# 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
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Label** & \(\mathbf{n}_{1.55\upmu\text{m}}\) & \(\mathbf{n}_{4\upmu\text{m}}\) \\ \hline GeS\({}_{2}\) based glass & 2.082 & 2.028 \\ Corning 9754 & 1.643 & 1.607 \\ Schott SF10 & 1.693 & 1.637 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Label and refractive index of the samples. The data for the 9754 and the SF10 come from the Corning and Schott datasheets, respectively. For GeS\({}_{2}\) data were measured using refracometry in total internal reflection mode. glasses show thresholds of about one order higher for both Type I threshold and spatial broadening regime. Taking into account the size of the laser beam, we can express the Type I thresholds in terms of fluence (J/cm\({}^{2}\)). In our typical writing conditions, at the scanning speed of 1 mm/s, we calculate the corresponding fluence per pulse in GeS\({}_{2}\) at 2.7 J/cm\({}^{2}\), in 9754 at 11.1 J/cm\({}^{2}\), and in SF10 at 13.3 J/cm\({}^{2}\). Moreover, the threshold of appearance of the different regimes depends on the number of pulses sometimes called the \"incubation effect\". Typically, one can find a decrease by a factor of 2 for oxide glasses, e.g., from 13.3 J/cm\({}^{2}\) to 6.6 J/cm\({}^{2}\) in SF10 when decreasing the speed down to 0.01 mm/s (i.e., \(1.2\times 10^{4}\) pulses/\(\upmu\)m). Following a similar trend in GeS\({}_{2}\), but more pronounced, the threshold decreases progressively from 4.4 J/cm\({}^{2}\) for 5 pulses/\(\upmu\)m (speed of 25 mm/s) to 2.7 J/cm\({}^{2}\) for 120 pulses/\(\upmu\)m and down to 0.4 J/cm\({}^{2}\) for \(1.2\times 10^{4}\) pulses/\(\upmu\)m (0.01 mm/s). We also note that the size (longitudinal) of the modified zone increases with the energy but not with the number of pulses; on the other hand, the \(\upDelta\)n increases with the number of pulses. In addition, the width of the laser tracks remains quite similar within the whole Type I regime. This contrasts with the so-called spatial broadening regime where the width increases progressively with the pulse energy. Therefore, we suggest here that Type I modifications are related to the accumulation of point defects leading to a glass network depolymerization, which favors a structural relaxation. For example, this is confirmed in GeS\({}_{2}\) by Raman measurements indicating that the refractive index increase is associated not only to point defects generation but rather to glass densification (e.g., an increase in the population of edge-sharing tetrahedra in GeS\({}_{2}\)) [20]. Then, in agreement with the literature, the measured Type I thresholds decrease when the number of pulses increases. Indeed, using moderate focusing (0.25 NA), [PERSON] et al. [21] observed \"incubation effects\" leading to a decrease in the damage threshold in oxide glasses when increasing the number of pulses, as observed during surface experiments [22; 23]. In the latter case, it has been demonstrated [24] that the effect of incubation is twofold: (i) it modifies the absorption by the accumulation of point defects, and (ii) it introduces new possibilities of energy deposition in the glass network, e.g., in addition to electron-phonon coupling. These possibilities are based on a coupling of electrons trapped in the lattice. The increase in defect concentration is proportional to the initial defect concentration and the increment in the number of pulses and also depends on the number of atomic sites likely to be affected during this accumulation process. Then, there is likely a second mechanism acting in the spatial broadening regime, which enlarges progressively the laser track's width. In a simplified view, the spatial extension of the laser tracks (beyond the light beam size) is likely related to a temperature increase with the pulse energy [25]. Now, if we increase the energy of the pulses, the maximum temperature reached at the laser focal point increases but the spatial distribution of heat (defined by the Fourier's law) remains unchanged.