Researchers demonstrate Rashba-like spin splitting in a perfectly symmetric plasmonic crystal, challenging conventional assumptions about the role of structural asymmetry
When a material does not have inversion symmetry, particles within it will move in different directions when they have different spin states. This is because the asymmetry of the material creates an effective electric field that couples the particles’ spin and momentum. The behaviour is called the Rashba effect, and it allows researchers to manipulate electron spins without requiring a magnetic field.
Surface plasmons are waves of oscillating electrons that travel along a metal surface when it interacts with light. In this work, the researchers investigated surface plasmons to see if they could create something like the Rashba effect but in a completely symmetrical material. To do this, they shone polarized light onto a periodic gold nanostructure (a plasmonic crystal) and measured how the resulting plasmon modes behaved, comparing what happened for left- and right-circularly polarized light.
They observed that differently polarized light caused the plasmon modes to split according to the light’s spin (polarization), mimicking Rashba spin splitting. Instead of being caused by the lack of symmetry in the material, the tightly focused light interacts with the anisotropic gold grating. The gold grating has lines running in one direction, meaning that light polarized along the grating lines behaves differently from light polarized across them. When light is tightly focused, its polarization is no longer identical everywhere, so the light experiences different combinations of birefringence and dichroism depending on its propagation direction. This creates a geometric phase gradient that gives left- and right-circularly polarized light opposite momentum shifts, causing the observed spin splitting. The researchers describe this mechanism as a geometric linear birefringence-linear dichroism effect.
The findings show that Rashba-like spin splitting does not necessarily require structural asymmetry and suggest that researchers may be able to achieve spin-controlled optical behaviour using simpler symmetric metasurfaces rather than increasingly complex asymmetric nanostructures.
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Quantum transport in Rashba spin–orbit materials: a review by Dario Bercioux and Procolo Lucignano (2015)