Observation of nonlinear rotational Doppler effect of light

Scientists at the University of Birmingham, in collaboration with the University of Paderborn, have proved experimentally for the first time the rotational Doppler effect in nonlinear optics nearly 50 years after its theoretical prediction by the Dutch Nobel Laureate Nicolaas Bloembergen.

Professor Shuang Zhang and Dr Guixin Li from the School of Physics & Astronomy, and their collaborator from the University of Paderborn, Professor Thomas Zentgraf, investigated a nonlinear process called second harmonic generation, in a rotating nonlinear crystal, in which two photons combine to form a higher energy photon. They observed a frequency shift of the higher energy photon that was three times the rotation frequency of the nonlinear crystal.

We experience the acoustic Doppler effect every day, such as an ambulance siren sounding higher in pitch as it approaches than when it is moving away. The effect applies to all kinds of waves, including light waves.

The rotational Doppler effect for rotary motion of objects leads to a shift of wavelength for a circularly polarized light depending on the rotational speed. It can be used in the measurement of rotational frequencies of air turbulence, molecules and astronomical objects. In contrast to the conventional rotational Doppler effect, the observed nonlinear rotational effect does not require the presence of birefringence in the rotating medium and therefore can be used in a broader range of applications.

The Doppler effect is used in a number of technological fields, including the velocity measurement of blood flow in the human body by ultrasound or speed measurement in traffic cameras.

The original paper has been published in the journal Nature Physics, and can be viewed online.