In recent years, condensed-matter systems have become a fertile playground for the discovery of new, emergent quasiparticles for which there exists no analogue in the standard model. The archetypal example is graphene whose low-energy electronic quasiparticles behave as so-called massless Dirac fermions – pseudorelativistic particles that exhibit remarkable transport properties, such the suppression of backscattering and anti-localization.
By translating this into the realm of metamaterials, researchers at Exeter University have have theoretically predicted the existence of two new quasiparticles, called type-I and type-II massless Dirac polaritons. These emerge in very elementary metasurfaces with honeycomb symmetry, which can be readily realized across the electromagnetic spectrum from arrays of plasmonic nanoparticles to microwave helical resonators (figure 1).
While these massless Dirac polaritons inherit some of the remarkable features found in graphene, they also offer unique tunabilty which is crucial for their practical implication in novel devices.
Charlie-Ray Mann, PhD student in our CDT in Metamaterials and lead author explains: “The true potential of these half-light, half-matter quasiparticles lies in their hybrid nature which provides additional tunable degrees of freedom that can be manipulated independently.”
By embedding the metasurface inside a cavity (figure 1), one can modify the fundamental properties of the emergent massless polaritons by simply changing the cavity height. This includes the ability to tune the dispersion from linear to quadratic, change the Berry phase, and invert the chirality (figure 2) – things that are impossible to achieve in graphene itself.
Charlie-Ray Mann adds: “Exploiting this tunability offers a plethora of opportunities to explore new pseudorelativistic phenomena for which there exists no analogue in condensed-matter systems. This can then be exploited in novel schemes for guiding and manipulating light below the diffraction limit.”
This theoretical work was recently published in Nature Communications: https://doi.org/10.1038/s41467-018-03982-7
Congratulations to Charlie-Ray and the co-authors!
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