Looking for very small things, researchers see the light

Extremely powerful, cheap, microscopes could be the result of new research that’s figured out a simple way to intensify light.

At the moment, to see small things like proteins or transistors in computer chips, you need an electron microscope, or even more expensive and inconvenient technology.

This hampers a lot of nanoscale science, particularly in medical research and computer chip manufacturing. Failures to see nanoscale faults in computer chips can cost billions of dollars.

But research published in Science Advances, by an international team of researchers, has landed on a method that could lead the way to much more simple magnification.

“If we look under a microscope, we can see pretty small objects, but not infinitely small,” says senior author Dr Sergey Kruk, a researcher at the Australian National University (ANU)’s Nonlinear Physics Centre.

“The limit is wavelengths of light. There is an equation which can work out exactly the smallest size you can see in any particular microscope, but loosely speaking, you can see objects as small as half of the wavelength of light.”

Need an explainer on light? Read: What is light?

Violet light waves have the shortest length for visible light, with a wavelength of around 400 nanometres (nm). This is also called high frequency visible light: the higher the frequency, the shorter the wavelength.

This means that it’s difficult to see anything smaller than 200nm: most molecules, and all atoms, are much smaller than that.

One avenue to work around this is to use non-visible light, with smaller wavelengths.

“If you use extreme ultraviolet light, 100 nanometres in wavelength, you might be able to see something that is about 50 nanometres large,” says Kruk.

But getting light with wavelengths this short isn’t easy.

“There are no natural sources of extreme ultraviolet light, and artificial sources are rare and extremely bulky and extremely expensive,” says Kruk.

“For example, synchrotrons can generate extreme ultraviolet light. But these machines can be anywhere from the size of a room to the size of a building or the size of a small town. Free electron lasers can generate extreme ultraviolet light, but again, these are very large and very expensive setups.

“So the only pathway that, in my understanding, we know today to get sources of extreme ultraviolet light at the tabletop or shoebox size, is a process called high harmonic generation. And that’s what we tried to pursue.”

The researchers are not yet at extreme ultraviolet light, but they have shown that they can turn lower frequency sources of light into higher frequencies.

“We started with a conventional light source, a laser – in our case infrared [light],” says Kruk.

“We shine short bursts of light pulses from the laser onto a single nanoparticle. And the nanoparticle generates multiples of a frequency of that laser. It generates twice the frequency, three times the frequency, four times the frequency, et cetera. In our case up to seven times the frequency was detected.”

What this looked like in reality was invisible, low-frequency infrared light becoming visible blue light.

Grayscale image of nanoparticle, with a micrometre bar for scale
One of the nanoparticles the researchers have developed, seen through an electron microscope. Credit: Dr Sergey Kruk/ANU

“We think that if we apply the same principles to a setup where we start from a red light, and we multiply the frequency by a factor of seven, that should bring us to extreme ultraviolet,” says Kruk.

“It’s a commercial laser, which can be fairly compact and fairly affordable. And then it is engineered from a nanoparticle which is a novelty of our research. Our team designed and fabricated those particles ourselves.”

There’s no physical reason they have to stop at seven multiplications either – that was just the highest number they could detect with the equipment they were using.

Next, the team is going to have a run at getting to extreme ultraviolet light, as well as seeing if they can demonstrate its use practically.

“We in particular interact with the School of Medical Research at ANU. So we will try to engage with biology and medical researchers to see something useful using those light sources,” says Kruk.

This would take around three years to achieve, Kruk believes, or about the size of a research grant or PhD project.

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