Observation of supersymmetric pseudo-Landau levels in strained microwave graphene

Using an array of coupled microwave resonators arranged in a deformed honeycomb lattice, we experimentally observe the formation of pseudo-Landau levels in the whole crossover from vanishing to large pseudomagnetic field strengths. This result is achieved by utilising an adaptable setup in a geometry that is compatible with the pseudo-Landau levels at all field strengths. The adopted approach enables us to observe the fully formed flat-band pseudo-Landau levels spectrally as sharp peaks in the photonic density of states and image the associated wavefunctions spatially, where we provide clear evidence for a characteristic nodal structure reflecting the previously elusive supersymmetry in the underlying low-energy theory. In particular, we resolve the full sublattice polarisation of the anomalous 0th pseudo-Landau level, which reveals a deep connection to zigzag edge states in the unstrained case. A deeper understanding of how the topological states of graphene-like structures affect their optical properties could pave the way for applications such as flat-band lasers and novel sensors. A material’s topological state can provide it with unusual properties. For example, topological insulators can act as insulators in their interior but allow currents to flow in opposite directions on their surface. Similar properties can also be engineered into optical systems. Combining theory led by Henning Schomerus from Lancaster University in the United Kingdom, and experiments carried out by Matthieu Bellec and Fabrice Mortessagne from Université Côte d’Azur in France, a team of researchers has demonstrated how the application of a deformation in a photonic analogue of graphene results in the formation of pseudo-Landau levels. The work shows that the material’s edge and bulk properties and sub-lattice polarisation are directly linked and provides a unifying principle that connects a wide variety of topological phenomena.