Realization of a Photonic Topological Z 2 -Insulator

The first experimental realization of topological insulators [1] constituted the advent of an entirely new field of physics. Their hallmark feature — scatter-free, unidirectional edge states — are of great interest for a wide range of application. Notably, the class of two-dimensional topological insulators is divided by the fermionic or bosonic characteristics of the underlying platform. Fermionic topological systems obey particle-hole as well as time-reversal symmetry, and are characterized by a Z 2 topological invariant [2]. On the other hand, topological insulators in bosonic systems were demonstrated in several systems, such as in photonics, acoustics, mechanics and matter waves — all of them relying on time reversal symmetry to be broken. Typically, these latter systems are topologically characterized by the Chern number.