# Laser-Assisted Etching of EagleXG Glass by Irradiation at Low Pulse-Repetition Rate
## 3 Experimental Results
### Analysis of the Irradiation Parameters
Significant etching selectivity was instead observed for the higher energies, though with different patterns in HF with respect to the KOH. In HF, we measured A.R.\(\sim\)10-20 in a broad range of \(E_{p}\) values, which is consistent with the experimental results. The results are shown in Fig. 1.  
Figure 1: (**a**) Pictorial representation of the two steps of the laser-assisted etching process. First (**left panel**), tracks are irradiated with a focused femtosecond laser beam, in the bulk of a glass sample and covering its full length. The irradiation is performed by translating the sample at constant speed with respect to the laser focus. Second (**right panel**), the sample is immersed in a solution of HF or KOH for a controlled time. (**b**) Typical outcome of the etching process of a straight irradiated track. A hollow cone is formed along the track, with basis radius \(r\) and length \(L\). The aspect ratio (A.R.) of the cone, defined as A.R. \(=L/r\), is a direct indication of the etching selectivity. Dimensions are not to scale. Figure 2: Characterisation of the A.R. of the etched cones as a function of the irradiation parameters: pulse energy \(E_{p}\), translation speed \(v_{t}\), and polarisation (parallel or orthogonal with respect to the translation direction). The repetition rate of the laser pulses is 25 kHz. The area of the coloured circles indicates the A.R. of the cones, as measured (**a**) after 10 min of etching in HF solution or (**b**) after 6 h in KOH solution. The size of the circles in the legend corresponds to a value of 10.  
of parameters; the two polarisations give results that are in the same order of magnitude, even if the parallel one consistently provides slightly lower selectivity than the orthogonal one. On the other hand, in KOH the etching results are less uniform across the explored energies and speeds, and a clear dependence on the laser polarisation can be noticed. In particular, while the orthogonal polarization gives very high values of A.R. (up to about 90), the A.R. hardly reaches 10 for the parallel one. On the basis of these preliminary results, we conducted a second experiment to better investigate the etching dynamics. In particular, we irradiated a set of tracks, performing a finer scan of the pulse energy between 0.6 \(\upmu\)J and 8.6 \(\upmu\)J, and with translation speeds again ranging from 0.5 mm/s to 4 mm/s. The polarisation of the writing beam was kept orthogonal to the translation direction, as it consistently provides the higher A.R. according to the first characterisation. For each couple of values of the parameters (\(E_{p}\) and \(v_{t}\)), we wrote two identical tracks. After fabrication, the sample was cut into several pieces, whose lateral facets were polished, and which were later etched separately in HF and KOH solutions. Figure 3 plots the measured A.R. of the etched cones, after 10 min etching in case of HF and after 6 h in case of KOH. A comparison between Figure 3a,b confirms the difference between the etching processes in the HF and KOH etchants, already visible in Figure 2. In detail, we observe that, in the case of HF etching, the measured A.R. at first increases with increasing pulse energy, then enters a sort of plateau in correspondence of \(E_{p}\simeq 2\)\(\upmu\)J. For higher energies the A.R. remains practically constant, reaching maximum values around 25. This corresponds to an etching rate of the irradiated track of about 60 \(\upmu\)m/min, namely 25 times larger than the  
Figure 3: Measured A.R. of the etched cones, for tracks irradiated with different pulse energy \(E_{p}\) and different translation speed \(v_{t}\). Laser polarisation was kept orthogonal to the translation direction. The plotted values correspond to the averages of the measured A.R. of two different cones, obtained by tracks irradiated with identical parameters. Panel (**a**) refers to 10-min etching in HF solution, while panel (**b**) refers to 6-h etching in KOH solution. etching rate of the unmodified substrate 2.5 \(\upmu\)m/min.