Why is sugar not transparent? Because light that penetrates a piece of sugar
is scattered, altered and deflected in a highly complicated way. However, as
a research team from TU Wien (Vienna) and Utrecht University (Netherlands)
has now been able to show, there is a class of very special light waves for
which this does not apply: for any specific disordered medium—such as the
sugar cube you may just have put in your coffee—tailor-made light beams can
be constructed that are practically not changed by this medium, but only
attenuated. The light beam penetrates the medium, and a light pattern
arrives on the other side that has the same shape as if the medium were not
there at all.
This idea of "scattering-invariant modes of light" can also be used to
specifically examine the interior of objects. The results have now been
published in the journal Nature Photonics.
An astronomical number of possible wave forms
The waves on a turbulent water surface can take on an infinite number of
different shapes—and in a similar way, light waves can also be made in
countless different forms. "Each of these light wave patterns is changed and
deflected in a very specific way when you send it through a disordered
medium," explains Prof. Stefan Rotter from the Institute of Theoretical
Physics at TU Wien.
Together with his team, Stefan Rotter is developing mathematical methods to
describe such light scattering effects. The expertise to produce and
characterize such complex light fields was contributed by the team around
Prof. Allard Mosk at Utrecht University. "As a light-scattering medium, we
used a layer of zinc oxide—an opaque, white powder of completely randomly
arranged nanoparticles," explains Allard Mosk, the head of the experimental
research group.
First, you have to characterize this layer precisely. You shine very
specific light signals through the zinc oxide powder and measure how they
arrive at the detector behind it. From this, you can then conclude how any
other wave is changed by this medium—in particular, you can calculate
specifically which wave pattern is changed by this zinc oxide layer exactly
as if wave scattering was entirely absent in this layer.
"As we were able to show, there is a very special class of light waves—the
so-called scattering-invariant light modes, which produce exactly the same
wave pattern at the detector, regardless of whether the light wave was only
sent through air or whether it had to penetrate the complicated zinc oxide
layer," says Stefan Rotter. "In the experiment, we see that the zinc oxide
actually does not change the shape of these light waves at all—they just get
a little weaker overall," explains Allard Mosk.
A stellar constellation at the light detector
As special and rare as these scattering-invariant light modes may be, with
the theoretically unlimited number of possible light waves, one can still
find many of them. And if you combine several of these scattering-invariant
light modes in the right way, you get a scattering-invariant waveform again.
"In this way, at least within certain limits, you are quite free to choose
which image you want to send through the object without interference," says
Jeroen Bosch, who worked on the experiment as a Ph.D. student. "For the
experiment we chose a constellation as an example: The Big Dipper. And
indeed, it was possible to determine a scattering-invariant wave that sends
an image of the Big Dipper to the detector, regardless of whether the light
wave is scattered by the zinc oxide layer or not. To the detector, the light
beam looks almost the same in both cases."
A look inside the cell
This method of finding light patterns that penetrate an object largely
undisturbed could also be used for imaging procedures. "In hospitals, X-rays
are used to look inside the body—they have a shorter wavelength and can
therefore penetrate our skin. But the way a light wave penetrates an object
depends not only on the wavelength, but also on the waveform," says Matthias
Kühmayer, who works as a Ph.D. student on computer simulations of wave
propagation. "If you want to focus light inside an object at certain points,
then our method opens up completely new possibilities. We were able to show
that using our approach the light distribution inside the zinc oxide layer
can also be specifically controlled." This could be interesting for
biological experiments, for example, where you want to introduce light at
very specific points in order to look deep inside cells.
What the joint publication of the scientists from the Netherlands and
Austria shows already is how important international cooperation between
theory and experiment is for achieving progress in this area of research.
Reference:
Pritam Pai et al. Scattering invariant modes of light in complex media, Nature
Photonics (2021).
DOI: 10.1038/s41566-021-00789-9
Tags:
Physics