Photonic skyrmions discovered

Skyrmions, ‘hedgehogs’ of electron spins, are well known in magnetic materials and were long considered for applications in spintronics and high-density data storage. However, electromagnetic waves also carry spin and orbital angular momenta. In a recent paper, the team of researchers from Shenzhen University in China and King’s College London and London Centre for Nanotechnology in the UK have discovered the skyrmion structures made of photon spins.

The team has shown a direct analogy between photonic spin structures observed in optical field with orbital angular momentum and skyrmions in magnetic materials. Interestingly, a free-space focused vector beam produces photonic skyrmion-like feature corresponding to a Bloch-type skyrmion in magnetic materials, while a waveguided beam leads to a Neel-type skyrmion. There are no skyrmion-like structures for unfocused propagating vector beams. This phenomenon is caused by strong spin-orbit coupling effects which result in an exchange of spin and orbital angular momenta and provide topological protection.

Even more interestingly and importantly for applications, the researchers demonstrated that the photonic spin structure of a focused vortex beam varies on the deep-subwavelength scales, down to 10 nm distances, greatly exceeding the diffraction limit. Diffraction always limits the size of spatial intensity distributions, however the direction of the field, defining its polarisation and spin, is not affected by diffraction and can be observed at much finer, deep-subwavelength scales. The application of the observed photonic skyrmions with such nanometric features may range from high-resolution imaging and precision metrology to quantum technologies and data storage where the analysis of the spin structure of the beam, not its intensity, can be applied to achieve deep-subwavelength optical field patterns.

In order to achieve this experimentally, the team constructed and validated a unique scanning probe experimental set-up capable of achieving the above-mentioned resolution in measuring the spin structure of the optical beams.

Professor Anatoly Zayats of King’s College London said: “electrons and photons are very different animals with different properties defining their behaviour, such as spin and statistics. However, in the specially designed environments photon exhibit very similar behaviour to electrons, such as, for example, topologically protected states (something unheard for photons until very recently). The demonstrated photonic skyrmions is another example of how well-known electron phenomena can be transposed into the photonic domain, where they can be used for developing new applications.”

The article “Deep-subwavelength features of photonic skyrmions in a confined electromagnetic field with orbital angular momentum” is published in Nature Physics,

DOI: 10.1038/s41567-019-0487-7