Role of ion migrations and structural reorganisations in femtosecond laser direct-written chalcogenide glass waveguides

Formation of femtosecond laser direct-written positive refractive index waveguides in Gallium Lanthanum Sulphide (GLS) glass is explained for the first time. Evidence of structural changes and ion migration are presented using Raman spectroscopy and electron probe microanalysis (EPMA), respectively. 2-D Raman spectra maps reveal a peak shift and full-width at half maximum variations in the symmetric vibrations of GaS4 main band. For the first time, the 2D map of the boson band was successfully used to identify and understand the material densification profile in a high refractive index glass waveguide. Finally EPMA provided the evidence of ion migration due to sulphur and the observation of an anion (S2-) migration causing material modification is also reported for the first time.