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2011-03-02 08:43:10
By Jason Palmer Science and technology reporter, BBC News
Wei Guo with "super-resolution" microscope The technique can see features
significantly smaller than prior efforts
UK researchers have demonstrated the highest-resolution optical microscope ever
- aided by tiny glass beads.
The microscope imaged objects down to just 50 billionths of a metre to yield a
never-before-seen, direct glimpse into the "nanoscopic" world.
The team says the method could even be used to view individual viruses.
Their technique, reported in Nature Communications, makes use of "evanescent
waves", emitted very near an object and usually lost altogether.
Instead, the beads gather the light and re-focus it, channeling it into a
standard microscope, allowing researchers to see with their own eyes a level of
detail that is normally restricted to indirect methods such as atomic force
microscopy or scanning electron microscopy.
Using visible light - the kind that we can see - to look at objects of this
size is, in a sense, breaking light's rules.
Normally, the smallest object that can be seen is set by a physical property
known as the diffraction limit.
Light waves naturally and inevitably "spread out" in such a way as to limit the
degree to which they can be focused - or, equivalently, the size of the object
that can be imaged.
At the surfaces of objects, these evanescent waves are also produced.
As the name implies, evanescent waves fade quickly with distance. But
crucially, they are not subject to the diffraction limit - so if they can be
captured, they hold promise for far higher resolution than standard imaging
methods can provide.
Going viral
"Previously, people including ourselves have been using microspheres for
focusing light for fabrication purposes, so we can machine features smaller
than the diffraction limit," explained Lin Li, of the University of
Manchester's Laser Processing Research Centre.
"It just came to my mind that if we reverse it, we might be able to see small
features as well, so that is the reason we carried out this piece of research,"
he told BBC News.
Professor Li and his colleagues used glass beads measuring between two and nine
millionths of a metre across, placed on the surfaces of their samples.
Simulated light propagation in optical microsphere (Nature Communications) The
beads gather up and re-focus light that normally fades away within nanometres
of the sample
The beads collect the light transmitted through the samples, gathering up the
evanescent waves and focusing them in such a way that a standard microscope
lens could pick them up.
The team imaged minuscule features in various solid samples and even the
nanometre-scale grooves in Blu-Ray discs to show that the approach's resolution
beat all previous records for optical microscopy.
But Professor Li thinks the technique holds great promise for biological
studies, for which the action at the nanoscale is difficult to see directly.
"The area we think will be of interest will be looking at cells, bacteria, and
even viruses," he said.
"Using the current technology, it is very time consuming; for example, using
fluorescence optical micoscopy, it takes two days to prepare one sample and the
success rate of that preparation is 10 to 20%. That illustrates the potential
gain by introducing a direct method of observing cells."
Ortwin Hess of Imperial College London said that "it's really quite fascinating
and exciting to see these effects coming together".
"If you use the fact that you do generate those (evanescent waves) and focus
them again, then you have a tight focal point that you wouldn't normally expect
to have," he told BBC News.
"It's quite a nice phenomenon that they've absolutely exploited."