The Doppler shift of sound or light waves from a moving source is familiar to physicists and non-physicists alike. Now, researchers in China and Australia have seen the more exotic inverse Doppler effect in light passing through a material made from tiny silicon rods. They say the result could enhance the use of the Doppler effect in all sorts of applications, from astronomy to medicine.

In the conventional Doppler effect, the frequency of waves that are emitted by, or bounce off, a moving object increases when the object is moving towards an observer and decreases when the object is moving away. This is because in the former case the waves become compressed as they travel towards the observer – and in the latter case the waves spread out.

In 1968 Soviet physicist Victor Veselago predicted that electromagnetic waves travelling through materials with a negative permittivity and a negative permeability would do the opposite. The frequency should drop for a source moving towards an observer and increase for a source moving away. This is because the magnitude of the Doppler effect is proportional to the refractive index of the medium through which the waves propagate. Whereas the refractive index of air and all other natural media is greater than (or equal to) one, the index of the artificial materials considered by Veselago was negative.

The inverse Doppler effect has already been observed at radio frequencies, by two physicists at BAE Systems in the UK in 2003. This work involved tuning the dispersion properties of an electrical transmission line, then bouncing a radio-frequency wave off a moving current pulse within the line and measuring the wave's frequency shift.

Optical observation a first

Now, a joint team led by Songlin Zhuang of the Shanghai University of Science and Technology and Min Gu of the Swinburne University of Technology in Australia has seen the effect at optical frequencies. To do this the researchers shone an infrared laser beam through a lattice of 2 ┬Ám diameter silicon rods attached to a moving platform and recorded the frequency shift of the light leaving the lattice. Being a photonic crystal, the lattice has a characteristic band-gap that forbids the passage of a narrow range of wavelengths, and the researchers say that by tuning the output of their laser so that its wavelength matched the edge of the bandgap they are able to negatively refract the laser light.

The challenge is proving that the light is inverse Doppler shifted as it passes through the photonic crystal. Not being able to position the source and the detector inside the crystal, the researchers had to find a way to subtract the normal Doppler shift that the light experienced as it travelled through the air outside the lattice. To do this, they use interferometry. They split the beam emerging from the laser into two components and adjust the path length of the beam not passing through the photonic crystal so that it experiences the same conventional Doppler shift as the beam passing through the crystal. The beat frequency resulting from the interference of the two beams reveals the frequency shift due only to the inverse Doppler effect.

According to Gu, the trick is to arrange the silicon rods to ensure the laser beam follows the simplest path through the photonic crystal. Otherwise, he says, it would have been too difficult to calculate the expected inverse Doppler shift and therefore impossible to compare theory with experiment. The team also carried out the same experiment using a normal zinc-selenide crystal instead of the photonic crystal, and saw the conventional Doppler shift as expected.

Practical applications

Gu says that his group's result is important scientifically, partly because of the fundamental role of the Doppler effect in physics and partly because it provides further experimental proof of the still-contested phenomenon of negative refraction. Plus, he adds, the latest work could have practical applications. For example, he says, it might lead to improved analyses of blood circulation, with the use of the inverse as well as the conventional Doppler effect potentially halving the number of measurements that have to be made when measuring the speed of complicated blood flows.

Vladimir Shalaev of Purdue University in the US describes the latest work as an "important breakthrough" that shows "how a fundamental phenomenon can manifest itself in an unusual way". He says the experiment was "rather smart and technically challenging", pointing out that it required the measurement of a tiny shift in frequency (about 10 Hz, compared with the central frequency of some 1013 Hz). And in terms of practical applications, he believes the research could benefit any technique currently exploiting the conventional Doppler effect, such as the Doppler cooling of atomic gases.

The work is described in Nature Photonics doi:10.1038/nphoton.2011.17.