Bringing Hidden Optical Imperfections To Light

TU Darmstadt-led study published in “Nature Communications”

Tiny manufacturing imperfections in optical components normally go unnoticed. Yet they can alter light in surprisingly significant ways. An international research team led by TU Darmstadt has shown that such imperfections can affect not only polarization – the direction in which light oscillates – but also the spatial shape of a light beam. The findings, now published in “Nature Communications”, could help make sensitive optical measurements even more precise in the future.

Light waves oscillate in a particular direction – a property known as polarization. Optical components can be used to selectively alter this direction of oscillation or filter out light with a particular polarization. So-called polarizers play an important role in this process. They can almost completely block light with a particular polarization. If two such filters are arranged at right angles to each other, ideally no light passes through.

Researchers use this principle in precision experiments to filter out strong, unwanted laser light and thereby make much weaker light signals visible. One example is the emission from a single quantum emitter – a tiny light source capable of emitting individual particles of light. A much stronger laser beam can obscure this weak emission.

A quarter-wave plate can additionally modify the polarization of light. Such components, made of quartz or polymer, are frequently used in optical experiments. In theory, a uniform quarter-wave plate should change the polarization of light but not the spatial shape of the light beam. This is precisely where the research team’s study begins.

Unexpected pattern in laser light
For the current study, the researchers placed an additional quarter-wave plate between the two polarizers. This allowed them to suppress the unwanted laser light significantly. At the same time, they observed an unexpected effect: the small amount of light that still passed through the setup no longer formed a round spot. Instead, it produced a two-lobed, dumbbell-shaped pattern with a dark gap in the center. When the team rotated the wave plate, the pattern rotated as well.

Because the altered light barely overlapped with the detector intended to capture it, only a very small amount was registered. This was precisely what made the filtering so much more effective: suppression of the unwanted laser light improved from a factor of around one hundred thousand with the two polarizers to around ten million with a quartz wave plate. With a polymer wave plate, the team achieved suppression by a factor of more than one billion.

The result contradicted theoretical expectations. A perfectly uniform quarter-wave plate positioned perpendicular to the laser beam should not produce such a pattern. The researchers therefore suspected that tiny variations within real optical components were responsible for the effect. To test this assumption, they systematically varied the diameter of the laser beam. An already known optical effect would have become weaker as the laser beam became wider. If, on the other hand, manufacturing imperfections on the surface of the wave plate were responsible, the effect should become stronger as the laser beam width increased.

The measurements confirmed the latter. As the diameter of the laser beam increased, the characteristic pattern became more pronounced. The researchers therefore attribute the effect to microscopic manufacturing imperfections that have hardly any impact during conventional use of the components. Further investigations using wave plates made of different materials and at different wavelengths, as well as numerical simulations, supported the findings.

The study thus demonstrates that, under real-world conditions, the polarization and spatial shape of a light beam can be coupled. The effect depends not only on the direction in which the light propagates, but also on the precise location at which it passes through an optical component. Effects that are barely measurable under ordinary conditions become apparent when light with the main polarization is strongly suppressed using crossed polarization optics.

More precise measurements of weak light
The findings connect fundamental physics with potential practical applications. In the future, the observed effect could help suppress laser light even more effectively in experiments designed to detect very weak light signals – for example, in studies of individual quantum emitters or in specialized microscopy techniques. The approach could also be used to detect and characterize very small manufacturing imperfections in precision optical components.

The study was led by TU Darmstadt’s Institute for Condensed Matter Physics and involved the Laboratoire Photonique, Numérique et Nanosciences (LP2N) at Université de Bordeaux – Institut d’Optique Graduate School – CNRS (France), eyeo BV (Netherlands) and attocube systems GmbH. The optical measurements were conceived and led by Dr. Wenze Lan, a postdoctoral researcher in Professor Bernhard Urbaszek’s “Hybrid Quantum Systems” research group, together with doctoral researcher Anton Lögl and bachelor’s student Duy Anh Tran. The theoretical model was developed by Dr. Wenze Lan, Anton Lögl and Dr. Peng Fu, an ERC postdoctoral researcher in the “Light in Complex Nanostructures” research group at LP2N.