A team of researchers from the University of Exeter has found an elegant technique to create artificial magnetic fields, which can make photons behave like charged particles in real magnetic fields. This can have a huge impact on future photonic devices.
The team of theoretical physicists led by Charlie-Ray Mann recently published their research in Nature Photonics journal.
So, let us first understand Lorentz force and get into details of the research
What is the Lorentz Force?
The Lorentz force (or electromagnetic force) is the combination of the electric and magnetic force on a point charge due to electromagnetic fields. So, basically, when a charged particles like electron 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 amazing phenomena like the beautiful Northern Lights and the famous quantum-Hall effect.
If we talk about photons, they can not be controlled using real magnetic fields as they are not charged.
However, the researchers 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.
A twelve years old discovery showed that electrons propagating through a strained graphene membrane behave as if they were subjected to a large magnetic field. This remarkable discovery inspired the researchers at Exeter. The problem with this strained graphene was that modifying the strain pattern with precision is extremely challenging
The team tried to solve this problem and in the process, they found an elegant solution to this problem.
Charlie-Ray Mann said “These metasurfaces, support hybrid light-matter excitations, called 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, known as a photonic cavity, 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.”
Dr Eros Mariani, the lead supervisor of the study, 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.”
Charlie-Ray Mann, Simon A. R. Horsley, Eros Mariani. Tunable pseudo-magnetic fields for polaritons in strained metasurfaces. Nature Photonics, 2020; DOI: 10.1038/s41566-020-0688-8
Press Release: University of Exeter