A new sprayable superlens made of transparent nanospheres can boost microscope magnification to see data grooves on a Blu-ray DVD.
Engineers in China and the UK sprayed spherical titanium dioxide nanoparticles onto a surface. Each “nanosphere” focused a narrow beam of light like a tiny torch onto the surface, letting the researchers snap images of structures previously invisible to a regular light microscope.
They presented their proof-of-concept superlens in Scientific Advances.
Microscopes that use light – optical microscopes – can produce marvellously detailed images of structures such as bacteria. But there’s a limit to how small an optical microscope can “see”.
A law outlined by German physicist Ernst Abbe in 1873 renders anything smaller than the smallest bacterium – around 200 nanometres – invisible to traditional optical light microscopes. It states that the width of a spot of light cannot be smaller than half of its wavelength.
For visible light, that’s around 200 nanometres – far too low a resolution to see something as small as the tiniest bacterium, let alone even smaller organisms such as most viruses.
Could there be a way of “upgrading” regular optical light microscopy to give it the resolution power to pick out previously invisible nanoscale structures?
Engineers have been working on this. A favourite ingredient in this field is the family of metamaterials – artificial materials that can steer light in weird and wonderful ways. (They are, for instance, a prime candidate for invisibility cloaks.)
In 2011, Zengbo Wang, then at the University of Manchester, and colleagues produced a metamaterial lens comprising transparent microspheres of silicon dioxide that, when sprayed on a sample, gave light microscopes the ability to pick out objects only 50 nanometres wide.
Now at Bangor University in Wales, Wang is part of a team that created a metamaterial lens with even higher resolution.
Wang, with Wen Fan and Limin Wu from Shanghai’s Fudan University and fellow Bangor engineer Bing Yan, dropped blobs of minuscule spheres of titanium dioxide, each only 15 millionths of a millimetre wide, onto surfaces. When viewed under a light microscope, they could pick up details down to 45 nanometres.
So how did they manage to get around Abbe’s limit?
Titanium dioxide nanospheres in liquid and spun in a centrifuge will settle into a tightly packed 3-D structure, like a box of golf balls fall into a neat pattern with a bit of jiggling.
When a beam of light is shone onto a drop of titanium dioxide nanospheres, it breaks up. Each sphere acts like its own lens, bending the light and directing it like a focused torch beam.
These millions of exceptionally thin beams of light illuminate the surface with powerful resolution – increasing the magnification of existing microscopes by around five times.
This effect boils down to the metamaterial’s refraction index.
Put a spoon in a glass of water and it looks slightly bent at the interface between the air and water. This is because water bends light differently to air – it has a different “refractive index”.
If it were possible to drop a spoon in a glass of their titanium dioxide metamaterial, “you’d see a larger bend where the spoon enters the material than you would looking at the same spoon in a glass of water”, Wang says.
Importantly, the titanium dioxide lens works over the whole visible light spectrum and is cheap and plentiful, so other labs will be able to replicate the work, Wang says: “The new lens will be used to see germs and viruses not previously visible.”
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