Gone, in the twinkling of an eye

A multinational collaboration is to use adaptive optics — telescope technology that takes the twinkle out of star observation — and refine it for use on a microscopic level.

The technique will use focused light to cancel distortions caused by the optical properties of a specimen, known as aberrations, to allow researchers to examine skin or brain tissue with a sharper focus.

The light will also be used to increase the accuracy of microscale engineering.

Oxford University engineers, with Japanese research institute RIKEN and Australia’s Swinburne University of Technology, are developing two optical systems — using similar components — for biomedical and engineering purposes. Oxford will develop both systems, with RIKEN and Swinburne using their expertise in developing optical fabrication methods.

Focused beam

Optical microscopes use a focused beam of light to view a specimen at high resolution. However, as the light passes through the specimen it travels at different speeds, distorting the optical wavefronts. So it is only possible to observe activity deep in a thick specimen at a low resolution. The Oxford team plans to correct this distortion.

Expensive adaptive optics techniques were originally developed to allow astronomers and the military to view stars and satellites without the distortions or aberrations caused by turbulence in the Earth’s atmosphere. This meant the naked eye could see ‘twinkling’ stars.

adaptive optics specialist Dr Martin Booth from Oxford said various techniques are used to increase the depth reached by a high-resolution microscope to allow for observation of cells in their natural environment.

‘Optical distortion is a limitation. At the moment you compromise on resolution, so you can image deeper but at low resolution or lower light efficiency. Or you can find a way of exciting tissue to look at it in thin tissue sections,’ he said.

Adaptive optics technology also has an engineering application on a sub-micrometer scale, with this project focusing mainly on photonic devices, such as those found in communications and optical computing. In these situations, the greater the overall size and complexity of the structures, the greater the effect of aberrations on the accuracy of the manufacturing process.

Focused light can be used to shape materials, or to initiate chemical reactions to produce photonic crystals blocks, as small as a few tens of nanometres in volume, that can be used to build polymer and metallic structures for nanotechnological devices in a block-by-block fashion.

Traditional optical systems have static components, such as lenses for focusing, mirrors for reflecting and scanning, and prisms for separating different wavelengths. These elements were clearly not compatible with the variable nature of specimens, which would produce a range of aberrations.

To correct these, a component such as a deformable mirror (an ordinary mirror with a changeable surface shape) or liquid crystal corrector, similar to the devices found in LCD projectors, can be used to balance a distortion by creating the opposite distorted shape. This will restore the quality of the focus.

Pre-shaped wavefront

‘What you want are perfect wavefronts, which aren’t distorted, so you need some element in there that will “unwrap” the distortion caused by the specimen,’ said Booth. ‘Essentially, if we’re focusing light into the specimen, then we would pre-shape the wavefront with the opposite shape to that introduced by the specimen.

‘This means that if the specimen introduces aberration x, we produce an aberration minus x, and when added together, we end up with zero aberration.’

Booth said there were two main reasons why this particular research was now taking place.

‘In terms of the technology, the devices that are required to carry out this research are now affordable, whereas previously they were only available for such things as astronomical research,’ said Booth. ‘Also, applications — such as microscopy, biomedical imaging and the optical manufacturing techniques — are getting to the stage where they require this type of technology.’

The researchers have a £900,000 grant for the five-year project, which they hope will result in a new breed of microscopes and widespread use in commercial applications.

Anh Nguyen