Could Quasiparticles Lead To Better Images?

Photons hitting certain surfaces can interact with electrons and form a quasiparticle. arleksey/shutterstock

From astronomy to medicine, every science struggles to obtain high quality images. There are always new telescopes, microscopes and endoscopes being developed, but technological advances are only half the battle. The other half is making sure that the light being detected is free of aberrations. The light emitted by an object can easily become distorted simply because the light moves through a medium before reaching the observer.

Air, water, and human tissue all distort the light waves and the images we could get from have lower quality and precision. There are different techniques to correct for distortion but they require knowing how the wavefront (any part of a light wave that was emitted at the same time) was distorted in the first place and although often successful, they cannot account for the most minute irregularities.  

So, researchers from University College Dublin developed a method to correct for smaller distortion than current techniques allow, using something known as quasiparticles. This could potentially be used to improve the resolution of images from chemical sensing to biomedicine, as well as complementing traditional methods such as adaptive optics in astronomy.

Once the distortions are precisely measured, it is possible to correct the shape of the wavefront and thus reconstruct the original light wave. The team used the peculiar physics phenomenon called a quasiparticle: a disturbance in a medium or material that behaves like a particle, but is not one. 

There are several known quasiparticles, and they are used to simplify complex physical interactions. For example, it's difficult to describe an electron moving in a semiconductor, such as the silicon chip in a computer, as it interacts with the other electrons and protons in the material; fortunately, thanks to the type of interactions, it can be approximated to a quasiparticle similar to the electron but with different mass moving through free space. So physicists can use the quasiparticle to describe the system.

The study, which is published in the journal Optica, considers the resonance behavior of surface plasmon polaritons (SPPs), a well-known type of quasiparticle. They are formed when photons and electrons interact, creating an electric field on the surface of some specific materials. When light hits the material, it forms a pulse (the SPP) that travels along the surface away from its point of origin; this pulse behaves like a moving particle, and describing it as such makes predicting its behaviour easier. The SPPs are strongly dependent on the photons that formed them, so any changes in the light produce a change in the SPP.

For their experiment, the researchers used a gold film sensor that can form SPPs on its surface. "We make use of the attenuation of the signal from the gold surface to simply convert the wavefront shape – or slope – into an intensity difference in a beam of light," said Dr Brian Vohnsen, lead author of the study, in a statement. "This change is easily captured with cameras that are sensitive to very minute changes in intensity."

The strength with which SPPs can be formed is intrinsically dependent on the angle at which light hits the surface. Distortions, even minuscule ones, slightly change the angle, which in turn affects the formation of the SPPs. The changes are measured by high-speed cameras and by taking two measurements at 90 degrees from one another, the researchers are able to fully reconstruct the original, undistorted light wave.

"I think it is exciting to combine a resonant phenomenon like SPP excitation with wavefront sensing," Dr Vohnsen told IFLScience. "The applications where it could be really exciting to use are in optical microscopy and in laser systems where, in combination with a wavefront corrector, it may achieve fast and accurate wavefront sensing and correction in adaptive optics systems for diffraction-limited performance."

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