# Laser-Assisted Etching of EagleXG Glass by Irradiation at Low Pulse-Repetition Rate
## 3 Experimental Results
### Morphology of the Etched Structures  
In Figure 5, we report representative pictures of the etched cones, taken with the optical microscope, which reveal a difference in the smoothness of their surfaces as the irradiation energy increases. In fact, despite maintaining a high A.R., the cones corresponding to the highest irradiation pulse-energies (beyond 6.6 \(\upmu\)J) show a rougher surface after etching, if compared to the cones corresponding to lower energies, which are uniformly smooth for their whole length. This observation applies both to cones etched in HF and KOH solution. We note that this increase in roughness comes together with the rebound of the A.R. observed in Figure 3b for \(E_{P}>6.6\)\(\upmu\)J. This might be due to a different substrate modification process, which comes into play when the pulse energy surpasses a given threshold. A smooth surface is preferable when dealing with the fabrication of microchannels for optofluidic circuits: a careful choice of the irradiation parameters should thus be made in order to match the requirements of the final device. A direct visual comparison of the roughness of the cones etched in HF or KOH is not easy, given the tiny diameter of the cones etched in KOH. In any case, it is worth noting that the quality of the unirradiated surfaces of the glass sample is also important in the applications, and we have observed  
Figure 4: (**a**) Microscope pictures of the cross-section of tracks irradiated with two different values of \(E_{p}\), and repetition rates ranging from 5 kHz to 200 kHz. Scalebars correspond to 5 \(\upmu\)m (all pictures in the same row have the same scale). The scan speed is scaled proportionally to the repetition rate, keeping as reference \(v_{t}=1\) mm/s at 25 kHz. (**b**) A.R. of the etched cones obtained from the above-illustrated tracks, as measured after 15 min immersion in HF solution. that this is indeed different after the exposure to the two etching agents. In fact, after immersion in HF solution, the surfaces that were polished to optical quality become slightly opaque and bumpy; on the contrary, such surface alterations are practically absent after KOH processing. It is interesting to examine the inlet of the cones etched in KOH solution at the optical microscope (see the rightmost panels of Figure 6). In fact, it is clearly noted that, while the laser-modified region extends transversally for several microns, the chemical agent only attacks a practically point-like feature placed towards the bottom of the irradiated track. Indeed, even after several hours of immersion in the solution, the rest of the cross-section looks practically unaltered. On the other hand, after several-minutes immersion in HF solution, the etched cone inlet is wider than the cross-section of the irradiated track (see, in particular, the pictures in Figure 6 corresponding to 10-20 min etching in HF). To gain further insight on the etching process in HF, we inscribed two sets of many identical tracks, corresponding to different irradiation parameters which give the best selectivity according to the data in Figure 3a, and observed how the excavation of the cones proceeds in small time steps. In detail, we inscribed tracks with laser polarization orthogonal to the writing direction, \(v_{t}=1\) mm/s and either \(E_{p}=2.2\)\(\upmu\) or \(E_{p}=8.6\)\(\upmu\). The sample was then cut in several pieces transversally to the tracks, the lateral facets were polished to optical quality, and pieces were immersed in HF solution for different times, up to 20 min. By inspecting the cone inlets at the optical microscope (Figure 6), it is evident that also in the case of HF the etching proceeds from the same point-like feature in the lower part of the inscribed track. Here, however, the other regions of the modified cross-section also show some etching enhancement with respect to the bulk, though in a lesser measure.