Spatially resolved correlation between glass structure and refractive index modifications resulting from irradiation of chalcogenide glass by femtosecond pulse train

A detailed analysis of 2D μ-Raman mapping of refractive index variation (∆n) induced by femtosecond filament in the bulk of a ternary (Ge-Ga-S) chalcogenide glass is presented. The results demonstrate strong structural modifications. S3Ga-GaS3 ethane like units are transformed into triclusters of Ga/S. Also the decrease of the population of GeS4/2 tetrahedrons implies the formation of mixed triclusters T/S where T=Ga and/or Ge. Finally, the refractive index variation is correlated to the structural modifications.


Introduction
In chalcogenide glass, femtosecond pulse can easily form a filament because of the high optical nonlinearities of the glass.From the resulting light-matter interaction, a refractive variation is produced with a complex morphology alternating positive and negative sign.In order to understand this behaviour, it is important to study the structural modifications that lead to the refractive index variation.
Therefore we perform a 2D µ-Raman analysis of ∆n.Four structural units are identified to contribute to the Raman spectra.By following the spatial evolution of the four contributions, a correlation between ∆n and the contribution of each group to ∆n is obtained.

Experimental procedure
The glass composition is 80 GeS 2 -20 Ga 2 S 3 and it has been synthesized according to classical melt and quenching technique [1].The sample is irradiated by a high repetition rate femtosecond Ti:Sa laser.The pulse energy is moderate (2 nJ) and the pulse duration is measured to be around ∼ 200 fs.The beam is focused inside the bulk of the sample by a X5 microscope objective and the exposure duration is set to 1 second without translation of the sample.A photography of ∆n is shown in figure 1 along with its quantitative reconstruction according the procedure described in [2].resolution of 1 µm in the direction perpendicular to the writing laser beam, and 2 µm in the parallel direction.The total number of Raman spectra is more than 11,000.On the basis of DFT calculation we determine the structural unit that contribute to the Raman spectra.We identify the presence of corner sharing and edge sharing tetrahedron of GeS 4 , S 3 Ga-GaS 3 ethane like unit and tricluster of Ga/S.Every group has a well resolved vibration mode from which the behaviour of its contribution can unambiguously be determined.All the Raman spectra are fitted considering only four parameters corresponding to the amplitude of each structural unit contribution.

Results
In fig.2, the amplitude of the contribution of the four structural units are shown.It is clearly seen that the laser irradiation induces a decrease of the population of GeS 4 tetrahedrons and a large increase of TS 1/3 S 3/p 3 triclusters.By comparing the ∆n (fig. 1) and the Raman mapping figures, it is clear that the formation of a positive ∆n is associated with the formation of triclusters.We attribute the negative ∆n to the formation of nanovoids resulting from Coulombian micro-explosion.

Conclusion
In conclusion we perform a 2D µ-Raman mapping of laser irradiation of chalcogenide glass.Light matter interaction results in strong chemical bonds reordering from S 3 Ga-GaS 3 ethane like units into GaS 1/3 S 3/p 3 triclusters.
The tail of positive ∆n is shown to be due to the higher efficiency of the tricluster tetrahedral packing leading to a higher density.As the number of free carriers generated in the filament increases, Coulombian micro-explosions occur and induce the formation of nanovoids responsible of the ∆n sign reversal.

Fig. 1 :
Fig. 1: Photography and reconstruction of ∆n induced y femtosecond pulse train in the bulk of 80 GeS 2 -20 Ga 2 S 3 glass.

Fig. 2 :
Fig. 2: Mapping of the relative contribution of the four groups into the Raman spectra.