## 3 Results and Discussion
### Laser-Induced Damage Threshold  
Surface LIDT measurement results using the R(1)-on-1 protocol at 1540 nm for 3.5 ns pulses of nonetched and oxygen-plasma-processed Co:MALO crystals are shown in Figure 6.  
Figure 4: TEM images of (**a**) plasma-unprocessed crystal, (**b**) plasma-etched to 100 nm, (**c**) plasma-etched to 250 nm and (**d**) plasma etched to 400 nm. The blue dotted line indicates the surface of samples. An etching of 50 nm already improved by 1.5 times the initial optical resistance of the plasma-unprocessed surface (\(55\pm 6\) J/cm\({}^{2}\)). However, a huge LIDT increase by \(\sim\)5-6 times up to \(\sim\)250-300 J/cm\({}^{2}\) was manifested for deeper etchings--100, 250 and 400 nm. It would be relevant to compare these obtained values to the results of other investigations, or to the bulk LIDT values of spinel crystal. However, to the best of our knowledge, such published results do not exist. Still, we may compare the obtained trend in LIDT values while changing the etching depth with previous research on etching fused-silica substrates (though oxygen plasma ion energies and current densities were not exactly the same in that investigation, which resulted in the highest LIDT increase) [17]. A 50 nm etching of FS already resulted in a twofold increase of the LIDT at 355 nm (single-shot tests), while a 220 nm depth etching resulted in a maximum increase of the LIDT (\(\sim\)15\(\times\) increase  
Figure 5: High-resolution TEM images of (**a**) plasma-unprocessed crystal, (**b**) plasma-etched to 100 nm, plasma-etched to 250 nm (**c**) and plasma-etched to 400 nm (**d**). The blue dotted line indicates the surface of samples. Insets present the electron diffractogram of a selected area. Figure 6: Surface LIDT (R(1)-on-1) for 1064 nm (\(\tau\) = 3.5 ns, d = 55 \(\upmu\)m) of unprocessed and plasma-etched to different depths Co:MALO crystals. comparing to the nonetched FS substrate). A much more pronounced effect of the plasma etching there might be related to the fact that in general, the absorption capabilities of polishing remnants are even stronger in the UV range. However, the LIDT of FS etched to ~550 nm decreased by about two times compared with the LIDT at d = 220 nm. We did not observe a drop in the LIDT in the present research of the plasma etching of Co-doped spinel. However, when using a longer (and deeper) etching time, another damage resistance trend might occur. This might be addressed in future experiments. [PERSON] et al. used a low-energy RF argon plasma etching of CsLiB\({}_{6}\)O\({}_{10}\) (CLBO) crystals and after a 60 nm etching, they observed a 10-time increase of their lifetime for 266 nm 100 Hz laser pulses with a 90-100 MW/cm\({}^{2}\) power density [20]. To our best knowledge, there are no other published results regarding alternative technologies for removing the damaged subsurface layer of magnesium aluminate spinel crystals. There was no clear correlation between the subsurface damages observed by TEM and the obtained laser damage resistance of crystals at 1540 nm. Though a 100 nm etched sample did not have a continuously damaged subsurface layer like a plasma-unprocessed crystal (Figure 3a), it still had some clearly seen subsurface defects (Figure 3b). Deeper-etched crystals did not show any damaged layer (Figures 3c and 4c,d), but their LIDTs were not higher. More detailed information about the damage initiating defect densities might be obtained by fitting damage probability data, which was discussed in the work by [PERSON] et al. [34]. Two types of defect ensembles could be identified in our case. They had completely different LIDTs: a low LIDT (55 J/cm\({}^{2}\) as clearly seen by fitting data for the nonetched sample (Figure 7a) and a high LIDT of 239 J/cm\({}^{2}\) (that ensemble is already seen in the damage probability data of 50 nm etched sample (Figure 7b)).