Lighting them all up, coherently

Abstract

Topological insulators in condensed matter physics are phases of matter that are insulating in their bulk
and have robust conductance on their edge, which is immune to disorder and defects. The topological
protection of this conductance is known to be a ubiquitous phenomenon, occurring in many physical
settings, such as photonics, cold atoms and mechanical or electrical systems. Recently, these ideas were
extended beyond their original context, introducing the concepts of topological insulators to nonHermitian systems, with gain or loss, and even to nonlinear settings. A particularly promising
development in this field is the topological insulator laser, invented by our group: an array of
semiconductor lasers utilizing fundamental features of topology to force injection locking, which makes
all emitters act as a single coherent laser. However, thus far, all topological insulator lasers emitted light
in the same plane as the topological transport mechanism locking them together.
I will present the topological insulator Vertical Cavity Surface Emitting Laser (VCSEL) array. The
topological properties of this VCSEL array are inherited from the geometry of the crystalline model. The
inter-cavity coupling lies in a plane perpendicular to the propagation direction of the light in the
microcavities, which permits the emitters to be organized in a few-wavelengths structure. The light
spends most of its time oscillating vertically, but the small in-plane coupling enabling topologicallyprotected transport suffices to force the emitters to act as a single coherent laser