# Photosensitivity of Infrared Glasses under Femtosecond Laser Direct Writing for mid-IR Applications
## 3 Results and Discussion  
### Determination of the optical modifications landscape in the energy-speed domain  
During laser direct writing, the non-linear absorption of laser energy by the material locally modifies its physical properties. Different types of permanent modifications can then appear according to the defined parameters. Three main types of modifications are defined in the literature [1; 10; 17]. Type I is defined as the appearance of a smooth, homogeneous, and permanent variation in the refractive index. The threshold for the appearance of the Type I change is at low energy. This type of modification allows 3D engineering of the index profiles with fine control of the refractive index variation. It is therefore a type of modification particularly interesting for the manufacture of optical components, especially in the field of imaging or for optical waveguides and gratings. Type II corresponds to an anisotropic modification of the refractive index whose orientation can be controlled by the writing laser polarization. In most oxide glasses, and according to specific conditions of pulse duration, repetition rate, pulse number and pulse energy, and self-assembled periodic nanostructures can appear and make the glass locally birefringent. With higher laser intensities, voids resulting from micro-explosions can appear in the material: this is the Type III modification. We can finally define the last type of modification appearing at high laser intensities. Often abusively called \"heat accumulation regime\" in the literature, this modification corresponds to a smooth and homogeneous variation of the refractive index with a significant increase in the size of the modified area compared to the size of the energy source. This spatially broadened Type I modification will therefore be referred to as the \"spatial broadening regime\" that described this phenomenon more accurately [18]. After the 2D micro-writing of the samples, we made an initial observation with an Olympus BX51 polarized optical microscope in order to determine the thresholds of appearance of the different types of modification. In the following, Type I is defined as a permanent refractive index contrast seen in natural light but not in crossed polarizers and whose size remains limited to the laser beam waist. In contrast, the spatial broadening regime is not necessarily a new type of modification because it is also detected as an isotropic refractive index contrast observed through optical microscopy (so in that sense it is quite similar to Type I). However, its threshold was defined here by an increase of 20% in the imprinted laser track width beyond the beam waist. Type II is detected under polarized optical microscopy and is characterized by the formation of a linear birefringence whose orientation is almost linearly dependent on the laser polarization. Figure 1 shows the different types obtained as a function of the scanning speed and the laser pulse energy in 9754 and GeS\({}_{2}\) glasses. SF10 has writing-type threshold graphs similar to those of 9754 in Figure 1. In particular, the energy thresholds needed for the appearance of Type I or spatial broadening regimes are similar: about 0.1 \(\upmu\)J for Type I and between 1.5 \(\upmu\)J and 2 \(\upmu\)J for the \"spatial broadening regime\". It should be noted, however, that 9754 is the only one, which shows the appearance of Type II, and none of the three materials showed Type III--voids formation. For a low writing speed (typ. 0.01 mm/s) self-assembled periodic nanostructures appear in 9754 glass, creating a birefringence whose neutral axes are oriented by the femtosecond laser polarization [19]. In contrast, we note that the chalcogenide glass has low Type I thresholds of a few nJ at low scanning speeds and a few 10 s nJ at high scanning speeds (\(>\)1 mm/s, up to 25 mm/s). The spatial broadening regime also appears for low energies of a few 10\({}^{-1}\)\(\upmu\)J. On the contr