The unravelling of one of the mysteries of electromagnetism could enable the design of antennas small enough to be integrated into an electronic chip, say researchers.
One of the biggest bottlenecks to miniaturisation in modern electronics is the fact that antennas remain far larger than electronic circuits, so ultra-small antennas could transform wireless communications and have been called the 'last frontier' of semiconductor design.
"Antennas, or aerials, are one of the limiting factors when trying to make smaller and smaller systems, since below a certain size, the losses become too great," said Professor Gehan Amaratunga of Cambridge University's Department of Engineering, who led the research.
The foundation of current understand of electromagnetic radiation comes from theories first proposed by James Clerk Maxwell in the 19th century, which state that electromagnetic radiation is generated by accelerating electrons.
But in new results published in the journal Physical Review Letters, Amaratunga’s team have proposed that electromagnetic waves are generated not only from the acceleration of electrons, but also from a phenomenon known as symmetry breaking.
The jumping off point for the groups theory was the fact that a dielectric solid, a material which normally acts as an insulator and so has electrons that are not free to move around, is able to emit radiowaves – dielectric resonators are already used as antennas in mobile phones.
"In dielectric aerials, the medium has high permittivity, meaning that the velocity of the radio wave decreases as it enters the medium," said Dr Dhiraj Sinha, the paper's lead author. "What hasn't been known is how the dielectric medium results in emission of electromagnetic waves. This mystery has puzzled scientists and engineers for more than 60 years."
To investigate this phenomenon the Cambridge team teamed up with researchers from the National Physical Laboratory and Cambridge-based dielectric antenna company Antenova to experiment with thin films of piezoelectric materials, a type of insulator which is deformed or vibrated when voltage is applied.
At a certain frequency the researchers found that these materials become not only efficient resonators, but efficient radiators as well, meaning that they can be used as aerials.
The researchers determined that the reason for this phenomenon is due to breaking of the symmetry of the electric field associated with the electron acceleration. When electronic charges are not in motion, there is symmetry of the electric field.
In addition, the team found that subjecting the piezoelectric thin films to an asymmetric excitation was able to break the symmetry of the system, resulting in a corresponding symmetry breaking of the electric field and the generation of electromagnetic radiation.
The electromagnetic radiation emitted from dielectric materials is due to accelerating electrons on the metallic electrodes attached to them, as Maxwell predicted, coupled with explicit symmetry breaking of the electric field.
"If you want to use these materials to transmit energy, you have to break the symmetry as well as have accelerating electrons – this is the missing piece of the puzzle of electromagnetic theory," said Amaratunga. "I'm not suggesting we've come up with some grand unified theory, but these results will aid understanding of how electromagnetism and quantum mechanics cross over and join up. It opens up a whole set of possibilities to explore."
The classical electromagnetism explanation of radiation being due to electron acceleration has no counterpart in quantum mechanics, where electrons are assumed to jump from higher to lower energy states. The Cambridge team’s discovery could help identify the points where the two fields overlap.
In addition the discovery points the way to future ultra-small antennas for applications in the Internet of Things, which will require billions of tiny wireless aerials to become a reality.
Piezoelectric materials can be made in thin film forms using materials such as gallium arsenide, which is already used to make amplifiers and filters available on the market.
"It's actually a very simple thing, when you boil it down," said Sinha. "We've achieved a real application breakthrough, having gained an understanding of how these devices work."