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Light manipulated with pseudo-magnetic field

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Physicists at the University of Exeter have demonstrated a technique for creating tuneable synthetic magnetic fields, which enable photons to mimic the behaviour of charged particles in magnetic fields.

When charged particles like electrons are in the presence of a magnetic field, they experience a Lorentz force, which influences their motion. The Lorentz force is responsible for many of the most important phenomena in physics, and the basis of many technologies.

However, only charged particles experience the Lorentz force, meaning that particles like photons (quanta of light) which are electrically neutral cannot be straightforwardly controlled using magnetic fields.

Now, a team of theoretical physicists from the University of Exeter have demonstrated that it is possible to create synthetic ‘magnetic fields’ for light by distorting honeycomb metasurfaces: surfaces with structures on a scale smaller than the wavelength of visible light, which can be used to manipulate the propagation of electromagnetic waves.

The physicists were inspired by a discovery ten years ago, when it was demonstrated that electrons propagating through a strained graphene membrane behave as though influenced by a large magnetic field. However, a drawback to this approach is that manipulating the synthetic magnetic field requires modifying the strain pattern with great precision; this is almost impossible with photonic structures.

The University of Exeter researchers discovered an elegant workaround which made use of hybrid light-matter particles trapped on the surface: “These metasurfaces support hybrid light-matter excitations, called polaritons, which are trapped on the metasurface,” said lead author of the Nature Photonics study, postgraduate student Charlie-Ray Mann. “They are then deflected by the distortions in the metasurface in a similar way to how magnetic fields deflect charged particles.”

“By exploiting the hybrid nature of the polaritons, we show that you can tune the artificial magnetic field by modifying the real electromagnetic environment surrounding the metasurface.”

The researchers embedded the metasurface between two mirrors to create a photonic cavity, then demonstrated that it was possible to tune the artificial field by changing only the width of the cavity. This removed the need to modify the tiny structures on the metasurface.

According to Mann, they even demonstrated that it is possible to switch off the synthetic field entirely at a critical width, without having to remove the distortion in the metasurface. This made it possible to emulate phenomena which naturally only involves charged particles with photons.

The researchers believe this study could have important implications for future photonic devices, as it provides a novel means for manipulating light below the diffraction limit.

Dr Eros Mariani, who supervised the study, commented: “Being able to emulate phenomena with photons that are usually thought to be exclusive to charged particles is fascinating from a fundamental point of view, but it could also have important implications for photonics applications.”

“We’re excited to see where this discovery leads, as it poses many intriguing questions which can be explored in many different experimental platforms across the electromagnetic spectrum.”

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