to lower frequencies1.

The Doppler effect in light is observed as a shift to shorter ‘bluer’ wavelengths when the light originates from objects moving toward us at great speed. This is known as a ‘blue-shift’, and the opposite ‘red-shift’ occurs when the source object is moving away.

Scientists use this phenomenon to determine the relative motion of stars, for example, or even airplanes.

To observe the inverse Doppler effect, the particles that make up a wave need to move in one direction, but the intensity variation of the light’s peaks and valleys in the wave shape the other.

There are natural materials in which the intensity variation moves slower than the particles in it, but materials in which they move in opposite directions do not exist in nature and can only be realized using artificial structures. So far, scientists have been unable to verify the inverse Doppler effect in the optical region because of the difficulty in measuring both frequency changes and sign (peak or valley) at the same time.

Here, the researchers used a very sensitive comparison between two laser beams. One beam was directed at a detector via a mirror, and a second identical beam was passed through a photonic crystal with negative refractive index, in which the reverse Doppler effect was predicted to occur.

The photonic crystal was mounted on a platform that moved toward the detector at a constant speed, and the Doppler shifts were measured from the interference pattern of the beams at the detector, which led to the first clear verification of the inverse Doppler effect at optical wavelengths.

In future, such experiments could be used to study other unusual Doppler effects in artificial materials.

Reference

• Chen, J.1, Wang, Y.1,2, Jia, B.3, Geng, T.1, Li, X.3, Feng, L.1, Qian, W.1, Liang, B.1, Zhang, X.1, Gu, M.3 & Zhuang, S.1 Observation of the inverse Doppler effect in negative-index materials at optical frequencies. Nature Photon. 5, 239–242 (2011).

Author affiliation

1. Shanghai Key Lab of Contemporary Optical System, Optical Electronic Information and Computer Engineering College, University of Shanghai for Science and Technology, Shanghai 200093 China

2. College of Physics and Communication Electronics, Jiangxi Normal University, Nanchang 330022 China

3. Center for Micro-Photonics and CUDOS, Swinburne University of Technology, Victoria 3122, Australia