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The Nanophotonics Orchestra Presents: Twisting to the Light of Nanoparticles To Reveal “Forbidden” Colors

Upon illumination with red light, third harmonic scattered light (in violet) reveals the twist of metal nanoparticles. Credit: Ventsislav Valev and Lukas Ohnoutek

Physicists at the University of Bath in the UK observe a new physical effect in chiral (twisted) nanoparticles.

Physics researchers at the University of Bath discover a new physical effect relating to the interactions between light and twisted materials – an effect that is likely to have implications for emerging new nanotechnologies in communications, nanorobotics, and ultra-thin optical components.

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 color or tone quality. All instruments have a timbre – when a musical note (sound with frequency fs) is played, the instrument creates harmonics (frequencies that are an integer multiple of the initial frequency, i.e. 2fs, 3fs, 4fs, 5fs, 6fs, etc.).

Similarly, when light of a certain color (with frequency fc) shines on materials, these materials can produce harmonics (light frequencies 2fc, 3fc, 4fc, 5fc, 6fc, etc.). 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 (2fc) 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 suppresses all of the even harmonics (2fs, 4fs, 6fs, etc.) and produces only odd harmonics (3fs, 5fs, 7fs, etc.). By contrast, a saxophone has a conical and curved shape which allows all harmonics and results in a richer, smoother sound. Somewhat similarly, when a specific type of light (circularly polarized) shines on metal nanoparticles dispersed in a liquid, the odd harmonics of light cannot propagate along the direction of light travel, and the corresponding colors are forbidden.

Now, an international team of scientists led by researchers from the Department of Physics at the University of Bath have found a way to reveal the forbidden colors, 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!

“To take a musical analogy, until now, scientists who study twisted molecules (DNADNA, or deoxyribonucleic acid, is a molecule composed of two long strands of nucleotides that coil around each other to form a double helix. It is the hereditary material in humans and almost all other organisms that carries genetic instructions for development, functioning, growth, and reproduction. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).”>DNA, amino acidsAmino acids are a set of organic compounds used to build proteins. There are about 500 naturally occurring known amino acids, though only 20 appear in the genetic code. Proteins consist of one or more chains of amino acids called polypeptides. The sequence of the amino acid chain causes the polypeptide to fold into a shape that is biologically active. The amino acid sequences of proteins are encoded in the genes. Nine proteinogenic amino acids are called “essential” for humans because they cannot be produced from other compounds by the human body and so must be taken in as food.”>amino acids, proteins, sugars, etc) and nanoparticles in water – the element of life – have illuminated them at a given frequency and have either observed that same frequency or its noise (inharmonic partial overtones). Our study opens up the study of the harmonic signatures of these twisted molecules. So, we can appreciate their ‘timbre’ for the first time.

“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. For instance, the ‘twist’ of nanoparticles can determine the value of information bits (for left-handed or right-handed twist). It is also present in the propellers for nanorobots and can affect the direction of propagation for a laser beam. Moreover, our method is applicable in tiny volumes of illumination, suitable for the analysis of natural chemical products that are promising for new pharmaceuticals but where the available material is often scarce.

PhD student Lukas Ohnoutek, also involved in the research, said: “We came very close to missing this discovery. Our initial equipment was not ‘tuned’ well and so we kept seeing nothing at the third-harmonic. I was starting to lose hope but we had a meeting, identified potential issues, and investigated them systematically until we discovered the problem. It is wonderful to experience the scientific method at work, especially when it leads to a scientific discovery!”

Professor Andrews added: ‘‘Professor Valev has led an international team to a real first in the 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.”

Reference: “Optical Activity in Third-Harmonic Rayleigh Scattering: A New Route for Measuring Chirality” by Lukas Ohnoutek, Hyeon-Ho Jeong, Robin Raffe Jones, Johannes Sachs, Ben J. Olohan, Dora-Maria Răsădean, Gheorghe Dan Pantoş, David L. Andrews, Peer Fischer and Ventsislav K. Valev, 15 September 2021, Laser & Photonics Reviews.
DOI: 10.1002/lpor.202100235

The research 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).

Source: SciTechDaily