19th August 2014

Atomic-force microscopes 20 times more sensitive

Laser physicists have found a way to make atomic-force microscope probes 20 times more sensitive and capable of detecting forces as small as the weight of an individual virus.

The technique – developed by researchers in the Quantum Optics Group of the Australian National University, Canberra – uses laser beams to cool a nanowire probe to minus 265 degrees Celsius.

“The level of sensitivity achieved after cooling is accurate enough for us to sense the weight of a large virus, 100 billion times lighter than a mosquito,” said Professor Ping Koy Lam, the leader of the Quantum Optics Group.

This could be used to improve the resolution of atomic-force microscopes, which are state-of-the-art tools for measuring nanoscopic structures and the tiny forces between molecules. Atomic force microscopes can achieve ultra-sensitive measurements of microscopic features by scanning a wire probe over a surface. However, such probes – around 500 times finer than a human hair – are prone to vibration.

“At room temperature the probe vibrates, just because it is warm, and this can make your measurements noisy,” said co-author Dr Ben Buchler. “We can stop this motion by shining lasers at the probe.”

Credit: Quantum Optics Group, Australian National University

The force sensor, pictured above, was a 200 nm-wide silver gallium nanowire coated with gold.

“The laser makes the probe warp and move due to heat. But we have learned to control this warping effect, and were able to use the effect to counter the thermal vibration of the probe,” said Giovanni Guccione, a PhD student on the team.

However, the probe cannot be used while the laser is on, as the laser effect overwhelms the sensitive probe. So the laser has to be turned off and any measurements quickly made before the probe heats up within a few milliseconds. By making measurements over a number of heating/cooling cycles, accurate values can be determined.

“We now understand this cooling effect really well,” says Harry Slatyer, another PhD student. “With clever data processing, we might be able to improve the sensitivity, and even eliminate the need for a cooling laser.”

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