How Photons Behave Like Electrons

Straining a honeycomb metasurface generates an artificial magnetic field for light which can be tuned by embedding the metasurface inside a cavity waveguide. (Source: U. Exeter)

Scientists have discovered an elegant way of mani­pulating light using a synthetic Lorentz force which in nature is responsible for many fasci­nating phenomena including the Aurora Borealis. A team of theo­retical physicists from the University of Exeter has pioneered a new technique to create tuneable arti­ficial magnetic fields, which enable photons to mimic the dynamics of charged particles in real magnetic fields. The team believe the new research could have impor­tant impli­cations for future photonic devices as it provides a novel way of mani­pulating light below the diffrac­tion limit.

When charged particles, like electrons, pass through a magnetic field they feel a Lorentz force due to their electric charge, which curves their tra­jectory around the magnetic field lines. This Lorentz force is responsible for many fascinating phenomena, ranging from the Northern Lights, to the famous quantum-Hall effect. However, because photons do not carry an electric charge, they cannot be straight­forwardly controlled using real magnetic fields since they do not experience a Lorentz force; a severe limitation that is dictated by the fundamental laws of physics.

The research team have shown that it is possible to create arti­ficial magnetic fields for light by distorting honeycomb meta­surfaces – ultra-thin 2D surfaces that are engineered to have structure on a scale much smaller than the wavelength of light. The team were inspired by a remarkable discovery ten years ago, where it was shown that electrons propa­gating through a strained graphene membrane behave as if they were subjected to a large magnetic field. The major drawback with this strain engi­neering approach is that to tune the artificial magnetic field one is required to modify the strain pattern with precision, which is extremely challenging, if not impossible, to do with photonic structures.

The Exeter physicists have proposed an elegant solution to overcome this funda­mental lack of tuna­bility. Charlie-Ray Mann explains: “These meta­surfaces, support hybrid light-matter excitations, polaritons, which are trapped on the metasurface. 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.”

For the study, the researchers embedded the metasurface between two mirrors and show that one can tune the artificial magnetic field by changing only the width of the photonic cavity, thereby removing the need to modify the distortion in the meta­surface. Charlie added: “We have even demonstrated that you can switch off the arti­ficial magnetic field entirely at a critical cavity width, without having to remove the distortion in the metasurface, something that is impossible to do in graphene or any system that emulates graphene. Using this mechanism you can bend the tra­jectory of the polaritons using a tunable Lorentz-like force and also observe Landau quanti­zation of the polariton cyclo­tron orbits, in direct analogy with what happens to charged particles in real magnetic fields. Moreover, we have shown that you can dras­tically recon­figure the polariton Landau level spectrum by simply changing the cavity width.”

Eros Mariani  said: “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.” (Source: U. Exeter)

Reference: C.-R. Mann et al.: Tunable pseudo-magnetic fields for polaritons in strained metasurfaces, Nat. Phot., online 14 September 2020; DOI: 10.1038/s41566-020-0688-8

Link: School of Physics and Astronomy (E. Mariani), University of Exeter, Exeter, UK

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