Unveiling pseudospin and angular momentum in photonic graphene

TL;DR

This paper demonstrates that pseudospin in photonic graphene can be directly measured as orbital angular momentum through vortex generation and topological charge flipping, confirming its physical observability.

Contribution

It provides experimental evidence that pseudospin in photonic graphene can manifest as measurable angular momentum, bridging a gap between theory and observation.

Findings

Pseudospin can be converted into orbital angular momentum.

Selective excitation of sublattices enables vortex generation.

Numerical solutions support the experimental results.

Abstract

Pseudospin, an additional degree of freedom inherent in graphene, plays a key role in understanding many fundamental phenomena such as the anomalous quantum Hall effect, electron chirality and Klein paradox. Unlike the electron spin, the pseudospin was traditionally considered as an unmeasurable quantity, immune to Stern-Gerlach-type experiments. Recently, however, it has been suggested that graphene pseudospin is a real angular momentum that might manifest itself as an observable quantity, but so far direct tests of such a momentum remained unfruitful. Here, by selective excitation of two sublattices of an artificial photonic graphene, we demonstrate pseudospin-mediated vortex generation and topological charge flipping in otherwise uniform optical beams with Bloch momentum traversing through the Dirac points. Corroborated by numerical solutions of the linear massless Dirac-Weyl equation, we show that pseudospin can turn into orbital angular momentum completely, thus upholding the belief that pseudospin is not merely for theoretical elegance but rather physically measurable.