Twisting in the light of nanoparticles

Physics researchers at the University of Bath in the UK are discovering a new physical effect linked to the interactions between light and twisted materials, an effect that may have implications for new emerging nanotechnologies in communications, nanorobotics and ultra-thin optical components.

In the 17th and 18th centuries, Italian master craftsman Antonio Stradivari produced musical instruments of legendary quality, and the most famous are his (so-called) Stradivarius violins. What makes the musical output of these musical instruments both beautiful and unique is their special timbre, also known as color or sound quality. All instruments have a timbre: when a musical note (sound of frequency fs) is played, the instrument creates harmonics (frequencies which are an integer multiple of the initial frequency, i.e. 2fs, 3fs, 4fs, 5fs, 6fs, etc.).

Likewise, when a light of a certain color (with the frequency fc) illuminates materials, these materials can produce harmonics (light frequencies 2fc, 3fc, 4fc, 5fc, 6fc, etc.). The harmonics of light reveal complex material properties that find applications in medical imaging, communications, and laser technology.

For example, virtually every green laser pointer is actually an infrared laser pointer whose light is invisible to the human eye. The green light 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 musical instruments as in shiny materials, certain frequencies are “off-limits”, that is, they cannot be heard or seen because the instrument or material actively cancels them out. Because the clarinet has a straight cylindrical shape, it removes all even harmonics (2fs, 4fs, 6fs, etc.) and only produces odd harmonics (3fs, 5fs, 7fs, etc.). In contrast, a saxophone has a conical and curved shape which allows all harmonics and gives a richer and smoother sound. Much the same way, when a specific type of light (circularly polarized) illuminates metallic nanoparticles dispersed in a liquid, the odd harmonics of the light cannot propagate in the direction of light travel and colors. corresponding are prohibited.

Today, 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, which is tantamount to the discovery of a new physical effect. To achieve this result, they “bent” 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 the harmonics of light was first formulated over 42 years ago. years, by a young doctoral student, David Andrews. David thought his theory was too elusive to be validated experimentally, but two years ago we demonstrated this phenomenon. Now, we have discovered that the torsion of nanoparticles can also be observed in odd harmonics of light. It is especially gratifying that the relevant theory has been provided by none other than our co-author and now well established professor — David Andrews!

“To take a musical analogy, so far scientists who study twisted molecules (DNA, amino acids, proteins, sugars, etc.) and nanoparticles in water – the element of life – have illuminated them at a given frequency and have frequency or its noise (inharmonic partial harmonics). Our study opens the study of the harmonic signatures of these twisted molecules. We can thus appreciate for the first time their “timbre”.

“From a practical point of view, our results offer a simple and user-friendly experimental method to achieve an unprecedented understanding of the interactions between light and twisted materials. Such interactions are at the heart of new emerging nanotechnologies in communications, nanorobotics and ultra-robotics. thin optical components. For example, the “twist” of nanoparticles can determine the value of the information bits (for left or right twist). It is also present in the propellers of nanorobots and can affect the direction of propagation of an In addition, our method is applicable in tiny volumes of illumination, suitable for the analysis of natural chemicals showing promise for new pharmaceuticals but where available material is often scarce.

PhD Student Lukas Ohnoutek, also involved in the research, said: “We almost missed this discovery. Our original equipment was not well “tuned” so we did not see anything at the third harmonic. I was starting to lose hope but we had a meeting, identified the potential issues and investigated them systematically until we found the problem. It’s wonderful to experience the scientific method at work, especially when it leads to scientific discovery! “

Professor Andrews added: “Professor Valev has led an international team towards a true first in the field of applied photonics. When he invited my participation, it brought me back to the theoretical work of my doctoral studies. It was amazing to see it come to fruition so many years later. “

The research is published in the journal Laser and photonics examinations.