Spotted: Ghost surface polaritons

For the first time, an international research team has demonstrated the existence of ghost hyperbolic surface polaritons, and it’s just about as mind-bending as you think.

“Polaritonics is the science and technology of exploiting strong interactions of light with matter, and it has revolutionised optical sciences in the past few years,” explains Andrea Alù, co-author of the new study from the City University of New York.

“Our discovery is the latest example of the exciting science and surprising physics that can emerge from exploring polaritons in conventional materials like calcite.”

First thing’s first: what’s a polariton? They fall under the classification of “quasiparticles”, which are disturbances within a medium that act like particles, even if they aren’t really one, and so can be treated as particle-like. Quasiparticles are an important concept in condensed matter physics because they play a role in the properties of matter.

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Specifically, polaritons are quasiparticles created by a strong coupling between electromagnetic waves and an electric or magnetic dipole-carrying excitation – essentially, when light interacts with matter in a specific way. They can be created in many different mediums by different types of electromagnetic radiation.

This new study, published in Nature, observed hyperbolic polaritons at the surface of bulk crystals of a common material, calcite.

The team were exploring how light interacted with calcite, and found unexpected responses from infrared polaritons. They demonstrated that calcite can support ghost polariton surface waves, which have features (including complex, out-of-plane momentum) unlike any other observed surface polariton to date.

“These types of polaritons can be tuned through their optical axis, introducing a new way of manipulation of polaritons,” says Cheng-Wei Qiu, from the National University of Singapore.

Co-author Weiliang Ma, from the Huazhong University of Science and Technology in China, adds: “Excitingly, we have shown ray-like nano-light propagation for up to 20 micrometres, a record long distance for polariton waves at room temperature.”

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These findings may be able to help researchers better control and enhance light-matter interactions at the nanoscale, which is essential in technologies across sensing, biomolecular and chemical diagnosis, signal processing, energy harvesting and more.

Calcite is commonly used in many other technologies, and the fact that it can naturally support ghost polaritons is a step forward for the above applications.

Qiu concludes: “We believe our findings will stimulate exploration of various optical crystals for nanoscale light manipulation.”

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