Probing topological invariants in the bulk of a non-Hermitian optical system

Understand

Topological insulators are insulating in the bulk but feature conducting states on their surfaces.

• Standard methods for probing their topological properties largely involve probing the surface, even though topological invariants are defined via the bulk band structure.
• Here, we utilize non-hermiticy to experimentally demonstrate a topological transition in an optical system, using bulk behavior only, without recourse to surface properties.
• This concept is relevant for a wide range of systems beyond optics, where the surface physics is difficult to probe.