Correlating structure with third-order optical nonlinearity of chalcogenide glasses within a germanium–sulfur binary system

In this work, a series of germanium–sulfur (GeS) chalcogenide glasses (ChGs) was prepared. The third-order optical nonlinear (TONL) properties of ChGs were measured using Z-scan technique at 800 nm. A rapid decrease of optical bandgap energy (Eopg) and rapid increase of nonlinear refractive index (n2) were observed at a mean coordination number (MCN) of 2.67, which is known as the second topological threshold. Raman spectra showed that the appearance of [Ge(Ge2S2)] tetrahedra in the glass network is responsible for the decrease of Eopg at the threshold MCN and the significant enhancement to the TONL properties of the GeS ChGs.