Scattering Microscope Peers Into the Nanoscale

Good news for fans of the super-small. Researchers have designed a light-scattering microscope that can see objects less than half the size of those visible with today's scopes. If perfected, the new device would allow researchers to see viruses and cells in unprecedented detail.

No matter how well the lens makers have done their jobs, a traditional microscope has its limits. It can't distinguish objects smaller than half the wavelength of the light used to illuminate them. So blue light, which at 400 nanometers has the shortest wavelength of any visible light, can't differentiate objects less than 200 nm apart (the flu virus is about 100 nm in diameter). Electron microscopes solve this problem by replacing the visible light with electron beams, which have a much shorter wavelength and a much smaller intrinsic blur. But they have their own limitations. Electrons pass straight through thin samples of most living tissue, making it invisible under an electron microscope unless it's prepared in a radical way (such as coating it in metal), which prevents researchers from viewing living organisms.

Now, researchers at the MESA+ Institute for Nanotechnology in Twente, the Netherlands, the Foundation for Fundamental Research on Matter Institute AMOLF in Amsterdam, and the University of Florence in Italy have come up with a new idea for a light microscope. First, they placed between the light source and the object a lens made from a slab of a crystalline substance called gallium phosphide. Light travels much more slowly in gallium phosphide than in air. Since the light waves in front slow down first, the waves get closer together and the wavelength shortens considerably when light enters the material. This design drastically reduces the theoretical minimum size of a visible object.

However, for complex technical reasons, no one has ever managed to get close to this theoretical minimum with a standard lens made of gallium phosphide. So the researchers took a different tack. They started by etching the lens with sulphuric acid, producing a frosted surface that, far from focusing the light, scattered it randomly in all directions. But now for the clever step: They used a computer to design an incoming light wave that, when scattered by the lens, would focus to a point. Although it sounds unlikely, this process of randomize and reconstruct produces a tighter focus than simply focusing the light with a traditional lens.

By capturing the light transmitted through the sample with a traditional light microscope, the researchers were able to construct a sharper image than would otherwise have been possible. Better still, by rotating the incoming wave, the team could focus the light at different points on the object, allowing them to create an ultra-sharp image of the whole sample by scanning the focus across it. The researchers still can't see nanoscale processes like viruses invading cells in living tissue samples. But they've showed that their approach works by imaging gold nanoparticles, achieving a resolution as small as 97 nm (twice as good as an ordinary light microscope). With a bit of tweaking and a more powerful light source, it should be possible to see details of living tissue with their new lens, they report this week in Physical Review Letters.

The study presents "an entirely novel approach to optical imaging," says theoretical solid state physicist John Pendry of Imperial College London, who in 2000 devised a special type of light microscope that can beat the 200 nm limit but can't see the inside of cells. "In my view this paper will be of very wide interest, particularly in the biosciences where resolution on this scale is valuable."

Posted in Physics