The magic of "Phonons"

Gold plasmonic nanostructures shaped like Swiss crosses can convert laser light into ultrahigh frequency (10GHz) sound waves. Credit: Berkeley Lab

You know a lot about Photons, quantum of light, but may be you haven't explored the intriguing "Phonons". These are "quantum of sound" (I am using quantum in a very loose way).

Researchers at the Lawrence Berkeley Laboratory have manage to create phonons by using laser pulses to hit gold plasmonic nano structures that as result vibrates generating phonons at a frequency of 10GHz. Not a sound you are likely to perceive.... Interesting way of converting light into sound!

Notice that phonons are not a manifestation of an electromagnetic field, they require a medium to propagate (like air or body tissue). This is an advantage if you are considering using waves to look at body parts. Whereas the electromagnetic field of light will be absorbed by tissue (and hence blocked at its surface) sound waves can penetrate in the tissue and this is why sound is used for diagnostic imaging.

The higher the frequency of sound waves the better the resolution. Present "echography" uses sounds waves at 20MHz. By increasing this frequency to 20GHz you get much better resolution.

Sound waves can also be used to inspect materials, like checking the integrity of an airplane wing.

The biggest challenges the researchers faced were how to generate the sound pulses and how to direct them. According to the one of the researchers, Kevin O'Brien:

“Through the interplay between phonons and localized surface plasmons, we can detect the spatial properties of complex phonon modes below the optical wavelength. This allows us to detect complex nanomechanical dynamics using polarization-resolved transient absorption spectroscopy.”

It may leave you confused, but it shows that there is a good deal of science and technology behind this. In lay man terms they have found a way to generate extremely high frequency sound pulses and at the same time a way to detect the reflection of these pulses (similarly to what happens in the radar where reflected waves are compared to the ones sent and their difference can be processed to identify objects and their structure).

Author - Roberto Saracco

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