Direct observation of photonic Landau levels and helical edge states in strained honeycomb lattices.

We report the realization of a synthetic magnetic field for photons and polaritons in a honeycomb lattice of coupled semiconductor micropillars. A strong synthetic field is induced in both the s and p orbital bands by engineering a uniaxial hopping gradient in the lattice, giving rise to the formation of Landau levels at the Dirac points. We provide direct evidence of the sublattice symmetry breaking of the lowest-order Landau level wavefunction, a distinctive feature of synthetic magnetic fields. Our realization implements helical edge states in the gap between n = 0 and n = ±1 Landau levels, experimentally demonstrating a novel way of engineering propagating edge states in photonic lattices. In light of recent advances in the enhancement of polariton–polariton nonlinearities, the Landau levels reported here are promising for the study of the interplay between pseudomagnetism and interactions in a photonic system. A honeycomb structured lattice composed of semiconductor micropillars carefully built up layer by layer can be used to make light mimic an exotic electronic behaviour known as the quantum Hall effect. An international team of researchers led by Omar Jamadi and Alberto Amo at the University of Lille in France developed the procedure, which allows photons to behave as if responding to magnetic fields, to which they are generally insensitive. The photons were induced to occupy quantized energy states known as Landau levels, in addition to adopting unusual topological arrangements. This opens what the researchers describe as “a new playground” for manipulating and exploring topological properties of light that had previously been inaccessible for study. Modifying the properties of the semiconductor micropillars could offer the possibility of studying photon nonlinearities in the honeycomb lattice.