Optical Frequency Combs have proved such a valuable tool for scientists their invention won the 2005 Physics Nobel Prize. Physicists are seeking a counterpart using sound. One team has found a way to achieve the goal using the bubble maker from a household fish tank, and think it may prove cheaper and more portable than alternatives.
Optical frequency combs produce light at evenly spaced wavelengths. Partially transparent objects affect the different frequencies in different ways, and comparisons can provide unprecedented insight into whatever is being studied. Dr Ivan Maksymov of Swinburne University compares the combs to a picket fence, with each frequency emitted analogous to a picket. “If you tried to build a fence on a bumpy road and didn't compensate for the bumps the top of the fence will repeat what is underneath,” Maksymov told IFLScience. “If you ran your hand along the top the differences in fence height would tell you about the road.”
Maksymov is keen to create a counterpart using ultrasound which could be used in places where light does not penetrate well, such as the deep ocean or the human body. However, optical frequency combs are usually made with a laser, whose single wavelength gets split to produce coherent light at millions of separate frequencies. Acoustic waves lack an obvious counterpart.
Recently, two other teams have published papers announcing acoustic frequency combs. One takes the obvious path – simply producing sound at many different wavelengths and beaming it at the target. The other vibrates an aluminum nitride-covered silicon wafer and has it interact with ultrasound waves, producing a range of discrete frequencies.
Maksymov's technique, announced in Scientific Reports, has more in common with the second approach, but still brings something new. Maksymov vibrated the bubbles produced by an aerator from an aquarium with single-frequency sound waves. The response was a set of sound waves with discrete, evenly spaced frequencies.
Indeed, Maksymov told IFLScience, the technique works so well the bubbles in a bottle of fizzy water could be used instead.
Small bubbles in the bloodstream are safe enough they are already used to transport drugs, Maksymov noted, for example across the blood-brain barrier to attack brain cancers. Consequently, it should be possible to peer deep into the human body by bouncing ultrasound off bubbles inside us and using the resulting frequency comb to create clearer pictures than any existing imaging method.
The same technique could be used to study underwater environments, for example providing accurate measurements of distances underwater. Indeed, Maksymov admitted, it's possible animals like dolphins – that are known to use sonar to investigate their surroundings – have been making frequency combs long before us. As far as he is aware, no one has investigated the possibility that this is the case.
The work was done while Maksymov and coauthors were trapped at home during Melbourne's COVID-19 lockdown, a lack of access to the lab contributing to the DIY nature of the equipment. There may be a payoff in the process, with combs produced this way being cheaper and more portable than those relying on higher-tech equipment, Maksymov thinks.
Lockdown-enforced home research also won Maksymov this year's IgNobel physics prize, thanks to his work turning drunk earthworms into Faraday-like waves.