Video: Looking through skin

Looking through skin (Image: Wonshik Choi/Korea University)

IT’S as if Humpty Dumpty was put back together again – stronger than before. Light that passes through an opaque medium, such as an eggshell, can be reassembled in sharper detail and over a wider field of view than if it passed through a transparent lens alone. Such “scattering” lenses could one day be used to see individual living cells, replacing biopsies and improving surgical precision.

In 2007, Allard Mosk and colleagues at Twente University in Enschede, the Netherlands, demonstrated that materials not normally transparent to optical wavelengths can be used to sharply focus what little light gets through. By correlating input and output light, the researchers calculated a “transmission matrix” that defines how light is scattered by disordered particles in such a material. They used this matrix to design the shape of the incoming light waves so that they scattered off the particles and came to a focus on the other side.

In 2010, the researchers showed that such a lens could focus light into a spot one-tenth the size of that produced by an ideal transparent lens of the same size, making the focus 10 times as sharp. In May, they imaged gold nanoparticles at a resolution of just 97 nanometres, to show that scattering lenses can image below the 200-nanometre limit of conventional optical lenses (Physical Review Letters, DOI: 10.1103/PhysRevLett.106.193905).


It is a painstaking process, though. To create an image with such high resolution, they had to take “zoomed in” snapshots of small regions of the nanoparticles and then stitch these images together. The technique “achieves a sub-100-nanometre resolution that puts a smile on our faces every day”, says Mosk. “But it does so at a cost of being rather slow and having a very small field of view of only a few micrometres.”

Now Wonshik Choi of Korea University in Seoul and colleagues have found a way to take pictures that are a thousand times as wide – up to several millimetres, though the resolution is lower than with Mosk’s method.

Their technique, detailed in a paper to appear in Physical Review Letters, measures the transmission matrix of a potential lens faster and in more detail than previous methods.

They used a 450-micrometre-thick slice of fresh rat skin as a lens. First they illuminated it from different angles, capturing around 20,000 images of the resulting transmitted light in 40 seconds. This provided a detailed 3D model of the skin sample’s transmission matrix.

Then they placed a microglial cell from a rat’s brain behind the skin-lens and took another image (see left image). They used pattern-matching software to sort through the 20,000 images to determine which regions of the matrix were in front of the cell. This allowed them to piece together a detailed picture of the cell (right image).

The technique provided a field of view five times as wide as a traditional lens. This is because rays of light reflected from the target at angles too wide to be intersected by normal lenses are bent into a detector by the scattering lens (see diagram). “Our system can perform wide-field imaging by converting a distorted image into a clean image using the recorded transmission matrix,” says Choi.

“It is very nice work,” says Jochen Aulbach at the FOM Institute for Atomic and Molecular Physics in Amsterdam, the Netherlands. “It brings the concept of scattering lenses a big step closer to application for wide-area microscopy.”

“I know of no other compact optical system that combines such high resolution with a field of view that large,” says Mosk. He hopes to see a hybrid system that combines his resolution with Choi’s speed and field of view. Ultimately, he says, the technique could improve surgeons’ views of what to cut during keyhole surgery. “Light scattering may seem detrimental to imaging, but in fact a scattering system can make an almost perfect lens.”