Tech Briefs Magazine - March 2021 - PIT-27

whether the object has moved a little to
the right or a little to the left. This wave
looks complicated and disordered but is
then modified by the turbid pane in such
a way that it arrives at the object in exactly the desired way and returns the greatest possible amount of information to
the experimental measuring apparatus.
The fact that the method actually works
was confirmed experimentally at Utrecht
University (Utrecht, Netherlands). Laser
beams were directed through a disor-

dered medium in the form of a turbid
plate. The scattering behavior of the
medium was thereby characterized. Then
the optimal waves were calculated in
order to analyze an object beyond the
plate - this succeeded, with a precision
in the nano-meter range.
Then the team carried out further
measurements to test the limits of
their method. The number of photons
in the laser beam was significantly
reduced to see whether one then still

When light gets deflected by a disordered structure it becomes difficult to estimate where the
target is located. (Credit: TU Wien)

turbance, no matter how small, must be
avoided.
But what can you do when you are
dealing with disturbances that cannot be
removed? Imagine a panel of glass that is
not perfectly transparent, but rough and
unpolished like a bathroom window.
Light can pass through, but not in a
straight line. The light waves are altered
and scattered, so we can't accurately see
an object on the other side of the window with the naked eye. The situation is
quite similar when you want to examine
tiny objects inside biological tissue: the
disordered environment disturbs the
light beam. The simple, regular straight
laser beam then becomes a complicated
wave pattern that is deflected in all
directions.
However, if you know exactly what the
disturbing environment is doing to the
light beam, you can reverse the situation. Then it is possible to create a complicated wave pattern, which gets transformed into exactly the shape needed to
correct for the disturbances, and hits
right where it can deliver the best result.
To achieve this, you don't even need to
know exactly what the disturbances are,
it's enough to first send a set of trial
waves through the system to study how
they are changed by the system.
The researchers developed a mathematical procedure that can then be
used to calculate the optimal wave from
this test data. It can be shown that for
various measurements there are certain
waves that deliver a maximum of information such as, for example, on the spatial coordinates at which a certain
object is located.
Take an object that is hidden behind a
turbid pane of glass: there is an optimal
light wave that can be used to obtain the
maximum amount of information about

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Tech Briefs Magazine - March 2021

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