Laser beams to make microscopes more sensitive

Laser physicists from the Australian National University (ANU) have found a way to make atomic-force microscope probes 20 times more sensitive. Their research has been published in the journal Nature Communications.

Atomic-force microscopes are state-of-the-art tools for measuring nanoscopic structures and the tiny forces between molecules. They achieve extraordinarily sensitive measurements of microscopic features by scanning a wire probe over a surface; however, the 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 study co-author Dr Ben Buchler.

“We can stop this motion by shining lasers at the probe.”

Researchers in the Quantum Optics Group at ANU’s Research School of Physics and Engineering used laser beams to cool the nanowire probe to -265°C. The force sensor used by the team was a 200 nm-wide silver gallium nanowire coated with gold.

PhD students Giovanni Guccione and Harry Slatyer examine their gold-coated silver gallium nanowire. Image: Quantum Optics Group, ANU.

“The laser makes the probe warp and move due to heat,” noted co-author Giovanni Guccione. “But we have learned to control this warping effect and were able to use the effect to counter the thermal vibration of the probe.”

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 cycles of heating and cooling, an accurate value can be found.

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

The silver gallium nanowire is 500 times finer than a human hair. Image: Quantum Optics Group, ANU.

The researchers stated that their high prevision and fast force microscopy results “will potentially benefit applications in biosensing, molecular metrology, subsurface imaging and accelerometry”. Co-author Harry Slatyer added, “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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