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Optical physics

Optical physics

Complex light waves measure hidden objects

16 Feb 2021
Hidden target
By calculating the optimal wave pattern for the incident laser light, it becomes possible to accurately measure objects located behind disordered structures. (Courtesy: TU Wien)

Researchers in Europe have developed a technique for obtaining accurate laser-based measurements of objects hidden behind complex scattering structures. They say that their mathematical formula, which calculates the optimal waveform needed for the scattering environment, can also be applied to other types of waves.

Laser beams are great for making precision measurements of objects. But only if the object can be seen clearly. If the object is behind or within a disordered environment – such as behind a cloudy pane of glass or embedded in complex biological tissue – these measurements become much more challenging. Both media will scatter and alter the light waves as they pass through, so the object can’t quite be seen, making it hard to obtain useful measurements.

The difficulty with such situations is that the scattering medium is overwhelmingly complex and unknown. This makes developing a universal approach for imaging through complex scattering systems challenging. To overcome this, Dorian Bouchet and colleagues at Utrecht University and TU Wien developed a mathematical formula that characterizes the scattering behaviour of the system.

Their analysis shows exactly how the disordered medium is affecting the light beam. This allows them to then create a complex wave pattern that gets scattered and altered by the disturbing environment to create the optimal beam of light needed to make the measurements. “You don’t even need to know exactly what the disturbances are,” explains Bouchet, now at Université Grenoble Alpes. “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 also tested this idea experimentally. At Utrecht University, they shone a continuous-wave solid-state laser emitting at 532 nm through a diffuser – the disordered medium. They then successfully measured objects on the other side with nanometre precision, after calculating the optimal light waves needed.

“We characterize the medium by successively sending 2400 plane waves with different propagation directions, and by measuring the light scattered from the medium for each one of them,” Bouchet tells Physics World. “We also measure how this scattered light changes as a function of the observable that we want to precisely measure. Remarkably, even though we first illuminate the medium with plane waves only, we are able to predict what will happen for any other kind of wave that we can use to illuminate the medium.”

The researchers say that because the results of the scattering analysis are universally applicable, they can be transferred to other types of waves, such as acoustic waves or those in the microwave regime.

The work was linked to a programme for nanometre-scale imaging of semiconductor structures, Bouchet tells Physics World, noting that the production of computer chips could be a good application for this technique as extremely precise measurements are indispensable. But the researchers also believe that it could have applications in biology. “However, we first need to address the question of what will happen for a medium that changes in time, for instance due to the flow of blood in vessels,” Bouchet says.

As well as investigating time-dependent media, the team is also trying to further increase the precision of measurements by using non-classical states of light. These are more difficult to generate, Bouchet says, but can in principle lead to more precise measurements.

The researchers report their findings in Nature Physics.

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