Experimental Realisation of PT-Symmetric Flat Bands

Non-Hermitian physics, and in particular its well-known representative of PT-symmetry, is a field of great interest in current research. Photonics allows for the implementation of its associated complex potentials by means of judiciously distributed refractive index, gain and loss. In this vein, femtosecond laser-written waveguide systems are a versatile platform to explore non-Hermitian physics in discrete settings. In light of the widespread use of light for rapid long-distance transmission, the capability to temporarily halt its propagation is desirable for a wide range of applications. In this vein, flat band structures are of particular importance. This work shows how flat bands can be synthesized by judiciously applying loss to the system. We experimentally investigate the emergence of a flat band at the systems exceptional point and observe the associated compact localised eigenmodes that can be excited at arbitrary locations of the periodic lattice.