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An experiment proves 50-year-old theory

The experimental physicist Prof. Thomas Zentgraf and his two colleagues from the University of Birmingham, Dr. Guixin Li and Prof. Shuang Zhang, proved for the first time experimentally the nonlinear rotational Doppler effect of light - nearly 50 years after its theoretical prediction by the Dutch Nobel Laureate Nicolaas Bloembergen.

The acoustic Doppler effect can be experienced day in the real world every: The siren of an ambulance sounds sharper when one moves closer to the emergency vehicle. When the vehicle moves away, the siren sounds lower. This is due to the change of wavelength of the sound-waves, which are compressed or stretched during the movement of the sound source and thus changing its pitch. The effect applies not only to acoustic waves, but also for all kinds of waves, including light waves. Similarly, for a star moving away from Earth, its emitted light wave is stretched, and then our eyes experience the red-shift, i.e. a longer wavelength of light. Conversely, a light wave emitted from a star approaching Earth will be compressed, which causes a blue shift of the light. Already in 1842, the Austrian physicist Christian Andreas Doppler predicted this optical effect in his paper "On the colored light of the double stars and certain other stars of heaven" and presented this phenomenon to the Royal Bohemian Society of Sciences in Prague. Three years later, the Dutch physicist Christoph H. D. Buys-Ballot observed the acoustic Doppler effect in a spectacular experiment. He used the fastest transportation tool at that time: the railway. A musician was playing trumpet on a moving rail car while the tones he played were listened by other musicians standing next to the train track. The displacement of the pitch in the tones they noticed when the train passed by is equivalent to the predictions for Doppler’s color shift of light.

Nowadays, the Doppler effect has made a number of technological achievements in fields such as e.g. the speed measurement in traffic cameras, the GPS or the velocity measurement of blood flow in the human body by ultrasound. In addition, the Doppler effect has a key role in some important quantum phenomenon like the broadening of spectral lines and the trapping and cooling of atoms with laser light.

In addition to the well-known Doppler effect for translational movements, There is also a rotational Doppler effect for rotary motion of objects. This effect leads to a shift of wavelength depending on the rotational speed and can be used in the measurement of rotational frequencies of air turbulence, molecules and astronomical objects.

Already in 1968, a few years after the invention of the laser, a new rotational Doppler effect was predicted by the later Nobel Laureate Nicolaas Bloembergen for rotating objects under the high intensities of illumination of laser light, i.e. a nonlinear rotational Doppler effect. Nearly 50 years later, this effect was demonstrated for the first time in the laboratory. "Due to the small wavelength shift for this non-linear effect, it is extremely difficult to observe it directly in an experiment" explained by Prof. Thomas Zentgraf. The reason for this is the low rotation speed of objects compared to the speed of light. This means that the wavelength shift of light as it passes through a rotating object is just in the range of a few trillionths part of the wavelength itself (1 trillionth = 0,000.000.000.001). Even in the laboratory such a small wavelength shift cannot be measured directly. "We have used a special superposition between two light waves, called interference" explains Prof. Zentgraf. The change of this superposition was then detected and from this the wavelength shift can be determined.

The poof of fundamental effects of physics, such as the nonlinear rotational Doppler effect, represents an important step in the examination of current theories for our view of the world. With the experiments at the University of Paderborn and the University of Birmingham, one further prediction has now been confirmed. In the future, this effect could find its broad application in the study of turbulence, laser plasmas and the rotational property of molecules.

The original paper has been published in the journal Nature Physics, and can be viewed on under following link:

dx.doi.org/10.1038/nphys3699