Scientists prove light harmonics theory after 40 years

Published by  News Archive

On 1st Nov 2021

Rays of light

Physicists have observed a new physical effect in chiral (twisted) nanoparticles – proving for the first time a theory put forward more than 40 years ago by UEA’s Prof David Andrews.

The University of Bath team discovered a new physical effect relating to the interactions between light and twisted materials.

But the theory behind the work was put forward by Prof Andrews as a young doctoral student, when he proposed that the chirality of nanoparticles could be observed through harmonics of light.

Prof Andrews, from UEA’s School of Chemistry, thought his theory was too elusive to ever be validated experimentally. But now, the effect is likely to have implications for emerging new nanotechnologies in communications, nanorobotics and ultra-thin optical components.

What are harmonics?

In the 17th and 18th centuries, the Italian master craftsman Antonio Stradivari produced musical instruments of legendary quality, and most famous are his (so-called) Stradivarius violins.

What makes the musical output of these musical instruments both beautiful and unique is their particular timbre, also known as tone colour or tone quality.

All instruments have a timbre – when a musical note is played, the instrument creates harmonics – or frequencies that are an integer multiple of the initial frequency.

Similarly, when light of a certain colour shines on materials, these materials can produce harmonics as well.

The harmonics of light reveal intricate material properties that find applications in medical imaging, communications and laser technology.

For instance, virtually every green laser pointer is in fact an infrared laser pointer whose light is invisible to human eyes. The green light that we see is actually the second harmonic of the infrared laser pointer and it is produced by a special crystal inside the pointer.

In both musical instruments and shiny materials, some frequencies are ‘forbidden’ – that is, they cannot be heard or seen because the instrument or material actively cancels them.

Because the clarinet has a straight, cylindrical shape, it supresses all of the even harmonics and produces only odd harmonics. By contrast, a saxophone has a conical and curved shape which allows all harmonics and results in a richer, smoother sound.

Similarly, when a specific type of light shines on metal nanoparticles dispersed in a liquid, the odd harmonics of light cannot propagate along the direction of light travel and the corresponding colours are forbidden.

Revealing ‘forbidden’ colours

Now, an international team of scientists led by researchers at the University of Bath have found a way to reveal the forbidden colours, amounting to the discovery of a new physical effect. To achieve this result, they ‘curved’ their experimental equipment.

Professor Ventsislav Valev, who led the research, said: “The idea that the twist of nanoparticles or molecules could be revealed through even harmonics of light was first formulated over 42 years ago, by a young PhD student – David Andrews.

“David thought his theory was too elusive to ever be validated experimentally but, two years ago, we demonstrated this phenomenon. Now, we discovered that the twist of nanoparticles can be observed in the odd harmonics of light as well.

“It’s especially gratifying that the relevant theory was provided by none other than our co-author and nowadays well-established professor – David Andrews!

“From a practical point of view, our results offer a straightforward, user-friendly experimental method to achieve an unprecedented understanding of the interactions between light and twisted materials.

“Such interactions are at the heart of emerging new nanotechnologies in communications, nanorobotics and ultra-thin optical components.

Professor Andrews said: ‘‘Professor Valev has led an international team to a real first in applied photonics. When he invited my participation, it led me back to theory work from my doctoral studies. It has been amazing to see it come to fruition so many years later.”

"When I first worked up this theory, it was done by exploiting some mathematical results which I had first derived during my PhD studies and only recently published.

"At the time, no one else had yet applied them. The application seemed obscure, and I was not surprised that its publication received rather little notice.

“I knew the experiments would be difficult; there were not even that many commercial lasers around, though the rapidly accelerating performance of new systems gave me a smidgeon of hope.

“Many decades later, it came as a sudden and enormously welcome surprise to find that Professor Valev's team had secured some very exciting preliminary results whose validation led to today's publication.

“And now we recognize the scope for some powerful applications of the new experimental method, way beyond my imagination all those years ago. All credit to the experimentalists for pursuing such an elusive goal."

'Optical Activity in Third-Harmonic Rayleigh Scattering: A New Route for Measuring Chirality’ is published in the journal Laser & Photonics Reviews. 

It was funded by The Royal Society, the Science and Technology Facilities Council (STFC) and the Engineering and Physical Science Research Council (EPSRC).

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