Femtosecond laser written devices inside a smartphone screen, in this case Gorilla Glass ®, have been recently demonstrated1 with a high potential of increasing the functionality of cellphones, consuming minimal space using the glass screen2. Even though low loss waveguides have been reported in this glass, the behavior of the refractive index of the glass subject to femtosecond laser radiation is not well understood. Here, we propose a study of that behavior by presenting the identification of two major transitions where the induced refractive index seems to decrease. The first transition occurs at lower fluence and is characterized by a single structure while the second one occurs at much higher fluences, and is well characterized by a double shell structure. In both these transitions, the refractive index at the center of the structure seems to decrease. However, it should be noted that between these two regimes, there is narrow regime in which light seem to be guide in the middle of the fs processed region, confirming the possibility of making a single pass waveguide. The fluence limits of each regime has been investigated as has the quality of the waveguide made by a single and multi-passes. The refractive index of the affected zones is mapped by a highly sensitive phase-interference technique.
 Lapointe, J., Gagné, M., Li M.J., and Kashyap R., "Making smart phones smarter with photonics," Op. Exp., Vol. 22, No. 13, pp. 15473-15483, (2014)
 Lapointe, J., Parent, F., Soares de Lima Filho, E., Loranger, S., and Kashyap R. "Toward the integration of optical sensors in smartphone screens using femtosecond laser writing," Optics Letters, Vol. 40, No. 23, pp. 5654-5657, (2015)