  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.
