Physics researchers at the University of Bath in the UK 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 supresses 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 Ph.D.
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 (DNA, 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.
Ph.D. 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."
The research is published in the journal Laser & Photonic Reviews.
Reference:
Lukas Ohnoutek et al, Optical Activity in Third‐Harmonic Rayleigh
Scattering: A New Route for Measuring Chirality, Laser & Photonics
Reviews (2021).
DOI: 10.1002/lpor.202100235
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