AFTER 60 years of trying, physicists have finally shown that the Doppler effect can be made to work in reverse. This may eventually allow electromagnetic radiation to be tuned to any desired region of the spectrum.
The normal Doppler effect is familiar enough: for example, sound waves from a police car’s siren rise in pitch as it approaches and fall as it recedes. That is because as the source moves nearer, each wave has slightly less far to travel than the one before it, so they arrive closer together in time than they left – at a higher frequency, in other words. The same effect occurs when waves are reflected from a target moving closer or further away.
Although it seems counter-intuitive, it was first suggested back in 1943 that it should also be possible to achieve the reverse effect, in which the frequency of radiation increases as the source recedes. Now Nigel Seddon and Trevor Bearpark of defence contractor BAe Systems in Bristol, UK, have done it, using a table-top electrical circuit strung together from a few hundred inductors and capacitors.
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The key was arranging the components in such a way that when a pulse of electromagnetic radiation travels through the circuit, the individual wavefronts making up the pulse actually travel backwards within the pulse, instead of forwards. The researchers hoped that this would make the pulse respond the opposite way to normal when it bounced off a moving target.
To test their set-up, they sent a large pulse of current into the circuit. This acted like a shock wave, travelling at about a tenth the speed of light. It was followed by a pulse of radio-frequency electrical waves moving at around a quarter the speed of light.
Sure enough, when the electrical waves bounced off the receding shock wave, they shifted up in frequency, not down.
Overall, the researchers saw a 20 per cent increase in frequency, and with further modifications they hope to at least double the frequency. That would enable them to shift microwave-frequency waves into the terahertz range, which holds great promise for medical imaging, or into gigahertz waves, which are used for scanning materials. Both types of waves are currently difficult and expensive to make.
Earlier this year, ¿ìè¶ÌÊÓƵ reported a computer simulation (24 May, p 14) which showed that a photonic crystal could dramatically shift the frequency of light passing through it. Seddon and Bearpark are the first to achieve such impressive shifts experimentally.