Ultra-small transducers produce ultra-high frequencies

This is the vision of Nottingham University scientists and engineers who have built ultrasonic transducers so small they are undetectable by the human eye.

The researchers claim the transducers are orders of magnitude smaller than current systems, adding that up to 500 of the smallest ones could be placed across the width of one human hair.

They can produce ultrasound of such a high frequency that its wavelength is smaller than that of visible light. Theoretically, they make it possible for ultrasonic images to take finer pictures than the most powerful optical microscopes.

Matt Clark, of the Applied Optics Group from the School of Electrical and Electronic Engineering at Nottingham University, explained this is because resolution, in optical or ultrasonic applications, is limited by wavelength.

‘The size of the smallest object you can see is directly proportional to the wavelength,’ he said. ‘With a large wavelength you can see big objects but not small objects. If you have a small wavelength you can see smaller and smaller things.

‘So in optical microscopy the resolution is limited by the wavelength of the light, which is about 500nm. For these transducers, the wavelengths will be about 100nm. You now have the potential to see things that are five times smaller than with optical microscopes.’

Clark described the transducers as sandwich- or shell-like structures with two reflective materials on the outside and a transparent material in the middle.

When they are struck by a pulse of laser light, they are set ringing at high frequency, which launches ultrasonic waves into the sample. When the transducer is excited by ultrasound, the distance between the outside reflective materials changes and that affects the amount of light that is reflected back. This is measured by an additional laser.

The devices can be constructed either by micro or nanolithography techniques similar to those used for microchips or by molecular self-assembly where the transducers are constructed chemically.

Clark said the transducers are nicknamed ‘chots’, which stands for ’cheap optical transducers’, because they ‘are essentially free to make’.

While the transducer will be able to spot smaller objects in the body, Clark warns this technology cannot overcome the problem of attenuation, which affects the distance ultrasonic signals can reach before weakening.

‘It’s a slightly complicated thing because now we’re at the position where with regular ultrasound in the body you can see about a millimetre,’ he said. ‘If you built a microscope using this technology you could see things down to about 100nm and a factor of 10,000 smaller. But there is a problem when going down in wavelength. The attenuation becomes much higher so you can’t see through very much stuff.’

Therefore, Clark’s team is narrowing the medical applications for the technology to cellular imaging.

‘In bone cancer, structural properties of cells - their stiffness - change,’ he said. ‘If you could take a sample of cells and measure their stiffness internally you could diagnose tumours that way.’

Clark added that it would be a step up from current techniques such as photo-acoustic imaging, which puts dyes, particles or stains inside a tissue and fires lasers to generate ultrasound. The problem with this technique, he said, is that the surrounding tissue filters signals, so there is no way to operate at anything above low frequencies.

The work by the Nottingham researchers received £850,000 worth of funding from the EPSRC last year and an additional grant of £350,000 to study the applications for this technology in the aerospace industry.

Clark said the technology could overcome one of the limiting factors for integrating nano-composite components in aircraft.

‘There’s no way of inspecting such a component at the moment,’ he added.

While laboratories have resources such as electron microscopes to study the integrity of nano-composites, they must slice up the material to get a good look at it. This would be completely impractical for a flight ground crew examining a fan blade.

Following additional testing and development of the transducers over the next couple of years, Clark expects to be preparing for the commercialisation of the technology.

‘We believe in five to 10 years there will be a strong demand from the aerospace industry to put nano-engineered components in their engines and airframes and they will need some way of inspecting them,’ he said.